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SC4605

Low Input, High Efficiency Synchronous, Step Down Controller

Semtech Corporation 

SC4605

POWER MANAGEMENT

Applications Information (Cont.)

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:

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:

CIN

= IOMAX

⋅

fs ⋅ (∆VI

D ⋅ (1− D)

− IOMAX ⋅ RCIN(ESR) )

PBOT _ TOTAL

=

I2

BOT _ RMS

⋅ RBOT _ ON

+ QGB ⋅ VGATE

⋅ fs

+ ID _ AVG ⋅ VF

Where:

D = VO/VI , duty ratio and

DVI = 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.

Power Mosfet Selection

The SC4605 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:

Where:

QGB = the total gate charge of the bottom MOSFET and

VF = the forward voltage drop of the bottom diode.

For a low voltage and high output current application such

as the 3.3V/1.5V@12A case, the conduction loss is of-

ten dominant and selecting low RDS(ON) MOSFETs will

noticeably improve the efficiency of the converter even

though they give higher switching losses.

The gate charge loss portion of the top/bottom MOSFET’s

total power loss is derived from the SC4605. This gate

charge loss is based on certain operating conditions (fs,

VGATE, and I ).

O

The thermal estimations have to be done for both

MOSFETs to make sure that their junction temperatures

do not exceed their thermal ratings according to their

total power losses PTOTAL, ambient temperature Ta and

their thermal resistance Rθja as follows:

Tj(max)

< Ta

+

PTOTAL

R θja

 2004 Semtech Corp.

9

www.semtech.com

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