SC1402 datasheet(8/18 Pages) SEMTECH

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SC1402

652 MITCHELL ROAD

NEWBURY PARK CA 91320

Multi-Output, Low-Noise Power Supply

Controller for Notebook Computers

December 15, 2000

Detailed Description

The SC1402 is a multiple-output, high efficiency, ver-

satile power supply controller designed to power bat-

tery operated systems. Four high current gate drive

outputs are supplied to control all MOSFETs in two

synchronous rectified buck converters. These buck

converters can be programmed to operate at either

fixed or adjustable output voltages. The power save

feature of the SC1402 achieves high efficiency over a

wide range of load current. The control and fault moni-

toring circuitry associated with each PWM controller

includes digital softstart, turn-on sequencing, voltage

error amplifier with slope compensation, pulse width

modulator, power save, over-current and over-voltage

and under voltage fault protection. Two linear regula-

tors and a precision reference voltage are also pro-

vided by the SC1402.

PWM Control Block

The two PWM control blocks for the 3V and 5V power

supply outputs are identical. The SC1402 employs

peak current mode control with slope compensation to

provide fast output response to load and line tran-

sients. The PWM control block consists of an analog

PWM modulator followed by PWM logic control. The

analog modulator combines the output current, slope

compensation signal and error voltage to generate a

PWM pulse train. The PWM logic uses the pulse train

from the modulator and other control signals to gener-

ate the output states for the high and low side gate

driver outputs. A block diagram of the PWM control

block is shown in Fig. 1.

An error amplifier generates the difference signal be-

tween the reference voltage and the feedback voltage

to generate the error voltage for the peak current mode

comparator. A nominal gain of 8 is used in the error

amplifier to increase the system loop gain and to re-

duce the load regulation error typically seen in low

loop-gain, current-mode controllers. The increased

gain in the voltage loop is compensated by pole-zero-

pole response of the voltage error amplifier. The cur-

rent feedback signal is summed with the slope com-

pensation signal and compared to the error voltage at

the PWM comparator.

When the power supply is operating in continuous con-

duction mode, the high-side MOSFET is turned on at

the beginning of each switching cycle. The high-side

MOSFET is turned off when the desired duty cycle is

reached. Active shoot-thruough protection delays the

turn-on of the low-side MOSFET until the phase node

drops below 2.5V. The low-side MOSFET remains on

until the beginning of the next switching cycle. Again,

active shoot-through protection ensures that the gate

to the low-side MOSFET has dropped low before the

high side MOSFET is turned on.

When PSAVE is enabled (low) and the output current

drops below 25% of its peak level, the PWM logic will

automatically enter PSAVE mode to improve efficiency.

When the controller enters power save, it increases the

regulation point by 0.8%, typically issuing one more

high side pulse as the converter enters PSAVE. The

PWM control then disables switching cycles until the

FB falls below the reference. At light loads the effec-

tive switching frequency will drop dramatically and effi-

ciency will increase because of the reduced gate

charge current required to switch the power stage.

Boosting the regulation point when entering PSAVE

gives the output improved dynamic regulation because

the output voltage is not allowed to droop below the

nominal regulation point. Load current steps, that

cause the converter to come out of PSAVE, will not

cause as large a negative dip in the output voltage.

Gate Drive/Control

The gate driver on the SC1402 are designed to switch

large MOSFETs up to 350kHz. The high side gate

driver is required to drive the gates of the high side

MOSFET above the V+ input. The supply for the gate

driver is generated by charging a bootstrap capacitor

from the VL supply when the low-side driver is on.

Monitoring circuit ensures that the bootstrap capacitor

is charged when coming out of shutdown or fault con-

ditions where the bootstrap capacitor may be depleted.

In continuos conduction mode, the low-side driver out-

put that controls the synchronous rectifier in the power

stage is on when the high-side driver is off. Under light

load conditions the inductor ripple current will approach

the point where it reverses polarity. This is detected by

the low side driver control and the synchronous rectifier

is turned off before the current reverses, preventing

energy drain from the output. The low-side driver oper-

ation is also affected by various fault conditions as de-

scribed in the Fault Protection section.

Internal Bias Supply

The VL linear regulator provides a 5V output that is

used to power the gate drivers, 2.5V reference, and

internal control sections of the SC1402. The regulator

is capable of supplying up to 50mA (including MOS-

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