0.991

92417

RT (k ) =

= 244 k

400 (kHz)

W

W

SW(shift)

8

6 A x 11 m

0.1 V

0.7 V

855 kHz

135 ns

60 V - 6 A x 92 m

0.7 V

W +

+

æ

ö

=

´

=

ç

÷

W +

è

ø

f

SW(max skip)

1

5 A x 11 m

5 V

0.7 V

708 kHz

135ns

60 V - 5 A x 92 m

0.7 V

W +

+

æ

ö

=

´

=

ç

÷

W +

è

ø

f

TPS54560

www.ti.com

SLVSBN0 – MARCH 2013

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. Calculations can be done with the aid of WEBENCH or the excel spreadsheet (slvc452) located

on the product page. For this example, we will start with the following known parameters:

Output Voltage

5 V

Transient Response 1.25 A to 3.75 A load step

ΔVOUT = 4 %

Maximum Output Current

5 A

Input Voltage

12 V nom. 7 V to 60 V

Output Voltage Ripple

0.5% of VOUT

Start Input Voltage (rising VIN)

6.5 V

Stop Input Voltage (falling VIN)

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 since 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 7 and Equation 8 should 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 TPS54560. For this example, the output voltage is 5 V and

the maximum input voltage is 60 V, which allows for a maximum switch frequency up to 708 kHz to avoid pulse

skipping from Equation 7. To ensure overcurrent runaway is not a concern during short circuits use Equation 8 to

determine the maximum switching frequency for frequency foldback protection. With a maximum input voltage of

60 V, assuming a diode voltage of 0.7 V, inductor resistance of 11 m

Ω, switch resistance of 92 mΩ, a current

limit value of 6 A and short circuit output voltage of 0.1 V, the maximum switching frequency is 855 kHz.

For this design, a lower switching frequency of 400 kHz is chosen to operate comfortably below the calculated

maximums. To determine the timing resistance for a given switching frequency, use Equation 5 or the curve in

Figure 6. The switching frequency is set by resistor R3 shown in Figure 35. For 400 kHz operation, the closest

standard value resistor is 243 k

Ω.

(23)

(24)

(25)

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