PROGRAMMABLE, HIGH PERFORMANCE

MULTI-PHASE, PWM CONTROLLER

SC1144

© 1999 SEMTECH CORP.

652 MITCHELL ROAD NEWBURY PARK CA 91320

Preliminary - August 24, 1999

10

The SC1144 is patent pending

BANDGAP REFERENCE/SOFT START

The precision internal bandgap reference provides a

stable, temperature compensated 2.05 V reference for

the error amplifier’s non-inverting input, the D

the VID digital to analog converter, the under-voltage

lock-out circuitry as well as the reference for the Bias

generator current sources. The error amplifiers non-

inverting input is connected to the bandgap reference

through a 3 kohm resistor, which is also externally con-

nected to a soft-start capacitor via the B

application of power, the non-inverting input of the er-

ror amplifier is held low for a short time allowing for the

decoder and PWM controller to settle. This prevents

any harmful output voltage overshoots upon start up or

after a converter shut-down (as a result of over-current

or the enable pin pulled low). The non-inverting input

then ramps up according to the RC time constant al-

lowing gradual rise of output voltage.

ISOLATED FEEDBACK GROUND

The feedback ground pin on the SC1144, FBG, is iso-

lated from the GND. The maximum voltage between

the two pins is set by the ESD diode forward voltage

drop. This allows the SC1144 to be configured for re-

mote sensing thus compensating for wiring and con-

nector voltage drops at high currents or during tran-

sients. Remote sensing is typically implemented by

using differential amplifiers. The use of this feature on

the SC1144 eliminates the need for additional compo-

nents.

OVER-CURRENT COMPARATOR

The SC1144 has an internal over-current comparator

designed to sense current in the input supply path.

The comparator inputs have a high common voltage

range thus allowing operation under all possible volt-

age/current combinations. The output of the compara-

tor pulls the B

error amplifier and causing the PWM outputs to enter

minimum duty cycle mode.

SYNCHRONOUS VS. ASYNCHRONOUS MODE

The SC1144 can be configured to drive Synchrounous

MSOFET drivers, such as the SC1405 for higher effi-

ciency. Refer to SC1405 datasheet for example de-

signs.

COMPONENT SELECTION AND LAYOUT CONSID-

ERATIONS

Care must be taken in component selection and PCB

layout in high frequency power appliactions. Refer to

SC1144EVB user manual for more application informa-

tion on component selection and layout guidlines as

well as example Artwork and Bill of materials.

ERROR AMPLIFIER CONT.

Fig. 3: Top Trace, Gate drive responding to removal of

transient load. Bottom trace, Load current transient

16A/div.

Fig. 2: Transient response, Top trace, Current sense

8A/div @ 30A/usec. Bottom trace, V