LM2673シリーズのレギュレータはモノリシックな集積回路で、降圧型(バック)スイッチング・レギュレータのすべてのアクティブ機能が搭載されており、優れたラインおよび負荷レギュレーション特性で3Aまでの負荷を駆動できます。低オン抵抗のDMOSパワー・スイッチを使用して、高い効率(90%超)を実現しています。このシリーズには、3.3V、5V、12Vの固定出力電圧のバージョンと、可変出力電圧のバージョンがあります。
SIMPLE SWITCHER®コンセプトにより、最小限の外付け部品で完全な設計を作成できます。高い固定周波数の発振器(260kHz)により、物理的に小さい部品を使用できます。LM2673で使用する標準インダクタは、いくつかの製造元から入手可能で、設計プロセスを大幅に簡素化できます。
他の特長として、ソフトスタート・タイミング・コンデンサを使用してレギュレータの電源を徐々にオンにするため、電源オン時の入力サージ電流が低いことが挙げられます。また、LM2673シリーズにはサーマル・シャットダウン機能が組み込まれており、フォルト状況でデバイスおよび負荷回路を保護するためパワーMOSFETスイッチの電流制限を抵抗によりプログラム可能です。出力電圧の定格許容誤差は±2%が保証されています。クロック周波数は±11%の許容誤差内に制御されます。
型番 | パッケージ | 本体サイズ(公称) |
---|---|---|
LM2673 | TO-263 (7) | 10.10mm×8.89mm |
TO-220 (7) | 14.986mm×10.16mm | |
VSON (14) | 6.00mm×5.00mm |
Changes from N Revision (April 2013) to O Revision
Changes from M Revision (April 2013) to N Revision
PIN | I/O | DESCRIPTION | ||
---|---|---|---|---|
NAME | TO-263, TO-220 |
VSON | ||
Switch output | 1 | 12, 13, 14 | O | Source pin of the internal High Side FET. This is a switching node. Attached this pin to an inductor and the cathode of the external diode. |
Input | 2 | 2, 3 | I | Supply input pin to collector pin of high side FET. Connect to power supply and input bypass capacitors CIN. Path from VIN pin to high frequency bypass CIN and GND must be as short as possible. |
CB | 3 | 4 | I | Boot-strap capacitor connection for high-side driver. Connect a high quality 100-nF capacitor from CB to VSW Pin. |
GND | 4 | 9 | — | Power ground pins. Connect to system ground. Ground pins of CIN and COUT. Path to CIN must be as short as possible. |
Current adjust | 5 | 6 | I | Current Limit adjust pin. Connect a resistor from this pin to GND to set the current limit of the part. |
FB | 6 | 7 | I | Feedback sense input pin. Connect to the midpoint of feedback divider to set VOUT for ADJ version or connect this pin directly to the output capacitor for a fixed output version. |
SS | 7 | 8 | I | Soft-start pin. Connect a capacitor from this pin to GND to control the output voltage ramp. If the feature not desired, the pin can be left floating |
NC | — | 1, 5, 10, 11 | — | No connect pins |
MIN | MAX | UNIT | ||
---|---|---|---|---|
Input supply voltage | 45 | V | ||
Soft-start pin voltage | –0.1 | 6 | V | |
Switch voltage to ground(3) | –1 | VIN | V | |
Boost pin voltage | VSW + 8 V | V | ||
Feedback pin voltage | –0.3 | 14 | V | |
Power dissipation | Internally Limited | |||
Soldering temperature | Wave, 4 s | 260 | °C | |
Infrared, 10 s | 240 | |||
Vapor phase, 75 s | 219 | |||
Storage temperature, Tstg | –65 | 150 | °C |
VALUE | UNIT | |||
---|---|---|---|---|
V(ESD) | Electrostatic discharge | Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001(1)(2) | ±2000 | V |
MIN | MAX | UNIT | |
---|---|---|---|
Supply voltage | 8 | 40 | V |
Junction temperature (TJ) | –40 | 125 | °C |
THERMAL METRIC(1) | LM2678 | UNIT | ||||
---|---|---|---|---|---|---|
NDZ (TO-220) | KTW (TO-263) | NHM (VSON) | ||||
7 PINS | 7 PINS | 14 PINS | ||||
RθJA | Junction-to-ambient thermal resistance | See (2) | 65 | — | — | °C/W |
See (3) | 45 | — | — | |||
See (4) | — | 56 | — | |||
See (5) | — | 35 | — | |||
See (6) | — | 26 | — | |||
See (7) | — | — | 55 | |||
See (8) | — | — | 29 | |||
RθJC(top) | Junction-to-case (top) thermal resistance | 2 | 2 | — | °C/W |
PARAMETER | TEST CONDITIONS | MIN(1) | TYP(2) | MAX(1) | UNIT | ||
---|---|---|---|---|---|---|---|
VOUT | Output voltage | VIN = 8 V to 40 V, 100 mA ≤ IOUT ≤ 5 A |
3.234 | 3.3 | 3.366 | V | |
over the entire junction temperature range of operation –40°C to 125°C | 3.201 | 3.399 | |||||
η | Efficiency | VIN = 12 V, ILOAD = 5 A | 86% |
PARAMETER | TEST CONDITIONS | MIN(1) | TYP(2) | MAX(1) | UNIT | ||
---|---|---|---|---|---|---|---|
VOUT | Output voltage | VIN = 8 V to 40 V, 100 mA ≤ IOUT ≤ 5 A |
4.9 | 5 | 5.1 | V | |
over the entire junction temperature range of operation –40°C to 125°C | 4.85 | 5.15 | |||||
η | Efficiency | VIN = 12 V, ILOAD = 5 A | 88% |
PARAMETER | TEST CONDITIONS | MIN(1) | TYP(2) | MAX(1) | UNIT | ||
---|---|---|---|---|---|---|---|
VOUT | Output voltage | VIN = 15 V to 40 V, 100 mA ≤ IOUT ≤ 5 A |
11.76 | 12 | 12.24 | V | |
over the entire junction temperature range of operation –40°C to 125°C | 11.64 | 12.36 | |||||
η | Efficiency | VIN = 24 V, ILOAD = 5 A | 94% |
PARAMETER | TEST CONDITIONS | MIN(1) | TYP(2) | MAX(1) | UNIT | ||
---|---|---|---|---|---|---|---|
VFB | Feedback voltage | VIN = 8 V to 40 V, 100 mA ≤ IOUT ≤ 5 A, VOUT programmed for 5 V |
1.186 | 1.21 | 1.234 | V | |
over the entire junction temperature range of operation –40°C to 125°C | 1.174 | 1.246 | |||||
η | Efficiency | VIN = 12 V, ILOAD = 5 A | 88% |
PARAMETER | TEST CONDITIONS | MIN | TYP | MAX | UNIT | ||
---|---|---|---|---|---|---|---|
DEVICE PARAMETERS | |||||||
IQ | Quiescent current | VFEEDBACK = 8 V for 3.3-V, 5-V, and ADJ versions, VFEEDBACK = 15 V for 12-V versions |
4.2 | 6 | mA | ||
VADJ | Current limit adjust voltage | 1.181 | 1.21 | 1.229 | V | ||
over the entire junction temperature range of operation –40°C to 125°C | 1.169 | 1.246 | |||||
ICL | Current limit | RADJ = 5.6 kΩ,(1) | 5.5 | 6.3 | 7.6 | A | |
over the entire junction temperature range of operation –40°C to 125°C | 5.3 | 8.1 | |||||
IL | Output leakage current | VIN = 40 V, soft-start pin = 0 V |
VSWITCH = 0 V | 1 | 1.5 | mA | |
VSWITCH = –1 V | 6 | 15 | |||||
RDS(ON) | Switch ON-resistance | ISWITCH = 5 A | 0.12 | 0.14 | Ω | ||
over the entire junction temperature range of operation –40°C to 125°C | 0.225 | ||||||
fO | Oscillator frequency | Measured at switch pin | 260 | kHz | |||
over the entire junction temperature range of operation –40°C to 125°C | 225 | 280 | |||||
D | Duty cycle | Maximum duty cycle | 91% | ||||
Minimum duty cycle | 0% | ||||||
IBIAS | Feedback bias current |
VFEEDBACK = 1.3 V ADJ version only |
85 | nA | |||
VSFST | Soft-start threshold voltage | 0.63 | V | ||||
over the entire junction temperature range of operation –40°C to 125°C | 0.53 | 0.74 | |||||
ISFST | Soft-start pin current | Soft-start pin = 0 V | 3.7 | µA | |||
over the entire junction temperature range of operation –40°C to 125°C | 6.9 |
Continuous Mode Switching Waveforms VIN = 20 V, VOUT = 5 V, ILOAD = 3 A L = 33 µH, COUT = 200 µF, COUTESR = 26 mΩ |
||
A: VSW Pin Voltage, 10 V/div | ||
B: Inductor Current, 1 A/div | ||
C: Output Ripple Voltage, 20 mV/div AC-Coupled |
Load Transient Response for Continuous Mode VIN = 20 V, VOUT = 5 V L = 33 µH, COUT = 200 µF, COUTESR = 26 mΩ |
||
A: Output Voltage, 100 mV//div, AC-Coupled. | ||
B: Load Current: 500-mA to 3-A Load Pulse |
Discontinuous Mode Switching Waveforms VIN = 20 V, VOUT = 5 V, ILOAD = 500 mA L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ |
||
A: VSW Pin Voltage, 10 V/div | ||
B: Inductor Current, 1 A/div | ||
C: Output Ripple Voltage, 20 mV/div AC-Coupled |
Load Transient Response for Discontinuous Mode VIN = 20 V, VOUT = 5 V, L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ | ||
A: Output Voltage, 100 mV/div, AC-Coupled. | ||
B: Load Current: 200-mA to 3-A Load Pulse |
The LM2673 provides all of the active functions required for a step-down (buck) switching regulator. The internal power switch is a DMOS power MOSFET to provide power supply designs with high current capability, up to 3 A, and highly efficient operation.
The design support WEBENCH, can also be used to provide instant component selection, circuit performance calculations for evaluation, a bill of materials component list and a circuit schematic for LM2673.
This is the output of a power MOSFET switch connected directly to the input voltage. The switch provides energy to an inductor, an output capacitor and the load circuitry under control of an internal pulse-width-modulator (PWM). The PWM controller is internally clocked by a fixed 260-kHz oscillator. In a standard step-down application the duty cycle (Time ON/Time OFF) of the power switch is proportional to the ratio of the power supply output voltage to the input voltage. The voltage on pin 1 switches between VIN (switch ON) and below ground by the voltage drop of the external Schottky diode (switch OFF).
The input voltage for the power supply is connected to pin 2. In addition to providing energy to the load the input voltage also provides bias for the internal circuitry of the LM2673. For ensured performance the input voltage must be in the range of 8 V to 40 V. For best performance of the power supply the input pin must always be bypassed with an input capacitor located close to pin 2.
A capacitor must be connected from pin 3 to the switch output, pin 1. This capacitor boosts the gate drive to the internal MOSFET above VIN to fully turn it ON. This minimizes conduction losses in the power switch to maintain high efficiency. The recommended value for C Boost is 0.01 µF.
This is the ground reference connection for all components in the power supply. In fast-switching, high-current applications such as those implemented with the LM2673, TI recommends that a broad ground plane be used to minimize signal coupling throughout the circuit.
A key feature of the LM2673 is the ability to tailor the peak switch current limit to a level required by a particular application. This alleviates the need to use external components that must be physically sized to accommodate current levels (under shorted output conditions for example) that may be much higher than the normal circuit operating current requirements.
A resistor connected from pin 5 to ground establishes a current (I(pin 5) = 1.2 V / RADJ) that sets the peak current through the power switch. The maximum switch current is fixed at a level of 37,125 / RADJ.
This is the input to a two-stage high gain amplifier, which drives the PWM controller. It is necessary to connect pin 6 to the actual output of the power supply to set the DC output voltage. For the fixed output devices (3.3-V, 5-V, and 12-V outputs), a direct wire connection to the output is all that is required as internal gain setting resistors are provided inside the LM2673. For the adjustable output version two external resistors are required to set the dc output voltage. For stable operation of the power supply it is important to prevent coupling of any inductor flux to the feedback input.
A capacitor connected from pin 7 to ground allows for a slow turnon of the switching regulator. The capacitor sets a time delay to gradually increase the duty cycle of the internal power switch. This can significantly reduce the amount of surge current required from the input supply during an abrupt application of the input voltage. If soft start is not required this pin must be left open circuited. See CSS Soft-Start Capacitor for further information regarding soft-start capacitor values.
NOTE
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
Power supply design using the LM2673 is greatly simplified by using recommended external components. A wide range of inductors, capacitors and Schottky diodes from several manufacturers have been evaluated for use in designs that cover the full range of capabilities (input voltage, output voltage, and load current) of the LM2673. A simple design procedure using nomographs and component tables provided in this data sheet leads to a working design with very little effort.
The individual components from the various manufacturers called out for use are still just a small sample of the vast array of components available in the industry. While these components are recommended, they are not exclusively the only components for use in a design. After a close comparison of component specifications, equivalent devices from other manufacturers could be substituted for use in an application.
Important considerations for each external component and an explanation of how the nomographs and selection tables were developed follows.
The inductor is the key component in a switching regulator. For efficiency the inductor stores energy during the switch ON time and then transfers energy to the load while the switch is OFF.
Nomographs are used to select the inductance value required for a given set of operating conditions. The nomographs assume that the circuit is operating in continuous mode (the current flowing through the inductor never falls to zero). The magnitude of inductance is selected to maintain a maximum ripple current of 30% of the maximum load current. If the ripple current exceeds this 30% limit the next larger value is selected.
The inductors offered have been specifically manufactured to provide proper operation under all operating conditions of input and output voltage and load current. Several part types are offered for a given amount of inductance. Both surface mount and through-hole devices are available. The inductors from each of the three manufacturers have unique characteristics.
The output capacitor acts to smooth the dc output voltage and also provides energy storage. Selection of an output capacitor, with an associated equivalent series resistance (ESR), impacts both the amount of output ripple voltage and stability of the control loop.
The output ripple voltage of the power supply is the product of the capacitor ESR and the inductor ripple current. The capacitor types recommended in the tables were selected for having low ESR ratings.
In addition, both surface mount tantalum capacitors and through-hole aluminum electrolytic capacitors are offered as solutions.
Impacting frequency stability of the overall control loop, the output capacitance, in conjunction with the inductor, creates a double pole inside the feedback loop. In addition the capacitance and the ESR value create a zero. These frequency response effects together with the internal frequency compensation circuitry of the LM2673 modify the gain and phase shift of the closed loop system.
As a general rule for stable switching regulator circuits it is desired to have the unity gain bandwidth of the circuit to be limited to no more than one-sixth of the controller switching frequency. With the fixed 260-kHz switching frequency of the LM2673, the output capacitor is selected to provide a unity gain bandwidth of 40 kHz (maximum). Each recommended capacitor value has been chosen to achieve this result.
In some cases multiple capacitors are required either to reduce the ESR of the output capacitor, to minimize output ripple (a ripple voltage of 1% of VOUT or less is the assumed performance condition), or to increase the output capacitance to reduce the closed-loop unity gain bandwidth, to less than 40 kHz. When parallel combinations of capacitors are required it has been assumed that each capacitor is the exact same part type.
The RMS current and working voltage (WV) ratings of the output capacitor are also important considerations. In a typical step-down switching regulator, the inductor ripple current (set to be no more than 30% of the maximum load current by the inductor selection) is the current that flows through the output capacitor. The capacitor RMS current rating must be greater than this ripple current. The voltage rating of the output capacitor must be greater than 1.3 times the maximum output voltage of the power supply. If operation of the system at elevated temperatures is required, the capacitor voltage rating may be de-rated to less than the nominal room temperature rating. Careful inspection of the manufacturer's specification for de-rating of working voltage with temperature is important.
Fast changing currents in high current switching regulators place a significant dynamic load on the unregulated power source. An input capacitor helps to provide additional current to the power supply as well as smooth out input voltage variations.
Like the output capacitor, the key specifications for the input capacitor are RMS current rating and working voltage. The RMS current flowing through the input capacitor is equal to one-half of the maximum DC load current so the capacitor should be rated to handle this. Paralleling multiple capacitors proportionally increases the current rating of the total capacitance. The voltage rating must also be selected to be 1.3 times the maximum input voltage. Depending on the unregulated input power source, under light load conditions the maximum input voltage could be significantly higher than normal operation. Consider this when selecting an input capacitor.
The input capacitor must be placed very close to the input pin of the LM2673. Due to relative high current operation with fast transient changes, the series inductance of input connecting wires or PCB traces can create ringing signals at the input terminal which could possibly propagate to the output or other parts of the circuitry. It may be necessary in some designs to add a small valued (0.1 µF to 0.47 µF) ceramic type capacitor in parallel with the input capacitor to prevent or minimize any ringing.
When the power switch in the LM2673 turns OFF, the current through the inductor continues to flow. The path for this current is through the diode connected between the switch output and ground. This forward biased diode clamps the switch output to a voltage less than ground. This negative voltage must be greater than –1 V so a low voltage drop (particularly at high current levels) Schottky diode is recommended. Total efficiency of the entire power supply is significantly impacted by the power lost in the output catch diode. The average current through the catch diode is dependent on the switch duty cycle (D) and is equal to the load current times (1-D). Use of a diode rated for much higher current than is required by the actual application helps to minimize the voltage drop and power loss in the diode.
During the switch ON time the diode will be reversed biased by the input voltage. The reverse voltage rating of the diode should be at least 1.3 times greater than the maximum input voltage.
The boost capacitor creates a voltage used to overdrive the gate of the internal power MOSFET. This improves efficiency by minimizing the on resistance of the switch and associated power loss. For all applications it is recommended to use a 0.01-µF, 50-V ceramic capacitor.
A key feature of the LM2673 is the ability to control the peak switch current. Without this feature the peak switch current would be internally set to 5 A or higher to accommodate 3-A load current designs. This requires that both the inductor (which could saturate with excessively high currents) and the catch diode be able to safely handle up to 5 A which would be conducted under load fault conditions.
If an application only requires a load current of 2 A or so the peak switch current can be set to a limit just over the maximum load current with the addition of a single programming resistor. This allows the use of less powerful and more cost-effective inductors and diodes.
The peak switch current is equal to a factor of 37,125 divided by RADJ. A resistance of 8.2 kΩ sets the current limit to typically 4.5 A. For predictable control of the current limit, TI recommends keeping the peak switch current greater than 1 A. For lower current applications 500-mA and 1-A switching regulators, the LM2674 and LM2672, are available.
When the power switch reaches the current limit threshold it is immediately turned OFF and the internal switching frequency is reduced. This extends the OFF time of the switch to prevent a steady-state, high-current condition. As the switch current falls below the current limit threshold, the switch turns back ON. If a load fault continues, the switch again exceeds the threshold and switch back OFF. This results in a low duty cycle pulsing of the power switch to minimize the overall fault condition power dissipation.
This optional capacitor controls the rate at which the LM2673 starts up at power on. The capacitor is charged linearly by an internal current source. This voltage ramp gradually increases the duty cycle of the power switch until it reaches the normal operating duty cycle defined primarily by the ratio of the output voltage to the input voltage. The soft-start turnon time is programmable by the selection of CSS.
The formula for selecting a soft-start capacitor is:
where
If this feature is not desired, leave the soft-start pin (pin 7) open circuited.
With certain soft-start capacitor values and operating conditions, the LM2673 can exhibit an overshoot on the output voltage during turnon. Especially when starting up into no load or low load, the soft-start function may not be effective in preventing a larger voltage overshoot on the output. With larger loads or lower input voltages during start-up this effect is minimized. In particular, avoid using soft-start capacitors between 0.033 µF and 1 µF.
When the output voltage is greater than approximately 6 V, and the duty cycle at minimum input voltage is greater than approximately 50%, the designer should exercise caution in selection of the output filter components. When an application designed to these specific operating conditions is subjected to a current limit fault condition, it may be possible to observe a large hysteresis in the current limit. This can affect the output voltage of the device until the load current is reduced sufficiently to allow the current limit protection circuit to reset itself.
Under current limiting conditions, the LM267x is designed to respond in the following manner:
If the output capacitance is sufficiently large, it may be possible that as the output tries to recover, the output capacitor charging current is large enough to repeatedly re-trigger the current-limit circuit before the output has fully settled. This condition is exacerbated with higher output voltage settings because the energy requirement of the output capacitor varies as the square of the output voltage (½ CV2), thus requiring an increased charging current.
A simple test to determine if this condition might exist for a suspect application is to apply a short circuit across the output of the converter, and then remove the shorted output condition. In an application with properly selected external components, the output recovers smoothly.
Practical values of external components that have been experimentally found to work well under these specific operating conditions are COUT = 47 µF, L = 22 µH. It should be noted that even with these components, for a device’s current limit of ICLIM, the maximum load current under which the possibility of the large current limit hysteresis can be minimized is ICLIM / 2. For example, if the input is 24 V and the set output voltage is 18 V, then for a desired maximum current of 1.5 A, the current limit of the chosen switcher must be confirmed to be at least 3 A.
Under extreme overcurrent or short-circuit conditions, the LM267X employs frequency foldback in addition to the current limit. If the cycle-by-cycle inductor current increases above the current limit threshold (due to short circuit or inductor saturation for example) the switching frequency is automatically reduced to protect the IC. Frequency below 100 kHz is typical for an extreme short-circuit condition.
Select the power supply operating conditions and the maximum output current and follow below procedures to find the external components for LM2673.
Using the nomographs and tables in this data sheet (or use the available design software at www.ti.com) a complete step-down regulator can be designed in a few simple steps.
Step 1: Define the power supply operating conditions:
Step 2: Set the output voltage by selecting a fixed output LM2673 (3.3-V, 5-V, or 12-V applications) or determine the required feedback resistors for use with the adjustable LM2673-ADJ
Step 3: Determine the inductor required by using one of the four nomographs, Figure 14 through Figure 17. Table 3 provides a specific manufacturer and part number for the inductor.
Step 4: Using Table 5 and Table 6 (fixed output voltage) or Table 9 and Table 10 (adjustable output voltage), determine the output capacitance required for stable operation. Table 1 and Table 10 provide the specific capacitor type from the manufacturer of choice.
Step 5: Determine an input capacitor from Table 5 and Table 8 for fixed output voltage applications. Use Table 1 or Table 2 to find the specific capacitor type. For adjustable output circuits select a capacitor from Table 1 or Table 2 with a sufficient working voltage (WV) rating greater than VIN maximum, and an RMS current rating greater than one-half the maximum load current (2 or more capacitors in parallel may be required).
Step 6:Select an appropriate diode from Table 4. The current rating of the diode must be greater than ILOAD maximum and the reverse voltage rating must be greater than VIN maximum.
Step 7: Include a 0.01-µF, 50-V capacitor for CBOOSTin the design and then determine the value of a soft-start capacitor if desired.
Step 8: Define a value for RADJ to set the peak switch current limit to be at least 20% greater than IOUT maximum to allow for at least 30% inductor ripple current (±15% of IOUT). For designs that must operate over the full temperature range the switch current limit should be set to at least 50% greater than IOUT maximum (1.5 × IOUT maximum).
CAPACITOR REFERENCE CODE | SURFACE MOUNT | ||||||||
---|---|---|---|---|---|---|---|---|---|
AVX TPS SERIES | SPRAGUE 594D SERIES | KEMET T495 SERIES | |||||||
C (µF) | WV (V) | Irms (A) | C (µF) | WV (V) | Irms (A) | C (µF) | WV (V) | Irms (A) | |
C1 | 330 | 6.3 | 1.15 | 120 | 6.3 | 1.1 | 100 | 6.3 | 0.82 |
C2 | 100 | 10 | 1.1 | 220 | 6.3 | 1.4 | 220 | 6.3 | 1.1 |
C3 | 220 | 10 | 1.15 | 68 | 10 | 1.05 | 330 | 6.3 | 1.1 |
C4 | 47 | 16 | 0.89 | 150 | 10 | 1.35 | 100 | 10 | 1.1 |
C5 | 100 | 16 | 1.15 | 47 | 16 | 1 | 150 | 10 | 1.1 |
C6 | 33 | 20 | 0.77 | 100 | 16 | 1.3 | 220 | 10 | 1.1 |
C7 | 68 | 20 | 0.94 | 180 | 16 | 1.95 | 33 | 20 | 0.78 |
C8 | 22 | 25 | 0.77 | 47 | 20 | 1.15 | 47 | 20 | 0.94 |
C9 | 10 | 35 | 0.63 | 33 | 25 | 1.05 | 68 | 20 | 0.94 |
C10 | 22 | 35 | 0.66 | 68 | 25 | 1.6 | 10 | 35 | 0.63 |
C11 | — | — | — | 15 | 35 | 0.75 | 22 | 35 | 0.63 |
C12 | — | — | — | 33 | 35 | 1 | 4.7 | 50 | 0.66 |
C13 | — | — | — | 15 | 50 | 0.9 | — | — | — |
CAPACITOR REFERENCE CODE | THROUGH HOLE | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
SANYO OS-CON SA SERIES | SANYO MV-GX SERIES | NICHICON PL SERIES | PANASONIC HFQ SERIES | |||||||||
C (µF) | WV (V) | Irms (A) | C (µF) | WV (V) | Irms (A) | C (µF) | WV (V) | Irms (A) | C (µF) | WV (V) | Irms (A) | |
C1 | 47 | 6.3 | 1 | 1000 | 6.3 | 0.8 | 680 | 10 | 0.8 | 82 | 35 | 0.4 |
C2 | 150 | 6.3 | 1.95 | 270 | 16 | 0.6 | 820 | 10 | 0.98 | 120 | 35 | 0.44 |
C3 | 330 | 6.3 | 2.45 | 470 | 16 | 0.75 | 1000 | 10 | 1.06 | 220 | 35 | 0.76 |
C4 | 100 | 10 | 1.87 | 560 | 16 | 0.95 | 1200 | 10 | 1.28 | 330 | 35 | 1.01 |
C5 | 220 | 10 | 2.36 | 820 | 16 | 1.25 | 2200 | 10 | 1.71 | 560 | 35 | 1.4 |
C6 | 33 | 16 | 0.96 | 1000 | 16 | 1.3 | 3300 | 10 | 2.18 | 820 | 35 | 1.62 |
C7 | 100 | 16 | 1.92 | 150 | 35 | 0.65 | 3900 | 10 | 2.36 | 1000 | 35 | 1.73 |
C8 | 150 | 16 | 2.28 | 470 | 35 | 1.3 | 6800 | 10 | 2.68 | 2200 | 35 | 2.8 |
C9 | 100 | 20 | 2.25 | 680 | 35 | 1.4 | 180 | 16 | 0.41 | 56 | 50 | 0.36 |
C10 | 47 | 25 | 2.09 | 1000 | 35 | 1.7 | 270 | 16 | 0.55 | 100 | 50 | 0.5 |
C11 | — | — | — | 220 | 63 | 0.76 | 470 | 16 | 0.77 | 220 | 50 | 0.92 |
C12 | — | — | — | 470 | 63 | 1.2 | 680 | 16 | 1.02 | 470 | 50 | 1.44 |
C13 | — | — | — | 680 | 63 | 1.5 | 820 | 16 | 1.22 | 560 | 50 | 1.68 |
C14 | — | — | — | 1000 | 63 | 1.75 | 1800 | 16 | 1.88 | 1200 | 50 | 2.22 |
C15 | — | — | — | — | — | — | 220 | 25 | 0.63 | 330 | 63 | 1.42 |
C16 | — | — | — | — | — | — | 220 | 35 | 0.79 | 1500 | 63 | 2.51 |
C17 | — | — | — | — | — | — | 560 | 35 | 1.43 | — | — | — |
C18 | — | — | — | — | — | — | 2200 | 35 | 2.68 | — | — | — |
C19 | — | — | — | — | — | — | 150 | 50 | 0.82 | — | — | — |
C20 | — | — | — | — | — | — | 220 | 50 | 1.04 | — | — | — |
C21 | — | — | — | — | — | — | 330 | 50 | 1.3 | — | — | — |
C22 | — | — | — | — | — | — | 100 | 63 | 0.75 | — | — | — |
C23 | — | — | — | — | — | — | 390 | 63 | 1.62 | — | — | — |
C24 | — | — | — | — | — | — | 820 | 63 | 2.22 | — | — | — |
C25 | — | — | — | — | — | — | 1200 | 63 | 2.51 | — | — | — |
INDUCTOR REFERENCE NUMBER | INDUCTANCE (µH) | CURRENT (A) | RENCO | PULSE ENGINEERING | COILCRAFT | ||
---|---|---|---|---|---|---|---|
THROUGH HOLE | SURFACE MOUNT | THROUGH HOLE | SURFACE MOUNT | SURFACE MOUNT | |||
L23 | 33 | 1.35 | RL-5471-7 | RL1500-33 | PE-53823 | PE-53823S | DO3316-333 |
L24 | 22 | 1.65 | RL-1283-22-43 | RL1500-22 | PE-53824 | PE-53824S | DO3316-223 |
L25 | 15 | 2 | RL-1283-15-43 | RL1500-15 | PE-53825 | PE-53825S | DO3316-153 |
L29 | 100 | 1.41 | RL-5471-4 | RL-6050-100 | PE-53829 | PE-53829S | DO5022P-104 |
L30 | 68 | 1.71 | RL-5471-5 | RL6050-68 | PE-53830 | PE-53830S | DO5022P-683 |
L31 | 47 | 2.06 | RL-5471-6 | RL6050-47 | PE-53831 | PE-53831S | DO5022P-473 |
L32 | 33 | 2.46 | RL-5471-7 | RL6050-33 | PE-53932 | PE-53932S | DO5022P-333 |
L33 | 22 | 3.02 | RL-1283-22-43 | RL6050-22 | PE-53933 | PE-53933S | DO5022P-223 |
L34 | 15 | 3.65 | RL-1283-15-43 | — | PE-53934 | PE-53934S | DO5022P-153 |
L38 | 68 | 2.97 | RL-5472-2 | — | PE-54038 | PE-54038S | — |
L39 | 47 | 3.57 | RL-5472-3 | — | PE-54039 | PE-54039S | — |
L40 | 33 | 4.26 | RL-1283-33-43 | — | PE-54040 | PE-54040S | — |
L41 | 22 | 5.22 | RL-1283-22-43 | — | PE-54041 | P0841 | — |
L44 | 68 | 3.45 | RL-5473-3 | — | PE-54044 | — | — |
L45 | 10 | 4.47 | RL-1283-10-43 | — | — | P0845 | DO5022P-103HC |
REVERSE VOLTAGE (V) | SURFACE MOUNT | THROUGH HOLE | ||
---|---|---|---|---|
3 A | 5 A OR MORE | 3 A | 5 A OR MORE | |
20 | SK32 | — | 1N5820 | — |
SR302 | ||||
30 | SK33 | MBRD835L | 1N5821 | — |
30WQ03F | 31DQ03 | |||
40 | SK34 | MBRB1545CT | 1N5822 | — |
30BQ040 | 6TQ045S | MBR340 | MBR745 | |
30WQ04F | 31DQ04 | 80SQ045 | ||
MBRS340 | SR403 | 6TQ045 | ||
MBRD340 | ||||
50 or more | SK35 | — | MBR350 | — |
30WQ05F | 31DQ05 | |||
SR305 |
For Continuous Mode Operation
Select the power supply operating conditions and the maximum output current and follow below procedures to find the external components for LM2673.
A system logic power supply bus of 3.3 V is to be generated from a wall adapter which provides an unregulated DC voltage of 13 V to 16 V. The maximum load current is 2.5 A. A soft-start delay time of 50 ms is desired. Through-hole components are preferred.
Step 1: Operating conditions are:
Step 2: Select an LM2673T-3.3. The output voltage has a tolerance of ±2% at room temperature and ±3% over the full operating temperature range.
Step 3: Use the nomograph for the 3.3-V device, Figure 14. The intersection of the 16-V horizontal line (VIN max) and the 2.5-A vertical line (ILOAD max) indicates that L33, a 22-µH inductor, is required.
From Table 3, L33 in a through-hole component is available from Renco with part number RL-1283-22-43 or part number PE-53933 from Pulse Engineering.
Step 4: Use Table 5 or Table 6 to determine an output capacitor. With a 3.3-V output and a 33-µH inductor there are four through-hole output capacitor solutions with the number of same type capacitors to be paralleled and an identifying capacitor code given. Table 1 or Table 2 provide the actual capacitor characteristics. Any of the following choices will work in the circuit:
Step 5:Use Table 5 or Table 8 to select an input capacitor. With 3.3-V output and 22 µH there are three through-hole solutions. These capacitors provide a sufficient voltage rating and an RMS current rating greater than 1.25 A (1/2 ILOAD max). Again using Table 1 or Table 2 for specific component characteristics the following choices are suitable:
Step 6: From Table 4 a 3-A or more Schottky diode must be selected. The 20-V rated diodes are sufficient for the application and for through-hole components two part types are suitable:
Step 7: A 0.01-µF capacitor will be used for CBOOST. For the 50-ms soft-start delay the following parameters are to be used:
Using VIN max ensures that the soft-start delay time will be at least the desired 50 ms.
Using the formula for CSS a value of 0.148 µF is determined to be required. Use of a standard value 0.22-µF capacitor will produce more than sufficient soft-start delay.
Step 8: Determine a value for RADJ to provide a peak switch current limit of at least 2.5 A plus 50% or 3.75 A.
Use a value of 10 kΩ.
OUTPUT VOLTAGE (V) | INDUCTANCE (µH) | SURFACE MOUNT | |||||
---|---|---|---|---|---|---|---|
AVX TPS SERIES | SPRAGUE 594D SERIES | KEMET T495 SERIES | |||||
NO. | C CODE | NO. | C CODE | NO. | C CODE | ||
3.3 | 10 | 4 | C2 | 3 | C1 | 4 | C4 |
15 | 4 | C2 | 3 | C1 | 4 | C4 | |
22 | 3 | C2 | 2 | C7 | 3 | C4 | |
33 | 2 | C2 | 2 | C6 | 2 | C4 | |
5 | 10 | 4 | C2 | 4 | C6 | 4 | C4 |
15 | 3 | C2 | 2 | C7 | 3 | C4 | |
22 | 3 | C2 | 2 | C7 | 3 | C4 | |
33 | 2 | C2 | 2 | C3 | 2 | C4 | |
47 | 2 | C2 | 1 | C7 | 2 | C4 | |
12 | 10 | 4 | C5 | 3 | C6 | 5 | C9 |
15 | 3 | C5 | 2 | C7 | 4 | C8 | |
22 | 2 | C5 | 2 | C6 | 3 | C8 | |
33 | 2 | C5 | 1 | C7 | 2 | C8 | |
47 | 2 | C4 | 1 | C6 | 2 | C8 | |
68 | 1 | C5 | 1 | C5 | 2 | C7 | |
100 | 1 | C4 | 1 | C5 | 1 | C8 |
OUTPUT VOLTAGE (V) | INDUCTANCE (µH) | THROUGH HOLE | |||||||
---|---|---|---|---|---|---|---|---|---|
SANYO OS-CON SA SERIES | SANYO MV-GX SERIES | NICHICON PL SERIES | PANASONIC HFQ SERIES | ||||||
NO. | C CODE | NO. | C CODE | NO. | C CODE | NO. | C CODE | ||
3.3 | 10 | 1 | C3 | 1 | C10 | 1 | C6 | 2 | C6 |
15 | 1 | C3 | 1 | C10 | 1 | C6 | 2 | C5 | |
22 | 1 | C5 | 1 | C10 | 1 | C5 | 1 | C7 | |
33 | 1 | C2 | 1 | C10 | 1 | C13 | 1 | C5 | |
5 | 10 | 2 | C4 | 1 | C10 | 1 | C6 | 2 | C5 |
15 | 1 | C5 | 1 | C10 | 1 | C5 | 1 | C6 | |
22 | 1 | C5 | 1 | C5 | 1 | C5 | 1 | C5 | |
33 | 1 | C4 | 1 | C5 | 1 | C13 | 1 | C5 | |
47 | 1 | C4 | 1 | C4 | 1 | C13 | 2 | C3 | |
12 | 10 | 2 | C7 | 2 | C5 | 1 | C18 | 2 | C5 |
15 | 1 | C8 | 1 | C5 | 1 | C17 | 1 | C5 | |
22 | 1 | C7 | 1 | C5 | 1 | C13 | 1 | C5 | |
33 | 1 | C7 | 1 | C3 | 1 | C11 | 1 | C4 | |
47 | 1 | C7 | 1 | C3 | 1 | C10 | 1 | C3 | |
68 | 1 | C7 | 1 | C2 | 1 | C10 | 1 | C3 | |
100 | 1 | C7 | 1 | C2 | 1 | C9 | 1 | C1 |
OUTPUT VOLTAGE (V) | INDUCTANCE (µH) | SURFACE MOUNT | |||||
---|---|---|---|---|---|---|---|
AVX TPS SERIES | SPRAGUE 594D SERIES | KEMET T495 SERIES | |||||
NO. | C CODE | NO. | C CODE | NO. | C CODE | ||
3.3 | 10 | 2 | C5 | 1 | C7 | 2 | C8 |
15 | 3 | C9 | 1 | C10 | 3 | C10 | |
22 | See(2) | See(2) | 2 | C13 | 3 | C12 | |
33 | See(2) | See(2) | 2 | C13 | 2 | C12 | |
5 | 10 | 2 | C5 | 1 | C7 | 2 | C8 |
15 | 2 | C5 | 1 | C7 | 2 | C8 | |
22 | 3 | C10 | 2 | C12 | 3 | C11 | |
33 | See(2) | See(2) | 2 | C13 | 3 | C12 | |
47 | See(2) | See(2) | 1 | C13 | 2 | C12 | |
12 | 10 | 2 | C7 | 2 | C10 | 2 | C7 |
15 | 2 | C7 | 2 | C10 | 2 | C7 | |
22 | 3 | C10 | 2 | C12 | 3 | C10 | |
33 | 3 | C10 | 2 | C12 | 3 | C10 | |
47 | See(2) | See(2) | 2 | C13 | 3 | C12 | |
68 | See(2) | See(2) | 2 | C13 | 2 | C12 | |
100 | See(2) | See(2) | 1 | C13 | 2 | C12 |
OUTPUT VOLTAGE (V) | INDUCTANCE (µH) | THROUGH HOLE | |||||||
---|---|---|---|---|---|---|---|---|---|
SANYO OS-CON SA SERIES | SANYO MV-GX SERIES | NICHICON PL SERIES | PANASONIC HFQ SERIES | ||||||
NO. | C CODE | NO. | C CODE | NO. | C CODE | NO. | C CODE | ||
3.3 | 10 | 1 | C7 | 2 | C4 | 1 | C5 | 1 | C6 |
15 | 1 | C10 | 1 | C10 | 1 | C18 | 1 | C6 | |
22 | See(2) | See(2) | 1 | C14 | 1 | C24 | 1 | C13 | |
33 | See(2) | See(2) | 1 | C12 | 1 | C20 | 1 | C12 | |
5 | 10 | 1 | C7 | 2 | C4 | 1 | C14 | 1 | C6 |
15 | 1 | C7 | 2 | C4 | 1 | C14 | 1 | C6 | |
22 | See(2) | See(2) | 1 | C10 | 1 | C18 | 1 | C13 | |
33 | See(2) | See(2) | 1 | C14 | 1 | C23 | 1 | C13 | |
47 | See(2) | See(2) | 1 | C12 | 1 | C20 | 1 | C12 | |
12 | 10 | 1 | C9 | 1 | C10 | 1 | C18 | 1 | C6 |
15 | 1 | C10 | 1 | C10 | 1 | C18 | 1 | C6 | |
22 | 1 | C10 | 1 | C10 | 1 | C18 | 1 | C6 | |
33 | See(2) | See(2) | 1 | C10 | 1 | C18 | 1 | C6 | |
47 | See(2) | See(2) | 1 | C13 | 1 | C23 | 1 | C13 | |
68 | See(2) | See(2) | 1 | C12 | 1 | C21 | 1 | C12 | |
100 | See(2) | See(2) | 1 | C11 | 1 | C22 | 1 | C11 |
Select the power supply operating conditions and the maximum output current and follow below procedures to find the external components for LM2673.
In this example, it is desired to convert the voltage from a two battery automotive power supply (voltage range of 20 V to 28 V, typical in large truck applications) to the 14.8-VDC alternator supply typically used to power electronic equipment from single battery 12-V vehicle systems. The load current required is 2 A (maximum). It is also desired to implement the power supply with all surface mount components. Soft start is not required.
Step 1: Operating conditions are:
Step 2: Select an LM2673S-ADJ. To set the output voltage to 14.9 V, two resistors need to be chosen (R1 and R2 in Figure 19). For the adjustable device the output voltage is set by the following relationship:
where
A recommended value to use for R1 is 1kΩ. In this example then R2 is determined to be:
R2 = 11.23 kΩ
The closest standard 1% tolerance value to use is 11.3 kΩ
This sets the nominal output voltage to 14.88 V which is within 0.5% of the target value.
Step 3:To use the nomograph for the adjustable device, Figure 17, requires a calculation of the inductor Volt • microsecond constant (E • T expressed in V • µS) from the following formula:
where
In this example this would be typically 0.15 Ω × 2 A or 0.3 V and VD is the voltage drop across the forward biased Schottky diode, typically 0.5 V. The switching frequency of 260 KHz is the nominal value to use to estimate the ON time of the switch during which energy is stored in the inductor.
For this example E • T is found to be:
Using Figure 17, the intersection of 27 V • µS horizontally and the 2-A vertical line (ILOAD max) indicates that L38, a 68-µH inductor, must be used.
From Table 3, L38 in a surface mount component is available from Pulse Engineering with part number PE-54038S.
Step 4: Use Table 9 or Table 10 to determine an output capacitor. With a 14.8-V output the 12.5-V to 15-V row is used and with a 68-µH inductor there are three surface mount output capacitor solutions. Table 1 or Table 2 provide the actual capacitor characteristics based on the C Code number. Any of the following choices can be used:
NOTE
When using the adjustable device in low voltage applications (less than 3-V output), if the nomograph, Figure 17, selects an inductance of 22 µH or less, Table 9 and Table 10 do not provide an output capacitor solution. With these conditions the number of output capacitors required for stable operation becomes impractical. TI recommends either a 33-µH or 47-µH inductor and the output capacitors from Table 9 or Table 10.
Step 5: An input capacitor for this example will require at least a 35-V WV rating with an RMS current rating of 1 A (1/2 IOUT max). From Table 1 or Table 2 it can be seen that C12, a 33-µF, 35-V capacitor from Sprague, has the required voltage and current rating of the surface mount components.
Step 6: From Table 4 aA 3-A Schottky diode must be selected. For surface mount diodes with a margin of safety on the voltage rating one of five diodes can be used:
Step 7: A 0.01-µF capacitor is used for CBOOST.
The soft-start pin will be left open circuited.
Step 8: Determine a value for RADJ to provide a peak switch current limit of at least 2 A plus 50% or 3 A.
Use a value of 12.4 kΩ.
OUTPUT VOLTAGE (V) | INDUCTANCE (µH) | SURFACE MOUNT | |||||
---|---|---|---|---|---|---|---|
AVX TPS SERIES | SPRAGUE 594D SERIES | KEMET T495 SERIES | |||||
NO. | C CODE | NO. | C CODE | NO. | C CODE | ||
1.21 to 2.5 | 33(1) | 7 | C1 | 6 | C2 | 7 | C3 |
47(1) | 5 | C1 | 4 | C2 | 5 | C3 | |
2.5 to 3.75 | 33(1) | 4 | C1 | 3 | C2 | 4 | C3 |
47(1) | 3 | C1 | 2 | C2 | 3 | C3 | |
3.75 to 5 | 22 | 4 | C1 | 3 | C2 | 4 | C3 |
33 | 3 | C1 | 2 | C2 | 3 | C3 | |
47 | 2 | C1 | 2 | C2 | 2 | C3 | |
5 to 6.25 | 22 | 3 | C2 | 3 | C3 | 3 | C4 |
33 | 2 | C2 | 2 | C3 | 2 | C4 | |
47 | 2 | C2 | 2 | C3 | 2 | C4 | |
68 | 1 | C2 | 1 | C3 | 1 | C4 | |
6.25 to 7.5 | 22 | 3 | C2 | 1 | C4 | 3 | C4 |
33 | 2 | C2 | 1 | C3 | 2 | C4 | |
47 | 1 | C3 | 1 | C4 | 1 | C6 | |
68 | 1 | C2 | 1 | C3 | 1 | C4 | |
7.5 to 10 | 33 | 2 | C5 | 1 | C6 | 2 | C8 |
47 | 1 | C5 | 1 | C6 | 2 | C8 | |
68 | 1 | C5 | 1 | C6 | 1 | C8 | |
100 | 1 | C4 | 1 | C5 | 1 | C8 | |
10 to 12.5 | 33 | 1 | C5 | 1 | C6 | 2 | C8 |
47 | 1 | C5 | 1 | C6 | 2 | C8 | |
68 | 1 | C5 | 1 | C6 | 1 | C8 | |
100 | 1 | C5 | 1 | C6 | 1 | C8 | |
12.5 to 15 | 33 | 1 | C6 | 1 | C8 | 1 | C8 |
47 | 1 | C6 | 1 | C8 | 1 | C8 | |
68 | 1 | C6 | 1 | C8 | 1 | C8 | |
100 | 1 | C6 | 1 | C8 | 1 | C8 | |
15 to 20 | 33 | 1 | C8 | 1 | C10 | 2 | C10 |
47 | 1 | C8 | 1 | C9 | 2 | C10 | |
68 | 1 | C8 | 1 | C9 | 2 | C10 | |
100 | 1 | C8 | 1 | C9 | 1 | C10 | |
20 to 30 | 33 | 2 | C9 | 2 | C11 | 2 | C11 |
47 | 1 | C10 | 1 | C12 | 1 | C11 | |
68 | 1 | C9 | 1 | C12 | 1 | C11 | |
100 | 1 | C9 | 1 | C12 | 1 | C11 | |
30 to 37 | 10 | No Values Available | 4 | C13 | 8 | C12 | |
15 | 3 | C13 | 5 | C12 | |||
22 | 2 | C13 | 4 | C12 | |||
33 | 1 | C13 | 3 | C12 | |||
47 | 1 | C13 | 2 | C12 | |||
68 | 1 | C13 | 2 | C12 |
OUTPUT VOLTAGE (V) | INDUCTANCE (µH) | THROUGH HOLE | |||||||
---|---|---|---|---|---|---|---|---|---|
SANYO OS-CON SA SERIES | SANYO MV-GX SERIES | NICHICON PL SERIES | PANASONIC HFQ SERIES | ||||||
NO. | C CODE | NO. | C CODE | NO. | C CODE | NO. | C CODE | ||
1.21 to 2.5 | 33(1) | 2 | C3 | 5 | C1 | 5 | C3 | 3 | C |
47(1) | 2 | C2 | 4 | C1 | 3 | C3 | 2 | C5 | |
2.5 to 3.75 | 33(1) | 1 | C3 | 3 | C1 | 3 | C1 | 2 | C5 |
47(1) | 1 | C2 | 2 | C1 | 2 | C3 | 1 | C5 | |
3.75 to 5 | 22 | 1 | C3 | 3 | C1 | 3 | C1 | 2 | C5 |
33 | 1 | C2 | 2 | C1 | 2 | C1 | 1 | C5 | |
47 | 1 | C2 | 2 | C1 | 1 | C3 | 1 | C5 | |
5 to 6.25 | 22 | 1 | C5 | 2 | C6 | 2 | C3 | 2 | C5 |
33 | 1 | C4 | 1 | C6 | 2 | C1 | 1 | C5 | |
47 | 1 | C4 | 1 | C6 | 1 | C3 | 1 | C5 | |
68 | 1 | C4 | 1 | C6 | 1 | C1 | 1 | C5 | |
6.25 to 7.5 | 22 | 1 | C5 | 1 | C6 | 2 | C1 | 1 | C5 |
33 | 1 | C4 | 1 | C6 | 1 | C3 | 1 | C5 | |
47 | 1 | C4 | 1 | C6 | 1 | C1 | 1 | C5 | |
68 | 1 | C4 | 1 | C2 | 1 | C1 | 1 | C5 | |
7.5 to 10 | 33 | 1 | C7 | 1 | C6 | 1 | C14 | 1 | C5 |
47 | 1 | C7 | 1 | C6 | 1 | C14 | 1 | C5 | |
68 | 1 | C7 | 1 | C2 | 1 | C14 | 1 | C2 | |
100 | 1 | C7 | 1 | C2 | 1 | C14 | 1 | C2 | |
10 to 12.5 | 33 | 1 | C7 | 1 | C6 | 1 | C14 | 1 | C5 |
47 | 1 | C7 | 1 | C2 | 1 | C14 | 1 | C5 | |
68 | 1 | C7 | 1 | C2 | 1 | C9 | 1 | C2 | |
100 | 1 | C7 | 1 | C2 | 1 | C9 | 1 | C2 | |
12.5 to 15 | 33 | 1 | C9 | 1 | C10 | 1 | C15 | 1 | C2 |
47 | 1 | C9 | 1 | C10 | 1 | C15 | 1 | C2 | |
68 | 1 | C9 | 1 | C10 | 1 | C15 | 1 | C2 | |
100 | 1 | C9 | 1 | C10 | 1 | C15 | 1 | C2 | |
15 to 20 | 33 | 1 | C10 | 1 | C7 | 1 | C15 | 1 | C2 |
47 | 1 | C10 | 1 | C7 | 1 | C15 | 1 | C2 | |
68 | 1 | C10 | 1 | C7 | 1 | C15 | 1 | C2 | |
100 | 1 | C10 | 1 | C7 | 1 | C15 | 1 | C2 | |
20 to 30 | 33 | No Values Available | 1 | C7 | 1 | C16 | 1 | C2 | |
47 | 1 | C7 | 1 | C16 | 1 | C2 | |||
68 | 1 | C7 | 1 | C16 | 1 | C2 | |||
100 | 1 | C7 | 1 | C16 | 1 | C2 | |||
30 to 37 | 10 | No Values Available | 1 | C12 | 1 | C20 | 1 | C10 | |
15 | 1 | C11 | 1 | C20 | 1 | C11 | |||
22 | 1 | C11 | 1 | C20 | 1 | C10 | |||
33 | 1 | C11 | 1 | C20 | 1 | C10 | |||
47 | 1 | C11 | 1 | C20 | 1 | C10 | |||
68 | 1 | C11 | 1 | C20 | 1 | C10 |
The LM2673 is designed to operate from an input voltage supply up to 40 V. This input supply must be well regulated and able to withstand maximum input current and maintain a stable voltage.
Board layout is critical for the proper operation of switching power supplies. First, the ground plane area must be sufficient for thermal dissipation purposes. Second, appropriate guidelines must be followed to reduce the effects of switching noise. Switch mode converters are very fast switching devices. In such cases, the rapid increase of input current combined with the parasitic trace inductance generates unwanted L di/dt noise spikes. The magnitude of this noise tends to increase as the output current increases. This noise may turn into electromagnetic interference (EMI) and can also cause problems in device performance. Therefore, take care in layout to minimize the effect of this switching noise. The most important layout rule is to keep the AC current loops as small as possible. Figure 20 shows the current flow in a buck converter. The top schematic shows a dotted line which represents the current flow during the top switch ON-state. The middle schematic shows the current flow during the top switch OFF-state. The bottom schematic shows the currents referred to as AC currents. These ac currents are the most critical because they are changing in a very short time period. The dotted lines of the bottom schematic are the traces to keep as short and wide as possible. This also yields a small loop area reducing the loop inductance. To avoid functional problems due to layout, review the PCB layout example. Best results are achieved if the placement of the LM2679 device, the bypass capacitor, the Schottky diode, RFBB, RFBT, and the inductor are placed as shown in the example. Note that, in the layout shown, R1 = RFBB and R2 = RFBT. TI also recommends using 2-oz. copper boards or heavier to help thermal dissipation and to reduce the parasitic inductances of board traces. See application note AN-1229 SIMPLE SWITCHER® PCB Layout Guidelines for more information.
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