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POWER MANAGEMENT

SC2596

Overview

Double Data Rate (DDR) SDRAM was defined by JEDEC

1997. Its clock speed is the same as previous SDRAM

but data transfer speed is twice than previous SDRAM.

By now, the requirement voltage range is changed from

3.3V to 2.5V or 1.8V; the power dissipation is smaller

than SDRAM. For above reasons, it is very popular and

widely used in M/B, N/B, Video-cards, CD ROM drives,

Disk drives.

Regarding the DDR power management solution, there

are two topologies can be selected for system designers.

One is switching mode regulator that has bigger sink/

source current capability, but the cost is higher and needs

more board space. Another solution is linear mode regu-

lator, which costs less, and needs less board space. For

two DIMM motherboards, system designers usually

choose the linear mode regulator for DDR power man-

agement solution.

Application Information

Thermal shutdown

The SC2596 has built-in thermal detected circuit to pre-

vent this device from over temperature and damage.

The SC2596 goes into shunt down mode when tem-

will release when the temperature of device drop down

AVCC and PVCC

AVCC and PVCC are the input supply pins for the SC2596.

AVCC is supply voltage for all the internal control circuitry.

The AVCC voltage has to be greater than its UVLO thresh-

old voltage (2.1V typical) to allow the SC2596 to be nor-

mal operation.

The PVCC pin provides the rail voltage from where the

VTT pin draws load current. There is a limitation between

AVCC and PVCC. The PVCC voltage must be less or equal

to AVCC voltage to ensure the correct VTT output voltage

regulation.

VSENSE

VSENSE pin is a feedback pin from VTT plane. VTT plane

is always a narrow and long strip plane in most

montherboard applications. This long strip plane will

cause a large trace inductance and trace resistance.

Consider the load transient condition, a fast load cur-

rent going through VTT strip plane will create a voltage

spike on VTT plane and a DC voltage drop for load current.

It is recommanded the VSENSE pin should be connected

center of VTT plane to improve regulation and transient

response.

A longer trace of VSENSE may pick up noise and cause

the error of load regulation. Hence designer should avoid

a longer trace between VSENSE to VTT plane. A 100nF

ceramic capacitor close to VSENSE pin is required.

VREF

VREF pin is an output pin to provid internal reference

voltage. System designer can use the voltage for

Northbridge chipset and memory. It is necessary to add

a ceramic capacitor (100nF) from VREF pin to ground

with shortest trace.

Typical Application Circuits & Waveforms

Four different application circuits are shown below in Fig-

ure 1, Figure 2, Figure 3 and Figure 4. Each circuit is

designed for a specific condition. See Note a. and b.

below for recommended power up sequencing.

Application_1: Standard SSTL-2 Application

The AVCC pin, PVCC pin and the V

together for SSTL-2 application (Figure 1). It only needs

a 2.5V power rail for normal operation. System designer

can save the PCB space and reduce the cost.

Figure 1: Standard SSTL-2 application.

10nF

EN

1.25V

VDDQ

Cout

220uF

1.25V

Cin1

1uF

SC2596

GND

1

EN

2

VSENSE

3

VREF

4

VDDQ

5

AVCC

6

PVCC

7

VTT

8

Cin2

100uF

VTT

Cref

100nF

2.5V

VREF

0