TPS54341_15 datasheet(26/48 Pages) TI

26 / 48 page

0.991

92417

RT (k ) =

= 163 k

600 (kHz)

SW(shift)

4.7 A x 21 m

0.1 V

0.7 V

1260 kHz

135 ns

42 V - 4.7 A x 87 m

0.7 V

W +

SW(max skip)

3.5 A x 21 m

3.3 V

0.7 V

712 kHz

135ns

42 V - 3.5 A x 87 m

0.7 V

W +

TPS54341

SLVSC61 – NOVEMBER 2013

www.ti.com

APPLICATION INFORMATION

Design Guide — Step-By-Step Design Procedure

This guide illustrates the design of a high frequency switching regulator using ceramic output capacitors. A few

parameters must be known in order to start the design process. These requirements are typically determined at

the system level. The necessary calculations can be done using WEBENCH or the excel spreadsheet

(SLVC452) located on the product. This design starts from the following known parameters.

Output Voltage

3.3 V

Transient Response 0.875 A to 2.625 A load step

ΔVOUT = 4 %

Maximum Output Current

3.5 A

Input Voltage

12 V nominal 6 V to 42 V

Output Voltage Ripple

0.5% of VOUT

Start Input Voltage (rising VIN)

5.75 V

Stop Input Voltage (falling VIN)

4.5 V

Selecting the Switching Frequency

The first step is to choose a switching frequency for the regulator. Typically, the designer uses the highest

switching frequency possible because this produces the smallest solution size. High switching frequency allows

for lower value inductors and smaller output capacitors compared to a power supply that switches at a lower

frequency. The switching frequency that can be selected is limited by the minimum on-time of the internal power

switch, the input voltage, the output voltage and the frequency-foldback protection.

Equation 12 and Equation 13 must be used to calculate the upper limit of the switching frequency for the

regulator. Choose the lower value result from the two equations. Switching frequencies higher than these values

results in pulse skipping or the lack of overcurrent protection during a short circuit.

The typical minimum on time, tonmin, is 135 ns for the TPS54341 device. For this example, the output voltage is

3.3 V and the maximum input voltage is 42 V, which allows for a maximum switch frequency up to 712 kHz to

avoid pulse skipping from Equation 12. To ensure overcurrent runaway is not a concern during short circuits use

Equation 13 to determine the maximum switching frequency for frequency foldback protection. With a maximum

input voltage of 42 V, assuming a diode voltage of 0.7 V, inductor resistance of 21 m

Ω, switch resistance of 87

Ω, a current-limit value of 4.7 A and short circuit output voltage of 0.1 V, the maximum switching frequency is

1260 kHz.

For this design, a lower switching frequency of 600 kHz is chosen to operate comfortably below the calculated

maximums. To determine the timing resistance for a given switching frequency, use Equation 10 or the curve in

Figure 6. The switching frequency is set by resistor R3 shown in Figure 46. For 600 kHz operation, the closest

standard value resistor is 162 k

Ω.

(28)

(29)

(30)

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