LP2996LQSLASHNOPB datasheet(8/28 Pages) TI1

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LP2996-N

SNOSA40J – NOVEMBER 2002 – REVISED MARCH 2013

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VSENSE

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

the termination resistors will connect to VTT 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 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 0.1uF ceramic capacitor placed next to the VSENSE 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 VTT

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, VDDQ 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.

VREF

VREF 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 VREF. 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.

VTT

VTT 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 VIN 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 VTT will tri-

state until the part returns below the hysteretic trip-point.

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