ACT6390/ACT6391
Rev PrA, 01-Sep-07
the cross-over frequency (unity gain-frequency)
should be less than one-fifth of the right-half-plane
zero f
Z(RHP)
, and lower than one-fifteenth of switch-
ing frequency f
sw
.
C
COMP2
,
C
COMP2
�½
C
OUT
R
ESR
R
COMP
(23)
f
Z
RHP
V
R
�½
IN 2 LOAD
2V
OUT
π
L
2
(16)
If the value of C
COMP2
calculated by
(23)
is smaller
than 10pF, C
COMP2
can be omitted.
For example:
Choose
f
C
�½
1
f
Z
RHP
, then calculate C
COMP
:
5
f
Z
RHP
C
COMP
�½
�½
V
FB
R
LOAD
G
M
1
D
V
OUT
R
CS
2
π
f
C
V
IN
V
FB
R
LOAD
G
M
2
R
CS
2
π
f
C
V
OUT
(17)
12V
250mA
57.8kHz
�½
2
2
12V
π
10
μ
H
1
f
Z
RHP
�½
11.56 kHz
5
3.3V
2
(24)
Choose
f
C
�½
C
COMP
�½
Select R
COMP
to meet the transient-droop require-
ments.
3.3V
1.24V 48
Ω
150
μ
S
�½
6.26nF
(25)
2
0.45
Ω
2
π
11.56kHz
12V
α
V
FB
G
M
R
COMP
�½
R
CS
V
OUT
I
OUT
K
1
V
IN
η
2
Choose C
COMP
= 6.8nF
Assume that 200mV of transient droop can be
accepted:
(18)
R
COMP
K
R
CS
V
OUT
I
OUT
1
2
�½
α
V
FB
G
M
V
IN
η
α
�½
(19)
200 mV
1
�½
12V
60
(26)
Where:
α
is the transient droop percentage which can be
accepted, calculated by:
α
�½
Δ
V
OUT
V
OUT
0.4
0.45
Ω
12V
250mA
1
2
R
COMP
�½
�½
186.3k
Ω
(27)
1
1.24V
150
μ
S
3.3V
0.85
60
Choose R
COMP
= 180kΩ
(20)
C
OUT
�½
K: is defined in equation
(4)
η:
is the typical efficiency.
V
FB
: is the feedback voltage, 1.24V
G
M
: is the trans-conductance of the error amplifier.
The output capacitor is chosen to set the output
pole for canceling the R
COMP,
C
COMP
zero.
R
COMP
C
COMP
180k
Ω
6.8nF
�½
25.5
μ
F
(28)
�½
R
LOAD
12V
0.25 A
C
OUT
can be chosen to be either 22µF or 33µF,
choose 33µF to reduce droop.
R
COMP
�½
R
LOAD
C
OUT
48
Ω
33
μ
F
�½
�½
233k
Ω
C
COMP
6.8nF
(29)
C
OUT
�½
R
COMP
C
COMP
R
LOAD
If a ceramic capacitor is used with an assumed
ESR of 20mΩ,
(21)
f
Z
ESR
�½
C
COMP2
is optional and can be used when the output
capacitor has significant ESR. The ESR will form a
zero as follows:
1
�½
241kHz
2
π
33
μ
F
20 m
Ω
(30)
f
Z(ESR)
> f
C
Since the zero frequency is greater than the pole
frequency ,C
COMP2
can be omitted.
If a tantalum capacitor is used, whose ESR is about
0.5Ω,
f
Z
ESR
�½
1
2
π
R
ESR
C
OUT
(22)
If this zero occurs at a higher frequency than the
cross-over frequency, it can be ignored. Otherwise,
it should be canceled with the pole set by capacitor
Innovative Products.
Active Solutions.
-7-
Copyright © 2007 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
