XR79110 Datasheet, PDF (14/20 Pages) Exar Corporation – 22V, 10A Synchronous Step Down COT Power Module

XR79110
only ceramic output capacitors are used for C_OUT then a
C_FF is necessary. Calculate C_FF from:
frequency can be ascertained. For example for V_IN=12V,
V_OUT=1.5V and IL=4A the required f is 500kHz.
C_FF
=
------------------------¹--------------------------
2    80kHz  R1
where:
R1 is the resistor that C_FF is placed in parallel with
80kHz is the location of the Zero formed by R1 and C_FF
Note that minimum required C_OUT is 90uF when using
ceramic capacitors.
When using capacitors with higher ESR, such as PANA-
SONIC TPE series, a C_FF is not required provided following
conditions are met:
1. The frequency of output filter LC double-pole f_LC should
be less than 15kHz.
2. The frequency of ESR Zero f_Zero,ESR should be at least
three times larger than f_LC.
As an example the application circuit has f_LC=8.3kHz and
f_Zero,ESR=48kHz.
Maximum Allowable Voltage Ripple at FB pin
Note that the steady-state voltage ripple at feedback pin FB
(V_FB,RIPPLE) must not exceed 50mV in order for the Module
to function correctly. If V_FB,RIPPLE is larger than 50mV then
C_OUT should be increased as necessary in order to keep
the V_FB,RIPPLE below 50mV.
Feed-Forward Resistor (R_FF)
Poor PCB layout can cause FET switching noise at the out-
put that may couple to the FB pin via C_FF. Excessive noise
at FB will cause poor load regulation. To solve this problem
place a resistor R_FF in series with C_FF. R_FF value up to 2%
of R1 is acceptable.
V_OUT versus frequency curves
V_OUT versus switching frequency (f) curves corresponding
to peak-to-peak inductor current ripple (IL) are plotted in
figure 12. For a particular V_IN, V_OUT and f the magnitude of
IL can be determined from figure 12. As an example, for
V_IN=12V, V_OUT=1.5V and f=400kHz the IL is 5A. Alter-
nately for a given V_IN, V_OUT and IL the required switching
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14 / 20
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Rev 2A