SC4603(2004) データシートの表示（PDF） - Semtech Corporation

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SC4603
(Rev.:2004)

Very Low Input, MHz Operation, High Efficiency Synchronous Buck

Semtech Corporation 

SC4603

POWER MANAGEMENT

Applications Information - (Cont.)

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

SC4603 applications.

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 SC4603 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.

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 ⋅

Vout ⋅ (Vin − Vout )

Vin2

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

converter efficiency. The input ripple voltage mainly de-

pends on the input capacitor’s ESR and its capacitance

for a given load, input voltage and output voltage. As-

suming that the input current of the converter is con-

stant, 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 = VO/VI , duty ratio and

∆VI = the given input voltage ripple.

For the top MOSFET, its total power loss includes its con-

duction loss, switching loss, gate charge loss, output ca-

pacitance loss and the loss related to the reverse

recovery of the bottom diode, shown as follows:

PTOP _ TOTAL

=

I2

TOP _ RMS

⋅ R TOP _ ON

+

ITOP _ PEAK

VGATE

⋅ VI

RG

⋅ fs

⋅

(QGD + QGS2 ) + QGT ⋅ VGATE ⋅ fs + (QOSS + Qrr ) ⋅ VI ⋅ fs

Where:

RG = gate drive resistor,

Q = the gate to drain charge of the top MOSFET,

GD

Q = the gate to source charge of the top MOSFET,

GS2

QGT = the total gate charge of the top MOSFET,

QOSS = the output charge of the top MOSFET, and

Q = the reverse recovery charge of the bottom diode.

rr

For the top MOSFET, it experiences high current and high

voltage overlap during each on/off transition. But for the

bottom MOSFET, its switching voltage is the bottom

diode’s forward drop during its on/off transition. So the

switching loss for the bottom MOSFET is negligible. Its

total power loss can be determined by:

PBOT _ TOTAL

=

I2

BOT _ RMS

⋅ RBOT _ ON

+ QGB ⋅ VGATE

⋅ fs

+ ID _ AVG ⋅ VF

Where:

Q = the total gate charge of the bottom MOSFET and

GB

V = the forward voltage drop of the bottom diode.

F

 2004 Semtech Corp.

10

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