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PM6641TR Datasheet(PDF) 38 Page - STMicroelectronics

Part # PM6641TR
Description  Monolithic VR for chipset and DDR2/3 supply for ultra-mobile PC (UMPC) applications
PDF  47 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

PM6641TR Datasheet(HTML) 38 Page - STMicroelectronics

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Components selection
PM6641
38/47
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
over-voltage 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 Vf is the output capacitor voltage after the load transient and Vi 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 CO and its equivalent series resistance (ESR) RES provide a low
frequency zero,
, the roll-off compensation pole must be added:
Equation 23
This pole is useful if the fzo zero is greater than about
.
However, the first assumption must relate the cross-over frequency, fCO, with the control
loop gain:
Equation 24
2
2
MAX
,
LOAD
2
min
,
OUT
Vi
Vf
I
L
C
−
⋅
=
(
)
(
)
()
O
O
ES
O
ES
O
C
Ro
C
Ro
C
C
C
L
m
LOOP
R
1
R
R
sC
1
R
sC
1
C
C
R
sC
sC
1
R
sC
K
g
)
s
(
G
+
+
+
⋅
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
+
+
α
=
ES
O
zo
R
C
2
1
f
π
=
C
RO
PRO
R
C
2
1
f
π
≅
5
f
0
C
()
()
()
O
ES
O
CO
O
C
L
m
O
O
ES
O
CO
C
CO
C
C
CO
L
m
CO
LOOP
R
R
C
f
2
R
R
K
g
R
R
R
C
f
2
C
f
2
R
C
f
2
K
g
f
2
j
G
+
π
α
=
+
π
⋅
π
π
α
≅
π



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