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SC1485

Dual Synchronous Buck Pseudo Fixed

Frequency Power Supply Controller

POWER MANAGEMENT

Revision: October 14, 2004

Description

Features

Applications

Typical Application Circuit

The SC1485 is a dual output constant on synchronous-

buck PWM controller intended for use in notebook

computers and other battery operated portable devices.

Features include high efficiency and a fast dynamic

response with no minimum on time. The excellent

transient response, means that SC1485 based solutions

will require less output capacitance than competing fixed

frequency converters.

The frequency is constant until a step in load or line voltage

occurs at which time the pulse density and frequency will

increase or decrease to counter the change in output or

input voltage. After the transient event, the controller

frequency will return to steady state operation. At light

loads, Power-Save Mode enables the SC1485 to skip

PWM pulses for better efficiency.

Each output voltage can be independently adjusted from

0.5V to VCCA. Two frequency setting resistors set the

on-time for each buck controller. The frequency can thus

be tailored to minimize crosstalk. The integrated gate

drivers feature adaptive shoot-through protection and

soft switching. Additional features include cycle-by-cycle

current limit, digital soft-start, over-voltage and under-

voltage protection, and a PGOOD output for each

controller.

Notebook computers

CPU I/O supplies

Handheld terminals and PDAs

LCD monitors

Network power supplies

Constant on-time for fast dynamic response

Programmable VOUT range = 0.5 – VCCA

VIN Range = 1.8V – 25V

DC current sense using low-side RDS(ON)

sensing or sense resistor

Resistor programmable frequency

Cycle-by-cycle current limit

Digital soft-start

Separate PSAVE option for each switcher

Over-voltage/Under-voltage fault protection

10uA Typical shutdown current

Low quiescent power dissipation

Two PGOOD indicators

1% Reference (2% system DC accuracy)

Efficiency >90%

Integrated gate drivers with soft switching

Separate enables

28 Lead TSSOP

Industrial temperature range

VBAT

SC1485

1

2

4

5

6

9

12

13

15

16

18

19

20

22

23

26

27

8

PGND1

DL1

ILIM1

LX1

DH1

TON2

FBK2

PGOOD2

PGND2

DL2

ILIM2

LX2

DH2

EN/PSV1

TON1

FBK1

PGOOD1

EN/PSV2

VOUT2

PGND1

R3

R7

PGOOD1

EN1

R5

PGOOD2

PGND1

Q2

+ C7

R8

R2

R6

R1

Q1

L2

Q3

VBAT

VOUT1

R4

EN2

L1

+

C8

Q4