VRM9.0 PROGRAMMABLE, MULTI-PHASE,

HIGH PERFORMANCE PWM CONTROLLER

SC1146

© 2000 SEMTECH CORP.

652 MITCHELL ROAD NEWBURY PARK CA 91320

July 25, 2000

9

The SC1146 is patent pending

The error amplifier has two inverting inputs, one for the

High Range, (VID4 set to logic high) and one input for

the Low Range. The effective gain of the error ampli-

fier stays constant in both ranges keeping the loop gain

constant regardless of output voltage range.

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 DAC

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 3Kohm resistor, which is also externally con-

nected to a soft-start capacitor via the BG

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

ror amplifier is held low for a short time, set by the RC

product of the soft-start capacitor and 3Kohm, allowing

for the decoder and PWM controller to settle. This pre-

vents 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

allowing gradual rise of output voltage.

OVER-CURRENT COMPARATOR

The SC1146 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 BG

error amplifier and causing the PWM outputs to enter

minimum duty cycle mode, regulating the output at a

minimum voltage. A current sense trace may be used

as the current sense resistor. This will utilize the trace

as input filter inductor, thus preventing conducted

emissions to input supply. For details of programming

the overcurrent comparator, refer to SC1144-EVB user

manual.

SC1146 Special features:

The SC1146 is a special version of the original

SC1144, 4-phase PWM controller, with the latest

VRM9.0 VID codes which allow programming lower

output voltages required by the latest Microprocessors.

Also a +12V supply is no longer required. The V

PWRGOOD pins can be connected to Vcc5v. This

eases the connection to “5V-only” MOSFET drivers,

such as the SC1405. The DRVs will swing to Vcc5v,

eliminating the need for +12V to +5V level shifting.

There is no Under-Voltage Lockout on V

ERROR AMPLIFIER CONT.

Fig. 3: Transient response, Top trace, Current sense

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

Fig. 4: Transient response, Top trace, Gate Drive

Bottom trace, Transient current 16A/div @ 30A/usec.