MAX1809EEE datasheet(13/17 Pages) MAXIM

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3A, 1MHz, DDR Memory Termination Supply

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the dissipation in the termination resistor is halved com-

pared to a termination scheme that connects the resistive

terminators to ground.

The VTT supply requires that it regulates to half the

memory voltage (VDDQ), tracks the changes of the

memory voltage, and is able to source and sink current

depending on the state of the bus. These requirements

are met in the MAX1809.

Discharging the Output in Shutdown

When SHDN is brought low after the controller has

been on for a while, the output may remain high if there

is no leakage or discharge path to bring the output

down. For DDR memory systems, keeping VTT at 1.25V

when VDDQ (2.5V) is shut down violates the DDR speci-

fications. This can result in the bus latching if the sys-

tem is subsequently turned on or possibly damaging

the memory subsystem.

When using the MAX1809 to generate the VTT output of

1.25V, several circuits are recommended to discharge

the output when the MAX1809 is shut down. These are

shown in Figure 9. Solution (a) is a diode added from

VTT to VDDQ so that VTT is discharged when VDDQ

goes low. Alternatively, solution (b) uses a small signal

transistor to discharge VTT when the MAX1809 is shut

down.

Startup in Sinking Mode

The MAX1809 will not startup until the feedback voltage

is made less than the external reference voltage when

power is applied or when the part is exiting shutdown.

In applications that cannot guarantee VFB < VEXTREF

Figure 10. Starting the MAX1809 in Sinking Mode with VOUT

>VEXTREF

MAX1809

CIN

VCC

INPUT VOLTAGE

(3V TO 5.5V)

SHDN

PGND

GND

SHDN

VTT =VDDQ/2

Ω

100

Ω

COUT

2.2

µF

0.01

µF

Figure 9. Discharging the Output of the MAX1809 in Shutdown

MAX1809

CIN

VCC

INPUT VOLTAGE

(3V TO 5.5V)

SHDN

EXTREF

(a)

(b)

PGND

GND

SHDN

2N7002

VTT = VDDQ/2

Ω

10k

Ω

10k

Ω

VDDQ

(2.5V)

VSSQ

COUT

0.1

µF

2.2

µF

0.1

µF

SHDN

PGND

VCC

GND

REF

GND

TOP VIEW

MAX1809

QSOP

EXTREF

TOFF

Pin Configurations (continued)

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