SLVS441 Data sheet

SLVS441D December 2002 – December 2014 TPS61042

PRODUCTION DATA.

9 Application and Implementation

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.

9.1 Application Information

9.1.1 Efficiency

The overall efficiency of the application depends on the specific application conditions and mainly on the selection of the inductor. A lower inductor value increases the switching frequency and switching losses yielding in a lower efficiency. A lower inductor dc resistance has lower copper losses, giving a higher efficiency. Therefore, the efficiency can typically vary ±5% depending on the selected inductor. Figure 1 and Figure 2 can be used as a guideline for the application efficiency. These curves show the typical efficiency powering four LEDs using a 4.7 µH inductor with just 1,2 mm height. The efficiency curve in Figure 1 and Figure 2 show the efficiency delivering the power to the LEDs rather than the overall converter efficiency and is calculated as:

Equation 3.

9.1.2 Setting The LED Current

The converter regulates the LED current by regulating the voltage across the current sense resistor (R_S). The voltage across the sense resistor is regulated to the internal reference voltage of V_(FB) = 252 mV.

Figure 12. Setting the LED Current

The LED current can be calculated:

Equation 4.

The current programming method is used when the brightness of the LEDs is fixed or controlled by a PWM signal applied to the CTRL pin. When using a PWM signal on the CTRL pin, the LED brightness is only dependent on the PWM duty cycle, independent of the PWM frequency, or amplitude, which simplifies the system.

Alternatively, an analog voltage can be used as well to control the LED brightness.

Figure 13. Setting the LED Current

In Figure 13 the LED current is determined by the voltage applied to R2 (V_(adj)) and the selection of R1, R2 and the sense resistor (R_S). In this configuration, the LED current is linear controlled instead of pulsed as in the configuration before. To select the resistor values following steps are required.

Select the voltage V_(adjmax) to turn the LEDs off. → V_(adjmax) (e.g. 3.3 V)
Select the voltage V_(adjmin) to turn the LEDs fully on. → V_(adjmin) (e.g. 0.0 V)
Select the maximum and minimum LED current I_O(max) and I_O(min). → (e.g. I_O(max) = 20 mA, I_O(min) = 0 mA)
Calculate R2 to achieve a feedback current in the range of I1 = 3 µA to 10 µA as the LEDs are fully turned on:

Equation 5.

Calculation of R1

Equation 6.

Calculation of the sense voltage (V_S) at maximum LED current

Equation 7.

Calculation of the required sense resistor (R_S)

Equation 8.

9.1.3 PWM Control With Separate Enable

The control pin (CTRL) combines the enable function as well as the PWM brightness control function in one pin. For some systems an independent enable function is required. One way to implement this is to use the brightness control configuration as shown in the previous section Figure 13.

Other possible solutions are shown in Figure 14, Figure 15, Figure 16.

Figure 14. Separate Enable and PWM Control Using a Schottky Diode

Figure 15. Separate Enable and PWM Control Using a Transistor

Figure 16. Separate Enable and PWM Control Using an AND Gate

9.1.4 Example Schematics

Figure 17. TPS61042 With Low LED Ripple Current and Higher Accuracy Using a 4.7 µF Output Capacitor

Figure 18. TPS61042 Powering 6 LEDs, Efficiency = 84%@V_I= 3.6 V/19 mA

Figure 19. TPS61042 Powering 8 LEDs, Efficiency = 81%@V_I = 3.6 V/18.6 mA

Figure 20. Adjustable Brightness Control Using an Analog Voltage

Figure 21. Alternative Adjustable Brightness Control Using PWM Signal

9.2 Typical Application

Figure 22. TPS61042 With 1,0 mm Total System Height. Efficiency = 82.7%@V_I = 3.0 V/19 mA

9.2.1 Design Requirements

Table 2. Design Parameters

DESIGN PARAMETER	Typical VALUE
Input Voltage	1.8 V to 6 V
Output Voltage	V_IN to 27.5 V
Dimming frequency	0.1 to 50 kHz

9.2.2 Detailed Design Procedure

9.2.2.1 Inductor Selection, Maximum Load Current, and Switching Frequency

The PFM peak current control scheme of the TPS61042 is inherently stable. The inductor value does not affect the stability of the regulator. The selection of the inductor together with the nominal LED current, input, and output voltage of the application determines the switching frequency of the converter.

The first step is to calculate the maximum load current the converter can support using the selected inductor. The inductor value has less effect on the maximum available load current and is only of secondary order. A good inductor value to start with is 4.7 µH. Depending on the application, inductor values down to 1.0 µH can be used. The maximum inductor value is determined by the maximum on time of the switch of 6 µs (typ). The peak current limit of 500 mA (typ) must be reached within this 6 µs for proper operation. The maximum load current of the converter is determined at the operation point where the converter starts to enter the continuous conduction mode. The converter must always operate in discontinuous conduction mode to maintain regulation.

Depending on the time period of the inductor current fall time being larger or smaller compared to the minimum off time of the converter (400ns typ), the maximum load current can be calculated.

Inductor fall time:

Equation 9.

For t_fall ≥ 400 ns

Equation 10.

for t_fall ≤ 400 ns

Equation 11.

with:

L = selected inductor value

η = expected converter efficiency. Typically between 70% to 85%

Equation 12.

(Peak inductor current as described in the peak current control section)

The above formula contains the expected converter efficiency that allows calculating the expected maximum load current the converter can support. The efficiency can be taken out of the efficiency graphs shown in Figure 1 and Figure 2 or 80% can be used as an accurate estimation.

If the converter can support the desired LED current, the next step is to calculate the converter switching frequency at the operation point, which must be ≤1 MHz. Also the converter switching frequency should be much higher than the applied PWM frequency at the CTRL pin to avoid non-linear brightness control. Assuming the converter shows no double pulses or pulse bursts (Figure 24, Figure 25) on the switch node (SW) the switching frequency at the operation point can be calculated as:

Equation 13.

with:

I_(LIM) = minimum switch current limit (500 mA typ)

L = selected inductor value

I_(LOAD)= nominal load or LED current

V_F = Rectifier diode forward voltage (typically 0.3 V)

The smaller the inductor value, the higher the switching frequency of the converter but the lower the efficiency.

The selected inductor must have a saturation current that meets the maximum peak current of the converter as calculated in the peak current control section. Use the maximum value for I_(LIM)(600mA) for this calculation.

Another important inductor parameter is the DC resistance. The lower the DC resistance the higher the efficiency of the converter. See Table 3 and Figure 22 to Figure 21 for inductor selection.

Table 3. Possible Inductor^⁽¹⁾

INDUCTOR VALUE	COMPONENT SUPPLIER	SIZE
10 µH	muRata LQH43CN100K01	4,5 mm×3,2 mm×2.6 mm
4.7 µH	muRata LQH32CN4R7M11	3,2 mm×2,5 mm×2,0 mm
10 µH	Coilcraft DO1605T-103MX	5,5 mm ×4,1 mm ×1,8 mm
4.7 µH	Sumida CDRH3D16-4R7	3,8 mm×3,8 mm×1,8 mm
3.3 µH	Sumida CMD4D11- 3R3	3,5 mm×5,3 mm×1,2 mm
4.7 µH	Sumida CMD4D11- 4R7	3,5 mm×5,3 mm×1,2 mm
3.3 µH	Sumida CMD4D11- 3R3	3,5 mm×5,3 mm×1,2 mm
4.7 µH	Coiltronics SD12-4R7	5,2 mm×5,2 mm×1,2 mm
3.3 µH	Coilcraft LPO1704-332M	6,6 mm×5,5 mm×1,0 mm
4.7 µH	Coilcraft LPO1704-472M	6,6 mm×5,5 mm×1,0 mm

(1) See Third-Party Products Disclaimer

9.2.2.2 Output Capacitor Selection and Line Regulation

For better output voltage filtering, a low ESR output capacitor is recommended. Ceramic capacitors have a low ESR value, but depending on the application, tantalum capacitors can be used.

The selection of the output capacitor value directly influences the output voltage ripple of the converter which also influences line regulation. The larger the output voltage ripple, the larger the line regulation, which means that the LED current changes if the input voltage changes. If a certain change in LED current gives a noticeable change in LED brightness, depends on the LED manufacturer and on the application. Applications requiring good line regulation ≤1%/V (typ) must use output capacitor values ≥1 µF.

See Table 3 and Figure 22 to Figure 21 for the selection of the output capacitor.

Assuming the converter does not show double pulses or pulse bursts on the switch node (SW), the output voltage ripple is calculated as (see Figure 24, Figure 25):

Equation 14.

with:

I_(LIM) = minimum switch current limit (400 mA typ)
L = selected inductor value
I_(LOAD) = nominal load current
ƒ_S= switching frequency at the nominal load current as calculated before.
V_F = rectifier diode forward voltage (0.3 V typ)
C_O= selected output capacitor
ESR = output capacitor ESR value

9.2.2.3 Input Capacitor Selection

For good input voltage filtering, low ESR ceramic capacitors are recommended. A 4.7 µF ceramic input capacitor is sufficient for most applications. For better input voltage filtering the capacitor value can be increased. Refer to Table 2 and Figure 22 to Figure 21 for input capacitor selection.

Table 4. Possible Input and Output Capacitors^⁽¹⁾

CAPACITOR	VOLTAGE RATING	COMPONENT SUPPLIER	COMMENTS
4.7 µF/X5R/0805	6.3 V	Tayo Yuden JMK212BY475MG	C_I
10 µF/X5R/0805	6.3 V	Tayo Yuden JMK212BJ106MG	C_I
100 nF		Any	C_O
220 nF		Any	C_O
470 nF		Any	C_O
1.0 µF/X7R/1206	25 V	Tayo Yuden TMK316BJ105KL	C_O
1.0 µF/X7R/1206	35 V	Tayo Yuden GMK316BJ105KL	C_O
4.7 µF/X5R/1210	25 V	Tayo Yuden TMK325BJ475MG	C_O

(1) See Third-Party Products Disclaimer

9.2.2.4 Diode Selection

To achieve high efficiency, a Schottky diode must be used. The current rating of the diode must meet the peak current rating of the converter as it is calculated in the peak current control section. Use the maximum value for I_(Lim)for this calculation. See Table 5 and Figure 22 to Figure 21 for the Schottky diode selection.

Table 5. Possible Diodes^⁽¹⁾

COMPONENT SUPPLIER	REVERSE VOLTAGE
ON Semiconductor MBR0530	30 V
ON Semiconductor MBR0520	20 V
Toshiba CRS02	30 V
Zetex ZHCS400	40 V

(1) See Third-Party Products Discalimer

9.2.3 Application Curves

Figure 23. Softstart

Figure 25. Bust Mode Operation

Figure 24. PFM Operation

Figure 26. PWM Dimming

9 Application and Implementation

9.1 Application Information

9.1.1 Efficiency

9.1.2 Setting The LED Current

9.1.3 PWM Control With Separate Enable

9.1.4 Example Schematics

9.2 Typical Application

9.2.1 Design Requirements

Table 2. Design Parameters

9.2.2 Detailed Design Procedure

9.2.2.1 Inductor Selection, Maximum Load Current, and Switching Frequency

Table 3. Possible Inductor(1)

9.2.2.2 Output Capacitor Selection and Line Regulation

9.2.2.3 Input Capacitor Selection

Table 4. Possible Input and Output Capacitors(1)

9.2.2.4 Diode Selection

Table 5. Possible Diodes(1)

9.2.3 Application Curves

Table 3. Possible Inductor^⁽¹⁾

Table 4. Possible Input and Output Capacitors^⁽¹⁾

Table 5. Possible Diodes^⁽¹⁾