4.5V to 28V Input, Synchronous PWM Buck Controllers

for DDR Termination and Point-of-Load Applications

______________________________________________________________________________________

19

peak inductor current at the maximum-defined load

Output-Capacitor Selection

The key selection parameters for the output capacitor

are the actual capacitance value, the ESR, the equiva-

lent series inductance (ESL), and the voltage-rating

requirements, which affect the overall stability, output

ripple voltage, and transient response.

The worst-case output ripple has three components:

variations in the charge stored in the output capacitor,

the voltage drop across the capacitor’s ESR, and ESL

caused by the current into and out of the capacitor.

This can be approximated by:

The output voltage ripple due to the ESR is:

The output voltage ripple due to the output capacitance is:

The output voltage ripple due to the ESL of the output

capacitor is:

After a load transient, the output voltage instantly

changes by ESR x

trollers respond within 100ns and try to regulate back to

the nominal output value.

Solid polymer or OSCON electrolytic capacitors are

recommended due to their low ESR and ESL at the

switching frequency. Higher output-current applications

require multiple output capacitors connected in parallel

to meet the output ripple-voltage requirements. Do not

exceed the capacitor’s voltage or ripple-current ratings.

Output-Capacitor Stability Consideration

Stability is determined by the value of the ESR zero rel-

ative to the switching frequency. To ensure stability, the

following condition must be met:

For a typical 300kHz application, the ESR zero frequen-

cy must be well below 95kHz, preferably below 50kHz.

Do not put high-value ceramic capacitors directly

across the feedback sense point without taking precau-

tions to ensure stability. Large ceramic capacitors can

have a high-ESR zero frequency and cause erratic,

unstable operation. However, it is easy to add enough

series resistance by placing the capacitors a couple of

inches downstream from the feedback sense point,

which should be as close as possible to the inductor.

The easiest method for checking stability is to apply a

very fast zero-to-max load transient and carefully

observe the output voltage-ripple envelope for over-

shoot and ringing. It can help to simultaneously monitor

the inductor current with an AC current probe. Do not

allow more than one cycle of ringing after the initial

step-response under- or overshoot.

Input-Capacitor Selection

drawn from the input supply and reduces noise injec-

tion. The source impedance to the input supply largely

requires high input capacitance. The input capacitor

imposed by the switching currents. The RMS input rip-

ple current is given by:

For optimal circuit reliability, choose a capacitor that

has less than 10

°C temperature rise at the peak ripple

current.

II

VV

V

V

RMS

LOAD

OUT

IN

OUT

IN

=×

×

()

-

f

RC

ESR

ESR

OUT

=

××

1

2

π

f

f

ESR

S

<

π

I

V

V

V

PP

IN

OUT

OUT

IN

-

S

-V

fL

=

×

×

V

I

Cf

RIPPLE C

PP

OUT

S

××

-

8

V

I

ESR

RIPPLE ESR

P P

-

VV

V

V

RIPPLE

RIPPLE ESR

RIPPLE C

RIPPLE ESL

()

( )

(

)

=+

+

II

LIR

I

PEAK

LOAD MAX

LOAD MAX

()

()

=+ 







×

2