XR79120 Datasheet, PDF(14/19 Page) Exar Corporation – 22V, 20A Synchronous Step Down COT Power Module

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XR79120 Datasheet, PDF (14/19 Pages) Exar Corporation – 22V, 20A Synchronous Step Down COT Power Module

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XR79120

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

ïIL for VIN=12V. As an example, for operating conditions of

VIN=12V, VOUT=1.5V and f=500kHz the current ripple is

about 6.5A. Note that maximum recommended peak-to-

peak ïIL is 10A. Therefore the maximum permissible VOUT

versus f corresponds to the top curve in figure 12. For

example with VIN=12V and f=500kHz maximum VOUT is

2.5V.

where:

R1 is the resistor that CFF is placed in parallel with

fLC is the frequency of output filter double-pole

fLC must be less than 13kHz when using ceramic COUT. If

necessary, increase 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 10kHz.

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 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.

Maximum recommended VOUT versus frequency

VOUT versus frequency curves corresponding to inductor

current ripple ïIL of 10A, 8A and 6A are plotted in figure 12.

These curves show the relationship between VOUT, f and

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