LM46002PWPR datasheet(29/52 Pages) TI

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OUT

ESR

(

0.5)

u u '

OUT

(1 D )

(1 r)

/ I

)

LM46002

www.ti.com

SNVSA13B – APRIL 2014 – REVISED SEPTEMBER 2014

9.2.2.5 Output Capacitor Selection

The device is designed to be used with a wide variety of LC filters. It is generally desired to use as little output

capacitance as possible to keep cost and size down. The output capacitor (s), COUT, should be chosen with

care since it directly affects the steady state output voltage ripple, loop stability and the voltage over/undershoot

during load current transients.

The output voltage ripple is essentially composed of two parts. One is caused by the inductor current ripple going

through the Equivalent Series Resistance (ESR) of the output capacitors:

ΔVOUT-ESR = ΔiL× ESR

(17)

The other is caused by the inductor current ripple charging and discharging the output capacitors:

ΔVOUT-C = ΔiL/ ( 8 × FS × COUT )

(18)

The two components in the voltage ripple are not in phase, so the actual peak-to-peak ripple is smaller than the

sum of the two peaks.

Output capacitance is usually limited by transient performance specifications if the system requires tight voltage

regulation in the presence of large current steps and fast slew rates. When a fast large load transient happens,

output capacitors provide the required charge before the inductor current can slew to the appropriate level. The

initial output voltage step is equal to the load current step multiplied by the ESR. VOUT continues to droop until

the control loop response increases or decreases the inductor current to supply the load. To maintain a small

over- or under-shoot during a transient, small ESR and large capacitance are desired. But these also come with

higher cost and size. Thus, the motivation is to seek a fast control loop response to reduce the output voltage

deviation.

For a given input and output requirement, the following inequality gives an approximation for an absolute

minimum output cap required:

(19)

Along with this for the same requirement, the max ESR should be calculated as per the following inequality

(20)

where

r = Ripple ratio of the inductor ripple current (

ΔIL / IOUT)

ΔVOUT = Target output voltage undershoot

D’ = 1 – Duty cycle

FS = Switching Frequency

IOUT = Load Current

A general guide line for COUT range is that COUT should be larger than the minimum required output capacitance

calculated by Equation 19, and smaller than 10 times the minimum required output capacitance or 1 mF. In

applications with VOUT less than 3.3 V, it is critical that low ESR output capacitors are selected. This will limit

potential output voltage overshoots as the input voltage falls below the device normal operating range. To

optimize the transient behavior a feed-forward capacitor could be added in parallel with the upper feedback

resistor. For this design example, three 47 µF,10 V, X7R ceramic capacitors are used in parallel.

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