LP2996LQ Datasheet, PDF (8/26 Pages) Texas Instruments – LP2996-N DDR Termination Regulator

LP2996-N
SNOSA40J – NOVEMBER 2002 – REVISED MARCH 2013
www.ti.com
V_SENSE
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 LP2996-N 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 0.1uF ceramic capacitor placed next to the V_SENSE pin can help
filter any high frequency signals and preventing errors.
SHUTDOWN
The LP2996-N 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 LP2996-N will drop, however, V_DDQ will always maintain its constant impedance of 100kΩ 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.
V_REF
V_REF provides the buffered output of the internal reference voltage VDDQ / 2. This output should be used to
provide the reference voltage for the Northbridge chipset and memory. Since these inputs are typically an
extremely high impedance, there should be little current drawn from V_REF. For improved performance, an output
bypass capacitor can be used, located close to the pin, to help with noise. A ceramic capacitor in the range of
0.1 µF to 0.01 µF is recommended. This output remains active during the shutdown state and thermal shutdown
events for the suspend to RAM functionality.
V_TT
V_TT is the regulated output that is used to terminate the bus resistors. It is capable of sinking and sourcing
current while regulating the output precisely to VDDQ / 2. The LP2996-N is designed to handle peak transient
currents of up to ± 3A with a fast transient response. The maximum continuous current is a function of V_IN and
can be viewed in the Typical Performance Characteristics section. If a transient is expected to last above the
maximum continuous current rating for a significant amount of time then the output capacitor should be sized
large enough to prevent an excessive voltage drop. Despite the fact that the LP2996-N is designed to handle
large transient output currents it is not capable of handling these for long durations, under all conditions. The
reason for this is the standard packages are not able to thermally dissipate the heat as a result of the internal
power loss. If large currents are required for longer durations, then care should be taken to ensure that the
maximum junction temperature is not exceeded. Proper thermal derating should always be used (please refer to
the Thermal Dissipation section). If the junction temperature exceeds the thermal shutdown point than V_TT will tri-
state until the part returns below the hysteretic trip-point.
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