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 2004 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2618

The output capacitor ESR also causes output voltage tran-

meet output transient criteria, the ESR value should be:

T

T

ESR

I

V

R

<

To meet both criteria, the smaller one of above two ESRs

is required.

The output capacitor value also contributes to load tran-

sient response. Based on a worst case where the induc-

tor energy 100% dumps to the output capacitor during

the load transient, the capacitance then can be calcu-

lated by:

2

2

T

T

V

I

L

C

⋅

>

Input Capacitor

Input Capacitor

Input Capacitor

Input Capacitor

Input Capacitor

The input capacitor should be chosen to handle the RMS

ripple current of a synchronous buck converter. This value

is given by:

2

2

)

(

)

1

(

IN

o

IN

RMS

I

I

D

I

D

I

−

⋅

+

⋅

−

=

rent, and D is the duty cycle. Choosing low ESR input

capacitors will help maximize ripple rating for a given size.

Bootstrap Circuit

Bootstrap Circuit

Bootstrap Circuit

Bootstrap Circuit

Bootstrap Circuit

The SC2618 uses an external bootstrap circuit to pro-

vide a voltage at BST pin for the top MOSFET drive. This

voltage is held up by a bootstrap capacitor. Typically, it is

recommended to use a 1uF ceramic capacitor with 25V

rating and a commonly available diode IN4148 for the

bootstrap circuit.

Filter for Vcc Supply Power

Filter for Vcc Supply Power

Filter for Vcc Supply Power

Filter for Vcc Supply Power

Filter for Vcc Supply Power

For the pin of Vcc, it is recommended to use a 1uF/25V

ceramic capacitor for decoupling. In addition, place a

small resistor (10 ohm) in between Vcc pin and the sup-

ply power for noise reduction.

Divider Ratio

Divider Ratio

Divider Ratio

Divider Ratio

Divider Ratio

The top resistor of the voltage divider can be chosen

from 5k to 15k. Then the bottom resistor is found from

top

O

bot

R

V

V

V

R

⋅

−

=

25

.

1

25

.

1

where 1.25V is the internal reference voltage of the

SC2618.

Component Selection (Cont.)