SC1402(2001) 데이터 시트보기 (PDF) - Semtech Corporation

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SC1402
(Rev.:2001)

Multi-Output, Low-Noise Power Supply Controller for Notebook Computers

Semtech Corporation 

SC1402

POWER MANAGEMENT

Detailed Description

The SC1402 is a multiple-output, high efficiency, versatile power

supply controller designed to power battery 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 cur-

rent. The control and fault monitoring circuitry associated with

each PWM controller includes digital softstart, turn-on sequenc-

ing, voltage error amplifier with slope compensation, pulse width

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

der voltage fault protection. Two linear regulators and a preci-

sion reference voltage are also provided 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 transients. The PWM control block consists

of an analog PWM modulator followed by PWM logic control.

The analog modulator combines the output current, slope com-

pensation 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 generate the output states for the

high and low side gate driver outputs. A block diagram of the

PWM control block is shown on page 8.

An error amplifier generates the difference signal between the

reference voltage and the feedback voltage to generate the er-

ror 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 reduce 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 current feedback signal is

summed with the slope compensation signal and compared to

the error voltage at the PWM comparator.

When the power supply is operating in continuous conduction

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, ac-

tive 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

effective switching frequency will drop dramatically and effi-

ciency will increase because of the reduced gate charge cur-

rent required to switch the power stage. Boosting the regula-

tion 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 ca-

pacitor is charged when coming out of shutdown or fault condi-

tions where the bootstrap capacitor may be depleted.

In continuous conduction mode, the low-side driver output 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

operation 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 MOSFET gate charge current). The VL

pin should be bypassed to GND with 4.7uF to supply the peak

current requirements of the gate driver outputs.

This regulator receives its input power from the V+ battery

input. Efficiency is improved by providing a boot-strapping mode

for the VL bias. When the 5V SMPS output voltage reaches

5V, internal circuitry detects this condition and turns on a PMOS

pass device between CSL5 and VL. The internal VL regulator

is then disabled and the VL bias is provided by the high effi-

ciency switch mode power supply.

The REF output is accurate to +/- 2% over temperature. It is

capable of delivering 5mA max and should be bypassed with

1uF minimum capacitor. Loading the REF output will reduce

the REF voltage slightly.

 2001 Semtech Corp.

10

www.semtech.com

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