XR79110EL-F datasheet(14/19 Pages) EXAR

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XR79110

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Rev 2B

only ceramic output capacitors are used for COUT then a

CFF is necessary. Calculate CFF from:

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 90uF 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=48kHz.

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 that 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.

VOUT versus frequency curves

VOUT versus switching frequency (f) curves corresponding

to peak-to-peak inductor current ripple (IL) are plotted in

figure 12. For a particular VIN, VOUT and f the magnitude of

IL can be determined from figure 12. As an example, for

VIN=12V, VOUT=1.5V and f=400kHz the IL is 5A. Alter-

nately for a given VIN, VOUT and IL the required switching

frequency can be ascertained. For example for VIN=12V,

VOUT=1.5V and IL=4A the required f is 500kHz.

CFF

 80kHz R1



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