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SC1563IMS

HIGH-EFFICIENCY STEP-DOWN DC-DC CONVERTER

Semtech Corporation 

VERY LOW DROPOUT, LOW

QUIESCENT 500mA REGULATOR

SC1563

PRELIMINARY - September 11, 2000

APPLICATIONS INFORMATION

Introduction

The SC1563 is intended for applications such as

graphics cards where high current capability and very

low dropout voltage are required. It provides a very

simple, low cost solution that uses little pcb real estate

and typically does not require a heatsink. Additional

features include an enable pin to allow for a very low

power consumption standby mode, and a fully ad-

justable output.

Component Selection

Input capacitor - a 1µF ceramic capacitor is recom-

mended. This allows for the device being some dis-

tance from any bulk capacitance on the rail. Addition-

ally, input droop due to load transients is reduced, im-

proving load transient response. Additional capaci-

tance may be added if required by the application.

Output capacitor - a minimum bulk capacitance of 1µF,

along with a 0.1µF ceramic decoupling capacitor is rec-

ommended. Increasing the bulk capacitance will im-

prove the overall transient response. The use of multi-

ple lower value ceramic capacitors in parallel to

achieve the desired bulk capacitance will not cause

stability issues. Although designed for use with ce-

ramic output capacitors, the SC1563 is extremely toler-

ant of output capacitor ESR values and thus will also

work comfortably with tantalum output capacitors.

External voltage selection resistors - the use of 1% re-

sistors, and designing for a current flow ≥ 10µA is rec-

ommended to ensure a well regulated output (thus R2

≤ 120kΩ).

Thermal Considerations

The power dissipation in the SC1563 is approximately

equal to the product of the output current and the input

to output voltage differential:

PD ≈ (VIN − VOUT ) • IO

The absolute worst-case dissipation is given by:

( ) PD (MAX ) = VIN (MAX ) − VOUT (MIN ) • IO (MAX ) + VIN (MAX ) • IQ (MAX )

For a typical scenario, VIN = 3.3V ± 5%, VOUT = 2.5V

and IO = 500mA, therefore:

VIN(MAX) = 3.465V, VOUT(MIN) = 2.450V and

IQ(MAX) = 1.75mA,

Thus PD(MAX) = 0.514W.

Using this figure, and assuming TA(MAX) = 85°C, we can

calculate the maximum thermal impedance allowable

to maintain TJ ≤ 150°C:

( ) ( ) R = TH(J−A )(MAX )

TJ(MAX ) − TA (MAX )

PD(MAX )

=

150 − 70

0.514

= 156 °C / W

This should be achievable using pcb copper area to aid

in conducting the heat away from the device. Internal

ground/power planes and air flow will also assist in re-

moving heat. For higher ambient temperatures addi-

tional board copper may be required. Use of the avail-

able MSOP-8 package will allow operation at full load

where higher input to output voltage differentials exist.

MARKING INFORMATION

MSOP-8

SOT-23-5L

63XX = SC1563, voltage option (example: 6318)

yyww = Datecode (example: 0008)

XXXX = Lot number (example: E01102)

© 2000 SEMTECH CORP.

6

652 MITCHELL ROAD NEWBURY PARK CA 91320

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