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SC1486

Dual Synchronous Buck Pseudo Fixed

Frequency DDR Power Supply Controller

POWER MANAGEMENT

Revision 3, October 2002

Description

Features

Applications

Typical Application Circuit

The SC1486 is a dual output constant on synchronous-

buck PWM controller optimized for cost effective mobile

DDR applications. Features include high efficiency, a fast

dynamic response with no minimum on time, a REFIN

input and a buffered REFOUT pin capable of sourcing

3mA. The excellent transient response means that

SC1486 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.

The output voltage of the first controller can be adjusted

from 0.5V to VCCA. In DDR applications, this voltage is

set to 2.5 volts. A resistor divider from the 2.5 volt supply

is used to drive the REFIN pin of the second controller. A

unity gain buffer drives the REFOUT pin to the same

potential as REFIN. The second controller regulates its

output to REFOUT. 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, overvoltage 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

VIN range = 1.8V – 25V

DC current sense using low-side RDS(ON) sensing

or sense resistor

Integrated reference buffer for VTT

Low power S3 state

Resistor programmable frequency

Cycle-by-cycle current limit

Digital soft-start

PSAVE option for VDDQ

Over-voltage/under-voltage fault protection

<20uA shutdown current

Low quiescent power dissipation

Two separate PGOOD indicators

Separate enable of each switcher

Integrated gate drivers with soft switching

Efficiency >90%

1% Internal reference

28 Lead TSSOP

Industrial temperature range

PGOOD1

EN1

+

C8

Q2

REFOUT

R10

L1

Q1

R3

PGND1

VBAT

VDDQ, 2.5V

+

C7

R11

R2

L2

VBAT

VDDQ/2

Q4

PGND1

VTT, 1.25V

SC1486

1

2

4

5

6

8

9

12

13

15

16

18

19

20

22

23

26

27

10

PGND1

DL1

ILIM1

LX1

DH1

REFIN

TON2

FBK2

PGOOD2

PGND2

DL2

ILIM2

LX2

DH2

EN/PSV1

TON1

FBK1

PGOOD1

REFOUT

Q3

R5

R4

PGOOD2

R1

R8