LP2996A

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SNOSCY7 – JUNE 2014

8 Applications and Implementation

8.1 Application Information

8.1.1 Input Capacitor

The LP2996A does not require a capacitor for input stability, but it 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

LP2996A 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.

8.1.2 Output Capacitor

The LP2996A has been designed to be insensitive of output capacitor size or ESR (Equivalent Series

Resistance). This allows the flexibility to use any capacitor desired. The choice for output capacitor will be

recommendation the output capacitor should be sized above 100 µF with a low ESR for SSTL applications with

DDR-SDRAM. The value of ESR should be determined by the maximum current spikes expected and the extent

at which the output voltage is allowed to droop. Several capacitor options are available on the market and a few

of these are highlighted below:

AL - It should be noted that many aluminum electrolytics only specify impedance at a frequency of 120 Hz, which

indicates they have poor high frequency performance. Only aluminum electrolytics that have an impedance

specified at a higher frequency (between 20 kHz and 100 kHz) should be used for the LP2996A. To improve the

ESR several AL electrolytics can be combined in parallel for an overall reduction. An important note to be aware

of is the extent at which the ESR will change over temperature. Aluminum electrolytic capacitors can have their

ESR rapidly increase at cold temperatures.

Ceramic - Ceramic capacitors typically have a low capacitance, in the range of 10 to 100 µF range, but they have

excellent AC performance for bypassing noise because of very low ESR (typically less than 10 m

Ω). However,

some dielectric types do not have good capacitance characteristics as a function of voltage and temperature.

Because of the typically low value of capacitance it is recommended to use ceramic capacitors in parallel with

another capacitor such as an aluminum electrolytic. A dielectric of X5R or better is recommended for all ceramic

capacitors.

Hybrid - Several hybrid capacitors such as OS-CON and SP are available from several manufacturers. These

offer a large capacitance while maintaining a low ESR. These are the best solution when size and performance

are critical, although their cost is typically higher than any other capacitor.

8.1.3 Thermal Dissipation

generating heat. To prevent damaging the part from exceeding the maximum allowable junction temperature,

care should be taken to derate the part dependent on the maximum expected ambient temperature and power

(1)

(2)

The

number of vias and the airflow.

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