LP2998MASLASHNOPB datasheet(4/33 Pages) TI1

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LP2998, LP2998-Q1

SNVS521K – DECEMBER 2007 – REVISED AUGUST 2014

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Pin Descriptions (continued)

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

LP2998 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 LP2998 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 LP2998 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.

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 LP2998 is designed to handle peak transient currents of up to ± 3 A

with a fast transient response. The maximum continuous current is a function of VIN and can be viewed in the Typical

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 LP2998 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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