LP2996AMRESLASHNOPB datasheet(3/29 Pages) TI1

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VDDQ

PVIN

AVIN

VSENSE

VREF

GND

VTT

GND

LP2996A

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SNOSCY7 – JUNE 2014

5 Pin Configuration and Functions

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Pin Functions

TYPE

DESCRIPTION

GND

Ground

Shutdown

VSENSE

Feedback pin for regulating VTT.

VREF

Buffered internal reference voltage of VDDQ/2

VDDQ

Input for internal reference equal to VDDQ/2

AVIN

Analog input pin

PVIN

Power input pin

VTT

Output voltage for connection to termination resistors

Exposed pad thermal connection. Connect to Ground.

5.1 Pin Descriptions

AVIN AND PVIN

AVIN and PVIN are the input supply pins for the LP2996A. AVIN is used to supply all the internal control circuitry. PVIN,

however, is used exclusively to provide the rail voltage for the output stage used to create VTT. These pins have the

capability to work off separate supplies depending on the application. Higher voltages on PVIN will increase the

maximum continuous output current because of output RDSON limitations at voltages close to VTT. The disadvantage

of high values of PVIN is that the internal power loss will also increase, thermally limiting the design. For SSTL-2

applications, a good compromise would be to connect the AVIN and PVIN directly together at 2.5 V. This eliminates the

need for bypassing the two supply pins separately. The only limitation on input voltage selection is that PVIN must be

equal to or lower than AVIN. It is recommended to connect PVIN to voltage rails equal to or less than 3.3 V to prevent

the thermal limit from tripping because of excessive internal power dissipation. If the junction temperature exceeds the

thermal shutdown than the part will enter a shutdown state identical to the manual shutdown where VTT is tri-stated and

VREF remains active.

VDDQ

VDDQ is the input used to create the internal reference voltage for regulating VTT. The reference voltage is generated

from a resistor divider of two internal 50 k

Ω resistors. This ensures that VTT will track VDDQ / 2 precisely. The optimal

implementation of VDDQ is as a remote sense. This can be achieved by connecting VDDQ directly to the 2.5 V rail at

the DIMM instead of AVIN and PVIN. This ensures that the reference voltage tracks the DDR memory rails precisely

without a large voltage drop from the power lines. For SSTL-2 applications VDDQ will be a 2.5 V signal, which will

create a 1.25 V termination voltage at VTT (See Electrical Characteristics Table for exact values of VTT over

temperature).

The purpose of the sense pin is to provide improved remote load regulation. In most motherboard applications the

VSENSE

termination resistors will connect to VTT in a long plane. If the output voltage was regulated only at the output of the

LP2996A then the long trace will cause a significant IR drop resulting in a termination voltage lower at one end of the

bus than the other. The VSENSE pin can be used to improve this performance, by connecting it to the middle of the bus.

This will provide a better distribution across the entire termination bus. If remote load regulation is not used then the

VSENSE pin must still be connected to VTT. Care should be taken when a long VSENSE trace is implemented in close

proximity to the memory. Noise pickup in the VSENSE trace can cause problems with precise regulation of VTT. A small

0.1 uF ceramic capacitor placed next to the VSENSE pin can help filter any high frequency signals and preventing errors.

SHUTDOWN

The LP2996A contains an active low shutdown pin that can be used to tri-state VTT. During shutdown VTT should not

be exposed to voltages that exceed AVIN. With the shutdown pin asserted low the quiescent current of the LP2996A

will drop, however, VDDQ will always maintain its constant impedance of 100 kΩ for generating the internal reference.

Therefore to calculate the total power loss in shutdown both currents need to be considered. For more information refer

to the Thermal Dissipation section. The shutdown pin also has an internal pull-up current, therefore to turn the part on

the shutdown pin can either be connected to AVIN or left open.

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