Step-Down Controllers with Prebias Startup,

Lossless Sensing, Synchronization, and OVP

22

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MOSFET Snubber Circuit

Fast switching transitions cause ringing because of res-

onating circuit parasitic inductance and capacitance at

the switching nodes. This high-frequency ringing

occurs at LX’s rising and falling transitions and can

interfere with circuit performance and generate EMI. To

dampen this ringing, a series RC snubber circuit is

added across each switch. Below is the procedure for

selecting the value of the series RC circuit.

Find the capacitor value (connected from LX to GND)

that reduces the ringing frequency by half.

equal to 1/3rd the value of the added capacitance above.

2

to tailor the desired damping and the peak voltage

excursion.

can be calculated as:

the specific application’s derating rule for the power

dissipation calculated.

Input Capacitor

The input filter capacitor reduces peak currents drawn

from the power source and reduces noise and voltage

ripple on the input caused by the circuit’s switching.

The input capacitor must meet the ripple-current

defined by the following equation:

their low ESR and ESL at high frequency with relatively

low cost. Choose a capacitor that exhibits less than 10°C

temperature rise at the maximum operating RMS current

for optimum long-term reliability. Ceramic capacitors with

an X5R or better temperature characteristic are recom-

mended. When operating from a soft input source, an

additional input capacitor (bulk bypass capacitor) may

be required to prevent input from sagging.

Output Capacitor

The key selection parameters for the output capacitor

are the actual capacitance value, the equivalent series

resistance (ESR), the equivalent series inductance

(ESL), and the voltage-rating requirements. These

parameters affect the overall stability, output voltage

ripple, and transient response. The 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. The maximum output voltage

ripple is estimated as follows:

The output voltage ripple as a consequence of the

ESR, ESL, and output capacitance is:

These equations are suitable for initial capacitor selec-

tion, but final values should be chosen based on a proto-

type or evaluation circuit. As a general rule, a smaller

current ripple results in less output voltage ripple. Since

the inductor ripple current is a factor of the inductor value

and input voltage, the output voltage ripple decreases

with larger inductance, and increases with higher input

voltages. Polymer, tantalum, or aluminum electrolytic

capacitors are recommended.

I

VV

fL

V

V

PP

IN

OUT

S

OUT

IN

−

×

×

V

I

Cf

RIPPLE C

PP

OUT

S

××

−

8

V

V

L

ESL

RIPPLE ESL

IN

V

I

ESR

RIPPLE ESR

P P

−

I

IV

V

V

V

RMS

LOAD

OUT

IN

OUT

IN

=

×−

()

PC

V

f

RSNUB

SNUB

IN

SW

=×

2

L

fC

PAR

R

PAR

=

1

2

2

π