out

out

ss

ssavg

C

V

0.8

T

I

´

´

>

out max

in

in

SW

I

0.25

V

C

f

´

D

=

´

TPS65320-Q1

SLVSAY9A – DECEMBER 2012 – REVISED APRIL 2013

www.ti.com

(30)

Table 3. Capacitor Types

VENDOR

VALUE (

μF)

EIA Size

VOLTAGE

DIALECTRIC

COMMENTS

1 to 2.2

100 V

1210

GRM32 series

1 to 4.7

50 V

Murata

1

100 V

1206

GRM31 series

1 to 2.2

50 V

X7R

1 to 4.7

50 V

1210

1

100 V

AVX

X7R dielectric series

1 to 4.7

50 V

1812

1 to 2.2

100 V

Slow-Start Capacitor

The slow-start capacitor determines the minimum amount of time it takes for the output voltage to reach its

nominal programmed value during power up. This is useful if a load requires a controlled voltage-slew rate. This

is also useful if the output capacitance is large and requires large amounts of current to charge the capacitor

quickly to the output voltage level. The large currents necessary to charge the capacitor may make the

TPS65320-Q1 device reach the current limit, or excessive current draw from the input power supply may cause

the input voltage rail to sag. Limiting the output voltage-slew rate solves both of these problems.

The slow-start time must be long enough to allow the regulator to charge the output capacitor up to the output

only allowing the average output current to be 3 A would require a 0.088-ms slow-start time.

After knowing the slow-start timen, one can calculate the slow start capacitor value using Equation 1. For the

example circuit, the slow-start time is not too critical, because the output-capacitor value is 2 × 22 µF, which

does not require much current to charge to 5 V. The example circuit has the slow-start time set to an arbitrary

value of 1 ms, which requires a 3.125-nF slow start capacitor. This design uses the next-larger standard value of

3.3 nF.

(31)

Bootstrap Capacitor Selection

Connect a 0.1-µF ceramic capacitor between the BOOT and SW pins for proper operation. TI recommends using

a ceramic capacitor with X5R or better-grade dielectric. The capacitor should have a 10-V or higher voltage

rating.

Output Voltage and Feedback Resistors Selection

The voltage divider of R2 and R3 sets the output voltage. For the example design, the selected value of R3 is 10

k

Ω, and the calculated value of R2 is 53.6 kΩ. Due to current leakage of the VFB1 pin, the current flowing

through the feedback network should be greater than 1

μA in order to maintain the output-voltage accuracy.

Choosing higher resistor values decreases the quiescent current and improves efficiency at low output currents,

but may introduce noise immunity problems.

Compensation

There are several methods used to compensate dc-dc regulators. The method presented here is easy to

calculate and ignores the effects of the slope compensation that is internal to the device. Ignoring the slope

compensation usually causes the actual crossover frequency to be lower than the crossover frequency used in

the calculations. This method assumes the crossover frequency is between the modulator pole and the ESR

zero, and the ESR zero is at least 10 times greater than the modulator pole.

24

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