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MAX8550
(Rev.:2004)

Integrated DDR Power-Supply Solutions for Desktops, Notebooks, and Graphic Cards

Maxim Integrated 

Integrated DDR Power-Supply Solutions for

Desktops, Notebooks, and Graphic Cards

Pin Description

PIN

NAME

FUNCTION

On-Time Selection-Control Input. This four-level logic input sets the nominal DH on-time. Connect to

GND, REF, AVDD, or leave TON unconnected to select the following nominal switching frequencies:

1

TON

TON = AVDD (200kHz)

TON = OPEN (300kHz)

TON = REF (450kHz)

TON = GND (600kHz)

Overvoltage/Undervoltage-Protection Control Input. This four-level logic input enables or disables the

overvoltage and/or undervoltage protection. The overvoltage limit is 116% of the nominal output

2

OVP/

UVP

(MAX8550)

voltage. The undervoltage limit is 70% of the nominal output voltage. Discharge mode is enabled when

OVP is also enabled. Connect the OVP/UVP pin to the following pins for the desired function:

OVP/UVP = AVDD (Enable OVP and discharge mode, enable UVP.)

OVP/UVP = OPEN (Enable OVP and discharge mode, disable UVP.)

OVP/UVP = REF (Disable OVP and discharge mode, enable UVP.)

OVP/UVP = GND (Disable OVP and discharge mode, disable UVP.)

N.C.

(MAX8551)

Do not connect; leave open.*

+2.0V Reference Voltage Output. Bypass to GND with a 0.1µF (min) capacitor. REF can supply 50µA

3

REF

for external loads. Can be used for setting voltage for ILIM. REF turns off when SHDNA, SHDNB, and

STBY are low.

Valley Current-Limit Threshold Adjustment for Buck Regulator. The current-limit threshold across PGND

and LX is 0.1 times the voltage at ILIM. Connect ILIM to a resistive divider, typically from REF to GND,

4

ILIM

to set the current-limit threshold between 25mV and 200mV. This corresponds to a 0.25V to 2V range at

ILIM. Connect ILIM to AVDD to select the 50mV default current-limit threshold. See the Setting the

Current Limit section.

Buck Power-Good Open-Drain Output. POK1 is low when the buck output voltage is more than 10%

5

POK1

above or below the normal regulation point or during soft-start. POK1 is high impedance when the

output is in regulation and the soft-start circuit has terminated. POK1 is low in shutdown.

LDO Power-Good Open-Drain Output. In normal mode, POK2 is low when either VTTR or VTTS is more

6

POK2

than 10% above or below the normal regulation point, which is typically REFIN / 2. In standby mode,

POK2 responds only to the VTTR input. POK2 is low in shutdown, and when VREFIN is less than 0.8V.

Standby. Connect to high for low-quiescent mode where the VTT output is disabled, but the VTTR

7

STBY

buffer is kept alive if SHDNB is high. POK2 takes input from only VTTR in this mode. PWM output can

be on or off, depending on the state of SHDNA.

Soft-Start Control for VTT and VTTR. Connect a capacitor (C9 in the Typical Applications Circuit) from

8

SS

SS to ground (see the Soft-Start Capacitor Selection section). Leave SS open to disable soft-start.

SS discharges to ground when SHDNB is low. See the POR, UVLO, and Soft-Start section.

Sensing Pin for Termination Supply Output. Normally connected to VTT pin to allow accurate regulation

9

VTTS

to half the REFIN voltage. Connected to a resistive divider from VTT to GND to regulate VTT to higher

than half the REFIN voltage.

10

VTTR

Termination Reference Voltage. VTTR tracks VREFIN / 2.

*The MAX8551 has no OVP or discharge-mode feature. Only UVP is available.

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