AN4086

Circuit description

Doc ID 023053 Rev 1

5/11

During a load transient, the output capacitor bank either supplies the load current, or

absorbs the energy stored in the inductor until the regulator reacts. The output voltage drop

that depends on the ESR (equivalent series resistance) and on the capacitive

charge/discharge is calculated according to the following:

Equation 5

where

MLCC capacitors typically have low ESR which is good to minimize the voltage ripple, but

they have low capacitance. Electrolytic capacitors have larger capacitance, which is good for

minimizing voltage changes during transients, but they also have higher ESR than MLCC

capacitors.

Ideally, a mix of electrolytic and MLCC capacitors can be used for minimal ripple as well as

minimizing voltage changes during transient loads.

●

Input capacitor selection

The major consideration when choosing an input capacitor is the input RMS current,

following:

Equation 6

.

Make sure the capacitor RMS current rating is well above the maximum operating RMS

current of the regulator. For long-term reliability, a good rule of thumb is to choose a

capacitor that will exhibit less than a 10 °C rise in temperature at max RMS current.

Most capacitor datasheets have plots that show RMS current vs. temperature.

Another consideration is the input ripple voltage - which is caused by the ESL

(equivalent series inductance) and ESR of the input capacitor and the dV/dt of the

switch node. Using low ESR and ESL ceramic capacitors are effective for lowering

input ripple voltage.

I

RMS

I

OUT

D1

D

–

()

⋅

⋅

=

I

RMS

I

OUT

2

------------

=