Datasheet

PM6641 Components selection
Doc ID 13510 Rev 3 39/47
From the definition of cross-over frequency, the value of the compensation resistor is
derived:
Equation 25
A good choice for the cross-over frequency is to assign f
CO
equal to .
The fixed parameters g
m
= 300 μs and K
L
= 4.4 s are design parameters, whereas the
feedback divider factor (α) is application dependant (see
Chapter 7.3: SW regulators control
loop on page 24
section for details).
After computing R
C
, the compensation capacitor can be designed in order to place the
compensation zero near the power stage pole:
Equation 26
The roll-off capacitance, as said previously, must compensate for the power stage high
frequency zero, when necessary:
Equation 27
As final step, it’s important to verify that the compensation zero is quite far from the cross-
over frequency. An empirical rule is satisfied if the following holds:
Equation 28
All these considerations are true if the cross-over frequency is quite lower than the switching
frequency, and the compensation zero and the power stage pole are far enough from
f
CO
.
OLm
OESOCO
CCOLOOP
RKg
)RR(Cf2
R1)f2j(G
α
+π
=⇒=π
10
f
SW
(
)
C
ESOO
C
R
RRC
C
+
=
C
ESO
RO
R
RC
C
=
5
f
RC2
1
f
CO
CC
ZC
≤
π
=