LP2998

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SNVS521I – DECEMBER 2007 – REVISED APRIL 2013

OUTPUT CAPACITOR

The LP2998 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 determined solely

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 (100 kHz) should be used for the LP2998. 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 tend to have rapidly increasing

ESR at cold temperatures.

Ceramic - Ceramic capacitors typically have a low capacitance, in the range of 10 to 100 µF. They also 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 type.

Thermal Dissipation

generation. To prevent damaging the part by exceeding the maximum allowable operating junction temperature,

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

(1)

(2)

The

of vias and the airflow. For instance, the

8x4 2-layer board with 1oz. copper, no airflow, and 0.5W dissipation at room temperature. This value can be

reduced to 151.2°C/W by changing to a 3x4 board with 2 oz. copper that is the JEDEC standard. Figure 18

shows how the

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