Datasheet

Components selection PM6641
38/47 Doc ID 13510 Rev 3
If ceramic capacitors are used, the output voltage ripple due to inductor current ripple is
negligible. Then the inductance could be smaller, reducing the size of the choke. In this case
it is important that output capacitor can adsorb the inductor energy without generating an
overvoltage condition when the system changes from a full load to a no load condition.
The minimum output capacitance can be chosen by the following equation:
Equation 21
where V
f
is the output capacitor voltage after the load transient and V
i
is the output capacitor
voltage before the load transient.
8.4 SW regulator compensation components selection
As described in section SW regulators control loop, the PM6641 switching regulators control
loop is:
Equation 22
If the output capacitor C
O
and its equivalent series resistance (ESR) R
ES
provide a low
frequency zero, , the roll-off compensation pole must be added:
Equation 23
This pole is useful if the f
zo
zero is greater than about .
However, the first assumption must relate the cross-over frequency,
f
CO
, with the control
loop gain:
Equation 24
22
MAX,LOAD
2
min,OUT
ViVf
IL
C
−
⋅
=
(
)
(
)
()
O
OESO
ESO
C
Ro
CRoC
CC
LmLOOP
R
1RRsC
1RsC
1
C
C
RsCsC
1RsC
Kg)s(G
++
+
⋅
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
++
+
α=
ESO
zo
RC2
1
f
π
=
CRO
PRO
RC2
1
f
π
≅
5
f
0C
()
() ()
OESOCO
OC
LmO
OESOCOCCO
CCCO
LmCOLOOP
RRCf2
RR
KgR
RRCf2Cf2
RCf2
Kgf2jG
+π
α=
+π⋅π
π
α≅π