XR76203 Datasheet, PDF(15/20 Page) Exar Corporation – 3A/5A/8A Synchronous Step Down COT Regulator

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XR76203 Datasheet, PDF (15/20 Pages) Exar Corporation – 3A/5A/8A Synchronous Step Down COT Regulator

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Feed-Forward Capacitor (CFF)

A feed-forward capacitor (CFF) may be necessary depend-

ing on the Equivalent Series Resistance (ESR) of COUT. If

only ceramic output capacitors are used for COUT then a

CFF is necessary. Calculate CFF from:

CFF

------------------------1-------------------------

2 ï´ ï° ï´ R1 ï´ 7 ï´ fLC

where:

R1 is the resistor that CFF is placed in parallel with

fLC is the frequency of output filter double-pole

fLC frequency must be less than 11kHz when using ceramic

COUT. If necessary, increase L and/or COUT in order to meet

this constraint.

When using capacitors with higher ESR, such as PANA-

SONIC TPE series, a CFF is not required provided following

conditions are met:

1. The frequency of output filter LC double-pole fLC should

be less than 11kHz.

2. The frequency of ESR Zero fZero,ESR should be at least

five times larger than fLC.

Note that if fZero,ESR is less than 5xfLC, then it is recom-

mended to set the fLC at less than 2kHz. CFF is still not

required.

Maximum Allowable Voltage Ripple at FB pin

Note that the steady-state voltage ripple at feedback pin FB

(VFB,RIPPLE) must not exceed 50mV in order for the Regula-

tor to function correctly. If VFB,RIPPLE is larger than 50mV

then COUT should be increased as necessary in order to

keep the VFB,RIPPLE below 50mV.

Feed-Forward Resistor (RFF)

Poor PCB layout can cause FET switching noise at the out-

put and may couple to the FB pin via CFF. Excessive noise

at FB will cause poor load regulation. To solve this problem

place a resistor RFF in series with CFF. RFF value up to 2%

of R1 is acceptable.

15 / 20

XR76203/5/8

exar.com/XR76203/5/8

Rev 1A

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