LP2996A Datasheet, PDF (3/29 Pages) Texas Instruments – DDR Termination Regulator

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5 Pin Configuration and Functions
SO PowerPAD
8 LEAD DDA
TOP VIEW
GND ¹
SD ²
VSENSE ³
VREF ⁴
GND
8 VTT
7 _PVIN
6 _AVIN
5 _VDDQ
LP2996A
SNOSCY7 – JUNE 2014
Pin Functions
PIN
TYPE
DESCRIPTION
1
GND
Ground
2
SD
Shutdown
3
VSENSE Feedback pin for regulating V_TT.
4
VREF
Buffered internal reference voltage of V_DDQ/2
5
VDDQ
Input for internal reference equal to V_DDQ/2
6
AVIN
Analog input pin
7
PVIN
Power input pin
8
VTT
Output voltage for connection to termination resistors
EP
Exposed pad thermal connection. Connect to Ground.
5.1 Pin Descriptions
AVIN AND PVIN
VDDQ
V_SENSE
SHUTDOWN
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 V_TT is tri-stated and
V_REF remains active.
VDDQ is the input used to create the internal reference voltage for regulating V_TT. The reference voltage is generated
from a resistor divider of two internal 50 kΩ resistors. This ensures that V_TT 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 V_TT (See Electrical Characteristics Table for exact values of V_TT over
temperature).
The purpose of the sense pin is to provide improved remote load regulation. In most motherboard applications the
termination resistors will connect to V_TT 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 V_SENSE 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
V_SENSE pin must still be connected to V_TT. Care should be taken when a long V_SENSE trace is implemented in close
proximity to the memory. Noise pickup in the V_SENSE trace can cause problems with precise regulation of V_TT. A small
0.1 uF ceramic capacitor placed next to the V_SENSE pin can help filter any high frequency signals and preventing errors.
The LP2996A contains an active low shutdown pin that can be used to tri-state VTT. During shutdown V_TT should not
be exposed to voltages that exceed AVIN. With the shutdown pin asserted low the quiescent current of the LP2996A
will drop, however, V_DDQ 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.
Copyright © 2014, Texas Instruments Incorporated
Product Folder Links: LP2996A
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