LP2998MASLASHNOPB datasheet(20/33 Pages) TI1

20 / 33 page

LP2998, LP2998-Q1

SNVS521K – DECEMBER 2007 – REVISED AUGUST 2014

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10 Power Supply Recommendations

There are several recommendations for the LP2998 input power supply. An input capacitor is not required but is

recommended for improved performance during large load transients to prevent the input rail from dropping. The

input capacitor should be located as close as possible to the PVIN pin. Several recommendations exist

dependent on the application required. A typical value recommended for AL electrolytic capacitors is 50 µF.

Ceramic capacitors can also be used, a value in the range of 10 µF with X5R or better would be an ideal choice.

The input capacitance can be reduced if the LP2998 is placed close to the bulk capacitance from the output of

the 2.5 V DC-DC converter. If the two supply rails (AVIN and PVIN) are separated then the 47 uF capacitor

should be placed as close to possible to the PVIN rail. An additional 0.1 uF ceramic capacitor can be placed on

the AVIN rail to prevent excessive noise from coupling into the device.

11 Layout

11.1 Layout Guidelines

1. The input capacitor for the power rail should be placed as close as possible to the PVIN pin.

2. VSENSE should be connected to the VTT termination bus at the point where regulation is required. For

motherboard applications an ideal location would be at the center of the termination bus.

3. VDDQ can be connected remotely to the VDDQ rail input at either the DIMM or the Chipset. This provides the

most accurate point for creating the reference voltage.

4. For improved thermal performance excessive top side copper should be used to dissipate heat from the

package. Numerous vias from the ground connection to the internal ground plane will help. Additionally these

can be located underneath the package if manufacturing standards permit.

5. Care should be taken when routing the VSENSE trace to avoid noise pickup from switching I/O signals. A 0.1

µF ceramic capacitor located close to the SENSE can also be used to filter any unwanted high frequency

signal. This can be an issue especially if long SENSE traces are used.

6. VREF should be bypassed with a 0.01 µF or 0.1 µF ceramic capacitor for improved performance. This

capacitor should be located as close as possible to the VREF pin.

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LP2998MA/NOPB Datenblatt(PDF) 20 Page - Texas Instruments