I

1*

V

OUT

V

IN

ESR)

1

8

C

OUT

f

[ 1.1DI

ESR

C

L

2

V

IN

DV

TPS75003

www.ti.com

SBVS052I – OCTOBER 2004 – REVISED AUGUST 2010

Diode Selection (Buck Controllers)

The diode is off when the PMOS is on, and on when the PMOS is off. Since it will be turned on and off at a

relatively high frequency, a Schottky diode is recommended for good performance. The peak current rating of the

current and low forward voltage at operating current will optimize efficiency. Equation 10 calculates the estimated

average power dissipation:

(10)

Output Capacitor Selection (Buck Controllers)

The output capacitor is selected based on output voltage ripple and transient response requirements. As a result

of the nature of the hysteretic control loop, a minimum ESR of a few tens of m

Ω should be maintained for good

operation unless a feed-forward resistor is used. Low ESR bulk tantalum or PosCap capacitors work best in most

applications. A 1.0mF ceramic capacitor can be used in parallel with this capacitor to filter higher frequency

spikes. The output voltage ripple can be estimated by Equation 11:

(11)

To calculate the capacitance needed to achieve a given voltage ripple as a result of a load transient from zero

output to full current, use Equation 12:

(12)

If only ceramic or other very low ESR output capacitor configurations are desired, additional voltage ripple must

be passed to the feedback pin. See Application Note SLVA210, Using Ceramic Output Capacitors with the

TPS6420x and TPS75003 Buck Controllers, available for download at www.ti.com, for detailed application

information.

ripple. For minimum on-time, the ripple adds to the voltage; for minimum off-time, it subtracts from the voltage.

Soft-Start Capacitor Selection (Buck Controllers)

The soft-start for BUCK1 and BUCK2 is not intended to be a precision function. However, the startup time (from

in start-up time increases rapidly. This variation can occur from unit to unit and even between the two BUCK

controllers in one device. Therefore, do not depend on the soft-start feature for sequencing multiple supplies if

BUCK1 is discussed in this section; it is identical to BUCK2. Soft-start is implemented on the buck controllers by

ramping current limit from 0 to its target value (set by R1) over a user-defined time. This time is set by the

external soft-start cap connected to pin SS1. If SS1 is left open, a small on-chip capacitor will provide a current

limit ramp time of approximately 250ms. Figure 25 shows the effects of R1 and SS1 on the current limit start-up

ramp.

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