SC4602A データシートの表示（PDF） - Semtech Corporation

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SC4602A

High Efficiency Synchronous, Step Down Controller

Semtech Corporation 

SC4602A/B

POWER MANAGEMENT

Applications Information (Cont.)

After the required inductor value is selected, the proper

selection of the core material is based on the peak in-

ductor current and efficiency requirements. The core

must be able to handle the peak inductor current IPEAK

without saturation and produce low core loss during the

high frequency operation.

IPEAK

= IOMAX

+

Ip −p

2

The power loss for the inductor includes its core loss and

copper loss. If possible, the winding resistance should

be minimized to reduce inductor’s copper loss. The core

loss can be found in the manufacturer’s datasheet. The

inductor’ copper loss can be estimated as follows:

PCOPPER

=

I2

LRMS

⋅ RWINDING

Where:

ILRMS is the RMS current in the inductor. This current can

be calculated as follows:

ILRMS = IOMAX ⋅

1+

1

3

⋅

∆I2

Output Capacitor Selection

Basically there are two major factors to consider in se-

lecting the type and quantity of the output capacitors.

The first one is the required ESR (Equivalent Series Re-

sistance) which should be low enough to reduce the volt-

age deviation from its nominal one during its load changes.

The second one is the required capacitance, which should

be high enough to hold up the output voltage. Before the

SC4602A/B regulates the inductor current to a new value

during a load transient, the output capacitor delivers all

the additional current needed by the load. The ESR and

ESL of the output capacitor, the loop parasitic inductance

between the output capacitor and the load combined

with inductor ripple current are all major contributors to

the output voltage ripple. Surface mount speciality poly-

mer aluminum electrolytic chip capacitors in UE series

from Panasonic provide low ESR and reduce the total

capacitance required for a fast transient response.

POSCAP from Sanyo is a solid electrolytic chip capacitor

which has a low ESR and good performance for high fre-

quency with a low profile and high capacitance. Above

mentioned capacitors are recommended to use in

SC4602A/B applications.

Input Capacitor Selection

The input capacitor selection is based on its ripple cur-

rent level, required capacitance and voltage rating. This

capacitor must be able to provide the ripple current by

the switching actions. For the continuous conduction

mode, the RMS value of the input capacitor can be cal-

culated from:

ICIN(RMS) = IOMAX ⋅

VO ⋅ (VI − VO )

V

2

I

This current gives the capacitor’s power loss as follows:

PCIN

=

I2

CIN( RMS )

⋅ RCIN(ESR)

This capacitor’s RMS loss can be a significant part of the

total loss in the converter and reduce the overall con-

verter efficiency. The input ripple voltage mainly depends

on the input capacitor’s ESR and its capacitance for a

given load, input voltage and output voltage. Assuming

that the input current of the converter is constant, the

required input capacitance for a given voltage ripple can

be calculated by:

CIN

= IOMAX

⋅

fs ⋅ (∆VI

D ⋅ (1− D)

− IOMAX ⋅ RCIN(ESR) )

Where:

D = V /V , duty ratio and

OI

DV = the given input voltage ripple.

I

Because the input capacitor is exposed to the large surge

current, attention is needed for the input capacitor. If

tantalum capacitors are used at the input side of the

converter, one needs to ensure that the RMS and surge

ratings are not exceeded. For generic tantalum capaci-

tors, it is wise to derate their voltage ratings at a ratio of

2 to protect these input capacitors.

Power Mosfet Selection

The SC4602A/B can drive a P-MOSFET at the high side

and an N-MOSFET synchronous rectifier at the low side.

The use of the high side P-MOSFET eliminates the need

for an external charge pump and simplifies the high side

gate driver circuit.

 2006 Semtech Corp.

11

www.semtech.com

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