XR79115 Datasheet, PDF(14/19 Page) Exar Corporation – 22V, 15A Synchronous Step Down COT Power Module

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XR79115 Datasheet, PDF (14/19 Pages) Exar Corporation – 22V, 15A Synchronous Step Down COT Power Module

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only ceramic output capacitors are used for COUT then a

CFF is necessary. Calculate CFF from:

CFF

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

2 ï´ ï° ï´ 80kHz ï´ R1

where:

R1 is the resistor that CFF is placed in parallel with

80kHz is the location of the Zero formed by R1 and CFF

Note that minimum required COUT is 140uF when using

ceramic capacitors.

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 15kHz.

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

three times larger than fLC.

As an example the application circuit has fLC=8.3kHz and

fZero,ESR=23.4kHz.

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

(VFB,RIPPLE) must not exceed 50mV in order for Module 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.

14 / 19

XR79115

exar.com/XR79115

Rev 1B

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