SC4609EVB データシートの表示（PDF） - Semtech Corporation

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SC4609EVB

Low Input, MHz Operation, High Efficiency Synchronous Buck

Semtech Corporation 

SC4609

POWER MANAGEMENT

Application Information (Cont.)

ICIN(RMS) = IOMAX ⋅

VOUT ⋅ (VIN − VOUT )

V

2

IN

Where:

IB = the boost current and

VD= discharge ripple voltage.

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 = VO/VI , duty ratio and

∆VI = the given input voltage ripple.

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.

Boost Capacitor Selection

The boost capacitor selection is based on its discharge

ripple voltage, worst case conduction time and boost

current. The worst case conduction time Tw can be esti-

mated as follows:

Tw

=

1

fs

⋅ Dmax

Where:

fs = the switching frequency and

Dmax = maximum duty ratio.

The required minimum capacitance for boost capacitor

will be:

Cboost

=

IB

VD

⋅ TW

With fs = 300kH, VD=0.3V and IB=50mA, the required

capacitance for the boost capacitor is:

Cboost

=

IB

VD

⋅

1

fs

⋅ Dmax

=

0.05

0.3

1

⋅ 300k

⋅ 0.95

= 528nF

Power MOSFET Selection

The SC4609 can drive an N-MOSFET at the high side

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

The use of the high side N-MOSFET will significantly re-

duce its conduction loss for high current. For the top

MOSFET, its total power loss includes its conduction loss,

switching loss, gate charge loss, output capacitance loss

and the loss related to the reverse recovery of the bot-

tom diode, shown as follows:

PTOP _ TOTAL

= I2TOP _ RMS

⋅ R TOP _ ON

+

ITOP _ PEAK

VGATE

⋅ VI

RG

⋅

fs

⋅

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

Where:

RG = gate drive resistor,

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

QGS2 = the gate to source charge of the top MOSFET,

QGT = the total gate charge of the top MOSFET,

QOSS = the output charge of the top MOSFET and

Qrr = the reverse recovery charge of the bottom diode.

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:

QGB = the total gate charge of the bottom MOSFET and

VF = the forward voltage drop of the bottom diode.

 2006 Semtech Corp.

10

www.semtech.com

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