8.2.2.1.1 STEP 1. Choose R_SENSE, R_SET, and R_IMON

The recommended range of the current-limit threshold voltage, V_{(VCC – SENSE)}, extends from 10 mV to 42 mV. Values near the low threshold of 10 mV may be affected by system noise. Values near the upper threshold of 42 mV may be too close to the minimum fast-trip threshold voltage of 52 mV. Values near the middle of this range help minimize both concerns.

To achieve high efficiency, the power dissipation in R_SENSE must be kept to a minimum. A R_SENSE of 2 mΩ develops a voltage of 24 mV at the specified peak current limit of 12 A, while dissipating only 200 mW at the rated 10-A current. This represents a 0.17% power loss.

For best performance, a current of approximately 0.5 mA (referring to the RECOMMENDED OPERATING CONDITIONS table) should flow into the SET pin and out of the IMON pin when the TPS24720 is in current limit. The voltage across R_SET nominally equals the voltage across R_SENSE, or 24 mV. Dividing 24 mV by 0.5 mA gives a recommended value of R_SET of 48 Ω. A 51.1-Ω, 1% resistor was chosen. Using Equation 3, the value of R_IMON must equal 1437 Ω, or as near as practically possible. A 1.43-kΩ, 1% resistor was chosen.

Equation 5.

8.2.2.1.2 STEP 2. Choose MOSFET M₁

The next design step is to select M₁. The TPS24720 is designed to use an N-channel MOSFET with a gate-to-source voltage rating of 20 V.

Devices with lower gate-to-source voltage ratings can be used if a Zener diode is connected so as to limit the maximum gate-to-source voltage the transistor sees.

The next factor to consider is the drain-to-source voltage rating, V_DS(MAX), of the MOSFET. Although the MOSFET only sees 12 V dc, it may experience much higher transient voltages during extreme conditions, such as the abrupt shutoff that occurs during a fast trip. A TVS may be required to limit inductive transients under such conditions. A transistor with a V_DS(MAX) rating of at least twice the nominal input power-supply voltage is recommended regardless of whether a TVS is used or not.

Next select the on-resistance of the transistor, r_DS(on). The maximum on-resistance must not generate a voltage greater then the minimum power-good threshold voltage of 140 mV. Assuming a current limit of 12 A, a maximum r_DS(on) of 11.67 mΩ is required. Also consider the effect of r_DS(on) on the maximum operating temperature T_J(MAX) of the MOSFET. Equation 6 computes the value of r_DS(on)(MAX) at a junction temperature of T_J(MAX). Most manufacturers list r_DS(on)(MAX) at 25°C and provide a derating curve from which values at other temperatures can be derived. Compute the maximum allowable on-resistance, r_DS(on)(MAX), using Equation 6.

Equation 6.

Taking these factors into consideration, the TI CSD16403Q5 was selected for this example. This transistor has a V_GS(MAX) rating of 16 V, a V_DS(MAX) rating of 25 V, and a maximum r_DS(on) of 2.8 mΩ at room temperature. During normal circuit operation, the MOSFET can have up to 10 A flowing through it. The power dissipation of the MOSFET equates to 0.24 W and an 9.6°C rise in junction temperature. This is well within the data sheet limits for the MOSFET. The power dissipated during a fault (e.g., output short) is far larger than the steady-state power. The power handling capability of the MOSFET must be checked during fault conditions.

8.2.2.1.3 STEP 3. Choose Power-Limit Value, P_LIM, and R_PROG

MOSFET M₁ dissipates large amounts of power during inrush. The power limit P_LIM of the TPS24720 should be set to prevent the die temperature from exceeding a short-term maximum temperature, T_J(MAX)2. The short-term T_J(MAX)2 could be set as high as 150°C while still leaving ample margin to the usual manufacturer’s rating of 175°C. Equation 7 is an expression for calculating P_LIM,

Equation 7.

where R_θJC is the junction-to-case thermal resistance of the MOSFET, r_DS(on) is the its resistance at the maximum operating temperature, and the factor of 0.8 represents the tolerance of the constant-power engine. For an ambient temperature of 50°C, the calculated maximum P_LIM is 29.3 W. From Equation 1, a 53.6-kΩ, 1% resistor is selected for R_PROG (see Equation 8).

Equation 8.

Power limit fold back (P_LIM-FB) is the ratio of operating current limit (I_LIM) and minimum power limited (regulated) current (when V_OUT = 0 V). Degradation of programmed power limit (P_LIM) accuracy and start up issues may occur if P_LIM-FB is too large. Equation 9 calculates V_{SNS-PL_MIN} (minimum sense voltage during power limit) and P_LIM-FB. To ensure reliable operation, verify that P_LIM-FB < 10 and V_SNS,PL,MIN > 3 mV.

Equation 9.

8.2.2.1.4 STEP 4. Choose Output Voltage Rising Time, t_ON, and Timing Capacitor C_T

The maximum output voltage rise time, t_ON, set by timer capacitor C_T must suffice to fully charge the load capacitance C_OUT without triggering the fault circuitry. Equation 10 defines t_ON for two possible inrush cases. Assuming that only the load capacitance draws current during startup,

Equation 10.

The next step is to determine the minimum fault-timer period. In Equation 10, the output rise time is t_ON. This is the amount of time it takes to charge the output capacitor up to the final output voltage. However, the fault timer uses the difference between the input voltage and the gate voltage to determine if the TPS24720 is still in inrush limit. The fault timer continues to run until V_GS rises 5.9 V (for V_VCC = 12 V) above the input voltage. Some additional time must be added to the charge time to account for this additional gate voltage rise. The minimum fault time can be calculated using Equation 11,

Equation 11.

where C_ISS is the MOSFET input capacitance and I_GATE is the minimum gate sourcing current of TPS24720, or 20 μA. Using the example parameters and the CSD16403Q5 data sheet in Equation 11 leads to a minimum fault time of 1.22 ms. This time is derived considering the tolerances of C_OUT, C_ISS, I_LIM, P_LIM, I_GATE, and V_VCC(MAX). The fault timer must be set to a value higher than 1.22 ms to avoid turning off during start-up, but lower than any maximum fault time limit determined by the device SOA curve (see Figure 39) derated for operating junction temperature.

For this example, select 7 ms to allow for variation of system parameters such as temperature, load, component tolerance, and input voltage. The timing capacitor is calculated in Equation 12 as 52 nF. Selecting the next-highest standard value, 56 nF, yields a 7.56-ms fault time.

Equation 12.

8.2.2.1.5 STEP 5. Calculate the Retry-Mode Duty Ratio

In retry mode, the TPS24720 is on for one charging cycle and off for 16 charge/discharge cycles, as can be seen in Figure 35. The first C_T charging cycle is from 0 V to 1.35 V, which gives 7.56 ms. The first C_T discharging cycle is from 1.35 V to 0.35 V, which gives 5.6 ms. Therefore, the total time is 7.56 ms + 33 x 5.6 ms = 192.36 ms. As a result, the retry mode duty ratio is 7.56 ms/192.36 ms = 3.93%.

8.2.2.1.6 STEP 6. Select R₁, R₂, and R₃ for UV and OV

Next, select the values of the OV and UV resistors, R₁, R₂, and R₃, as shown in the typical application diagram on the front page. From the TPS24720 electrical specifications, V_OVTHRESH = 1.35 V and V_ENTHRESH = 1.35 V. V_OV is the overvoltage trip voltage, which in this case is 14 V. V_UV is the undervoltage trip voltage, which for this example equals 10.8 V.

Equation 13.

Equation 14.

Assume R₃ is 1 kΩ and use Equation 13 to solve for (R₂ + R₃). Use Equation 14 and the (R₂ + R₃) from Equation 13 to solve for R₂ and finally for R₃. From Equation 13, (R₂ + R₃) = 9370.4 Ω. From Equation 14,
R₂ = 296 Ω and R₁ = 9.074 kΩ. Scaling all three resistors by a factor of ten to use less supply current for these voltage references and using standard 1% resistor values gives R₁ = 90.9 kΩ, R₂ = 2.94 kΩ, and R₃ = 10 kΩ.

8.2.2.1.7 STEP 7. Choose R_GATE, R₄, R₅, R₆, and C₁

In the typical application diagram on the front page, the gate resistor, R_GATE, is intended to suppress high-frequency oscillations. A resistor of 10 Ω serves for most applications, but if M₁ has a C_ISS below 200 pF, then 33 Ω is recommended. Applications with larger MOSFETs and very short wiring may not require R_GATE. R₄, R₅, and R₆ are required only if PGb, FLTb, and FFLTb are used; these resistors serve as pullups for the open-drain output drivers. The current sunk by each of these pins should not exceed 2 mA ( referring to the Recommended Operating Conditions). C₁ is a bypass capacitor to help control transient voltages, unit emissions, and local supply noise while in the disabled state. Where acceptable, a value in the range of 0.001 μF to 0.1 μF is recommended.

8.2.2.2.1 Use of PGb

Use the PGb pin to control and coordinate a downstream dc/dc converter. If this is not done, then a long time delay is needed to allow C_OUT to fully charge before the converter starts. An undesirable latch-up condition can be created between the TPS24720 output characteristic and the dc/dc converter input characteristic if the converter starts while C_OUT is still charging; using the PGb pin is one way to avoid this.

8.2.2.2.2 Output Clamp Diode

Inductive loads on the output may drive the OUT pin below GND when the circuit is unplugged or during a current-limit event. The OUT pin ratings can be satisfied by connecting a diode from OUT to GND. The diode should be selected to control the negative voltage at the full short-circuit current. Schottky diodes are generally recommended for this application.

8.2.2.2.3 Gate Clamp Diode

The TPS24720 has a relatively well-regulated gate voltage of 12 V–15.5 V with a supply voltage V_VCC higher than 4 V. A small clamp Zener from gate to source of M₁ is recommended if V_GS of M₁ is rated below 12 V. A series resistance of several hundred ohms or a series silicon diode is recommended to prevent the output capacitance from discharging through the gate driver to ground.

8.2.2.2.4 High-Gate-Capacitance Applications

Gate voltage overstress and abnormally large fault-current spikes can be caused by large gate capacitance. An external gate clamp Zener diode is recommended to assist the internal Zener if the total gate capacitance of M₁ exceeds about 4000 pF.

8.2.2.2.5 Bypass Capacitors

It is a good practice to provide low-impedance ceramic capacitor bypassing of the VCC and OUT pins. Values in the range of 10 nF to 1 µF are recommended. Some system topologies are insensitive to the values of these capacitors; however, some are not and require minimization of the value of the bypass capacitor. Input capacitance on a plug-in board may cause a large inrush current as the capacitor charges through the low-impedance power bus when inserted. This stresses the connector contacts and causes a short voltage sag on the input bus. Small amounts of capacitance (e.g., 10 nF to 0.1 µF) are often tolerable in these systems.

8.2.2.2.6 Output Short-Circuit Measurements

Repeatable short-circuit testing results are difficult to obtain. The many details of source bypassing, input leads, circuit layout and component selection, output shorting method, relative location of the short, and instrumentation all contribute to variation in results. The actual short itself exhibits a certain degree of randomness as it microscopically bounces and arcs. Care in configuration and methods must be used to obtain realistic results. Do not expect to see waveforms exactly like those in the data sheet; every setup differs.