AND8079 datasheet(2/20 Pages) ONSEMI

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AND8079/D

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Many industry experts have predicted that DDR memory

will soon become the standard for desktop computers, with

notebooks shortly behind. Next generation DDR-II

generation systems are likely to have a lower Vdd voltage of

1.8 V with a Vtt and VREF voltage equal to 900 mV. This

lower voltage will be required to satisfy the consumer’s

requirement for more memory without a large increase in

required power.

Supply Voltage (Vdd)

The Vdd 2.5 V power supply is created with the NCP1571

low voltage synchronous buck controller. The NCP1571

controller contains the required circuitry for a synchronous

N-channel MOSFET buck regulator. The V2t control

method is used to achieve a fast 200 ns transient response

and an output regulation of

±1.0%. The IC operates at a fixed

internal frequency of 200 kHz. In addition, the NCP1571

provides the following features: undervoltage lockout

protection, programmable soft start, power good signal with

delay and overvoltage protection. Note the NCP1570 and

NCP1571 are functionally and pin for pin equivalent. The

NCP1571’s under voltage lockout operation (UVLO)

feature has been modified for applications that require a

parallel standby power supply in addition to the main power

supplied by the buck converter.

Termination Supply Voltage (Vtt) and Reference

Voltage (VREF)

The Vtt supply voltage is equal to one half of the Vdd

voltage, or approximately 1.25 V. Operational amplifiers

U2A and U2B function as voltage followers to create the Vtt

voltage. The input to U2B is created by the resistive voltage

divider formed by R5 and R6 and divides the 2.5 V Vdd

supply by two to form the VREF reference voltage. Also, U2B

provides filtering to remove any of the high frequency

switching noise that is results from the synchronous buck

converter. The Vtt output of the circuit formed by U2A and

transistors Q4 and Q5 tracks the voltage at the non-inverting

terminal by virtue of the voltage follower circuit

configuration. Thus, the output of voltage of the Vtt supply

is referenced to 50% of the 2.5 V Vdd supply, rather than an

absolute 1.25 V reference.

The sink and source ability of the Vtt supply is provided

by MOSFETs Q4 and Q5 which are used to extend the

current capability of the operational amplifier circuit. When

the Vtt supply is in the current sinking mode of operation, Q4

is “OFF” and Q5 is “ON”. The output of U2A will be at a

negative voltage (i.e. –5.0 V) to control the Vgs of the

P-channel MOSFET (Q5) in order to maintain the Vtt

voltage of 1.25 V. In a similar manner, when the Vtt supply

is in the current sourcing mode of operation, Q4 is “ON” and

Q5 is “OFF”. The output of U2A will reach a positive voltage

(i.e. + 4.5 V) to control the Vgs of the N-channel MOSFET

(Q5) in order to maintain the Vtt voltage of 1.25 V. Resistor

R7 is used to isolate the output of U2B from Vtt and the bulk

capacitor C20.

The slew rate of the operational amplifier and the ability

of the bulk capacitors to hold the voltage at 1.25 V under the

load conditions control the transient response of the Vtt

control loop. Note that the bulk capacitors maintain the Vtt

voltage at approximately 1.25 V; therefore, the operational

amplifier is only required to slew its output a relatively small

amount; therefore, the relatively slow slew rate of the

LM358 operational amplifiers is not a limiting factor in the

design.

Standby Power Operation

The demonstration PCB has the provision of providing a

low power standby mode of operation to the DDR memory

system. This mode could be used to provide a 2.5 V low

current standby voltage to the memory ICs when the main

5.0 V input power is not available. A MC33375 (U3)

300 mA low dropout voltage regulator (LDO) was chosen

for the design to provide the 2.5 V standby power. The

MC33375 has an ON/OFF enable pin and is available in a

SOT-223 package. The performance of the standby

regulator was not verified.

Q1, a N-Channel MOSFET, serves as a diode to prevent

current flow back to the main 5.0 volt input power supply

during the standby mode. The MOSFET was chosen instead

of a Schottky diode in order to minimize the voltage drop

and power consumption of the diode.

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