XR75100 Datasheet, PDF(12/15 Page) Exar Corporation – 40V Synchronous Step Down COT Controller

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XR75100 Datasheet, PDF (12/15 Pages) Exar Corporation – 40V Synchronous Step Down COT Controller

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XR75100

Programming the Soft-start

Place a capacitor CSS between the SS and GND pins to

program the soft-start. In order to program a soft-start time

of TSS, calculate the required capacitance CSS from the

following equation:

CSS

TSS

ï´

ï¦

ï¨

1--0--0-.--6-ï--V-A---ï¸ï¶

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 must be less than 11kHz when using ceramic COUT. If

necessary, increase COUT and/or L 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.

Feed-Forward Resistor (RFF)

Poor PCB layout and/or extremely fast switching FETs can

cause switching noise at the output 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 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.

12 / 15

exar.com/XR75100

Rev 1C

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