34712 datasheet(10/23 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34712

FUNCTIONAL DESCRIPTION

INTRODUCTION

FUNCTIONAL DESCRIPTION

INTRODUCTION

In modern microprocessor/memory applications, address

commands and control lines require system level termination

to a voltage (VTT) equal to 1/2 the memory supply voltage

(VDDQ). Having the termination voltage at midpoint, the power

supply insures symmetry for switching times. Also, a

reference voltage (VREF) that is free of any noise or voltage

variations is needed for the DDR SDRAM input receiver,

VREF is also equal to 1/2 VDDQ. Varying the VREF voltage will

effect the setup and hold time of the memory. To comply with

DDR requirements and to obtain best performance, VTT and

VREF need to be tightly regulated to track 1/2 VDDQ across

voltage, temperature, and noise margins. VTT should track

any variations in the DC VREF value (VTT = VREF +/- 40 mV),

(See Figure 4) for a DDR system level diagram.

The 34712 supplies the VTT and a buffered VREF output.

To ensure compliance with DDR specifications, the VDDQ line

is applied to the VREFIN pin and divided by 2 internally

through a precision resistor divider. This internal voltage is

then used as the reference voltage for the VTT output. The

same internal voltage is also buffered to give the VREF

voltage at the VREFOUT pin for the application to use without

the need for an external resistor divider. The 34712 provides

the tight voltage regulation and power sequencing/tracking

required along with handling the DDR peak transient current

requirements. Buffering the VREF output helps its immunity

against noise and load changes.

The 34712 utilizes a voltage mode synchronous buck

switching converter topology with integrated low RDS(ON)

(45 mΩ) N-channel power MOSFETs to provide a VTT voltage

with an accuracy of less than ±2.0 %. It has a programmable

switching frequency that allows for flexibility and optimization

over the operating conditions and can operate at up to

1.0 MHz to significantly reduce the external components size

and cost. The 34712 can sink and source up to 3.0 A of

continuous current. It provides protection against output over

current, overvoltage, undervoltage, and overtemperature

conditions. It also protects the system from short circuit

events. It incorporates a power-good output signal to alert the

host when a fault occurs.

For boards that support the Suspend-To-RAM (S3) and

the Suspend-To-Disk (S5) states, the 34712 offers the STBY

and the SD pins respectively. Pulling any of these pins low,

puts the IC in the corresponding state.

By integrating the control/supervisory circuitry along with

the Power MOSFET switches for the buck converter into a

space-efficient package, the 34712 offers a complete, small-

size, cost-effective, and simple solution to satisfy the needs

of DDR memory applications.

Besides DDR memory termination, the 34712 can be used

to supply termination for other active buses and graphics card

memory. It can be used in Netcom/Telecom applications like

servers. It can also be used in desktop motherboards, game

consoles, set top boxes, and high end high definition TVs.

Figure 4. DDR System Level Diagram

FUNCTIONAL PIN DESCRIPTION

REFERENCE VOLTAGE INPUT (VREFIN)

The 34712 will track 1/2 the voltage applied at this pin.

REFERENCE VOLTAGE OUTPUT (VREFOUT)

This is a buffered reference voltage output that is equal to

1/2 VREFIN. It has a 10.0 mA current drive capability. This

output is used as the VREF voltage rail and should be filtered

against any noise. Connect a 0.1 µF, 6 V low ESR ceramic

filter capacitor between this pin and the GND pin and

between this pin and VDDQ rail. VREFOUT is also used as the

reference voltage for the buck converter error amplifier.

FREQUENCY ADJUSTMENT INPUT (FREQ)

The buck converter switching frequency can be adjusted

by connecting this pin to an external resistor divider between

VDDI and GND pins. The default switching frequency (FREQ

pin connected to ground, GND) is set at 1.0 MHz.

SIGNAL GROUND (GND)

Analog ground of the IC. Internal analog signals are

referenced to this pin voltage.

VDDQ

VTT

VREF

BUS

DDR Memory Controller

DDR Memory Input Receiver

34712 Datasheet(PDF) 10 Page - Freescale Semiconductor, Inc

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