8.2.1 Design Requirements

This design uses the parameters listed in Table 8-1.

Table 8-1. Design Example Specifications

DESIGN PARAMETER

CONDITION

MIN

TYP

MAX

UNIT

VIN

Voltage range

8

12

16

V

VOUT

Output voltage

3.3

V

ILOAD

Output load current

25

A

VRIPPLE

Output voltage ripple

VIN = 12 V, IOUT = 12 A

26

mVPP

V TRANS

Output voltage undershoot and

overshoot after load step

IOUT = 5 A to 15 A step, 1 A/µs slew rate

17

mV

IOVER

Output overcurrent

27.5

A

tSS

Soft-start time

0.55

ms

fSW

Switching frequency

1.4

MHz

Light load operating mode

Skip-

mode

TA

Operating temperature

25

°C

8.2.2 Detailed Design Procedure

The external component selection is a simple process using D-CAP4 mode. Select the external components

using the following steps.

8.2.2.1 Output Voltage Setting Point

The output voltage is programmed by the voltage-divider resistors, R1 and R2, shown in Equation 7. Connect R1

between the FB pin and the output, and connect R2 between the FB pin and GOSNS. The recommended R2

value is 10 kΩ, but it can also be set to another value between the range of 1 kΩ to 20 kΩ. Determine R1 for the

TPS54KB20 by using Equation 7.

§

·

§

·

u

: u

:

¨

¸

¨

¸

©

¹

©

¹

OUT

REF

1

2

REF

V

V

3.3 V

0.9 V

R

R

10 k

26.7 k

V

0.9 V

(7)

8.2.2.2 Choose the Switching Frequency, the Operation Mode, and the Compensation Ramp

The switching frequency and operation mode are configured by the resistor on MSEL pin. Select one of three

switching frequencies: 800 kHz, 1.1 MHz, or 1.4 MHz. Refer to Table 7-1 for the relationship between the

switching frequency, operation mode, compensation ramp and, RMSEL.

Switching frequency selection is a tradeoff between higher efficiency and smaller system solution size. Lower

switching frequency yields higher overall efficiency but relatively bigger external components. Higher switching

frequencies cause additional switching losses which impact efficiency and thermal performance. For this design,

connect MSEL pin to AGND using a 178-kΩ resistor to set the switching frequency to 1.4 MHz, and set the light

load operating mode as Skip-mode (DCM) and the compensation RAMP4.

When selecting the switching frequency of a buck converter, the minimum on-time and minimum off-time must

be considered. Equation 8 calculates the maximum fSW before being limited by the minimum on-time. When

hitting the minimum on-time limits of a converter with D-CAP4 control, the effective switching frequency changes

to keep the output voltage regulated. This calculation ignores resistive drops in the converter to give a worst

case estimation.

TBD

(8)

Equation 9 calculates the maximum fSW before being limited by the minimum off-time. When hitting the minimum

off-time limits of a converter with D-CAP4 control, the operating duty cycle maxes out and the output voltage

begins to drop with the input voltage. This equation requires the DC resistance of the inductor, RDCR, selected in

the following step so this preliminary calculation assumes a resistance of 2.2 mΩ. If operating near the maximum

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TPS54KB20

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