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LM20124, LM20124-Q1

SNVS507F – OCTOBER 2007 – REVISED MARCH 2013

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conditions. A wide range of output capacitors may be used with the LM20124 that provide excellent performance.

The best performance is typically obtained using ceramic, SP, or OSCON type chemistries. Typical trade-offs are

that the ceramic capacitor provides extremely low ESR to reduce the output ripple voltage and noise spikes,

while the SP and OSCON capacitors provide a large bulk capacitance in a small volume for transient loading

conditions.

When selecting the value for the output capacitor the two performance characteristics to consider are the output

voltage ripple and transient response. The output voltage ripple can be approximated by using the formula shown

below.

(4)

Where,

used in the design. The amount of output ripple that can be tolerated is application specific; however a general

recommendation is to keep the output ripple less than 1% of the rated output voltage. Keep in mind ceramic

capacitors are sometimes preferred because they have very low ESR; however, depending on package and

voltage rating of the capacitor the value of the capacitance can drop significantly with applied voltage. The output

capacitor selection will also affect the output voltage droop during a load transient. The peak droop on the output

voltage during a load transient is dependent on many factors; however, an approximation of the transient droop

ignoring loop bandwidth can be obtained using the following equation.

(5)

output voltage drop ignoring loop bandwidth considerations,

tolerance and voltage coefficient of the capacitor needs to be examined when designing for a specific output

ripple or transient drop target.

Good quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the

switch current during the on-time. In general it is recommended to use a ceramic capacitor for the input as they

provide both a low impedance and small footprint. One important note is to use a good dielectric for the ceramic

capacitor such as X5R or X7R. These provide better over temperature performance and also minimize the DC

voltage derating that occurs on Y5V capacitors. For most applications, a 22 µF, X5R, 6.3V input capacitor is

sufficient; however, additional capacitance may be required if the connection to the input supply is far from the

PVIN pins. The input capacitor should be placed as close as possible PVIN and PGND pins of the device.

Non-ceramic input capacitors should be selected for RMS current rating and minimum ripple voltage. A good

approximation for the required ripple current rating is given by the relationship:

(6)

As indicated by the RMS ripple current equation, highest requirement for RMS current rating occurs at 50% duty

cycle. For this case, the RMS ripple current rating of the input capacitor should be greater than half the output

current. For best performance, low ESR ceramic capacitors should be placed in parallel with higher capacitance

capacitors to provide the best input filtering for the device.

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