Datasheet

ADP1822 Data Sheet
Rev. D | Page 18 of 24
Choose the compensation capacitor to set the compensation
zero, f
ZC
, to the lesser of 1/4 of the crossover frequency or 1/2 of
the LC resonant frequency, or
( )( )
COMPCOMP
SWCO
ZC
RC
ff
f
π2
1
404
===
(36)
or
( )( )
COMPCOMP
LC
ZC
RC
f
f
π2
1
2
==
(37)
Solving for C
COMP
,
( )( )
COMPCO
COMP
Rf
C
π2
4
=
(38)
In terms of the switching frequency and combining the constants,
( )( )
COMPSW
COMP
Rf
C
37.6
≅
(39)
or
( )( )
COMPLC
COMP
Rf
C
π2
2
=
(40)
or whichever is greater.
Choose the feed-forward capacitor, C
FF
, to set the feed-forward
zero at 1/7 of the crossover frequency
7
CO
ZFF
f
f =
(41)
or
( ) ( )
FF
TOP
CO
CR
f
π2
7
=
(42)
Simplifying and solving for C
FF
,
( ) ( )
SW
TOP
FF
fR
C
14.11
=
(43)
Choose the feed-forward resistor, R
FF
, to set the condition
( )
( )( )
FFFF
CO
CR
f
π27
1
=
(44)
Simplifying and solving for R
FF
,
( ) ( )
FF
SW
FF
Cf
R
227.0
=
(45)
Compensation Using Both the ESR and Feed-Forward Zeros
If the output capacitor ESR zero frequency falls between 1/2 of
the crossover frequency to 2× the crossover frequency, use the
circuit shown in Figure 19, such that the ESR zero, along with a
feed-forward network, stabilizes the regulator. In this case, the
feed-forward zero is set to 1/7 of the crossover frequency, and
the feed-forward pole is set to the same frequency as the ESR zero.
Choose the compensation resistor, R
COMP
, to set the crossover
frequency by
( ) ( )( )( )
( )
2
LC
IN
CO
ZFF
RAMP
TOP
COMP
fV
ffVR
R =
(46)
where f
ZFF
is the feed-forward zero frequency and is 1/7 of the
crossover frequency. Simplify the following equation:
( ) ( )
( )
( )
IN
SW
TOP
COMP
V
CLfR
R
2
0705.0≅
(47)
Choose the compensation capacitor to set the compensation
zero, f
ZC
, to 1/2 of the LC resonant frequency, or
( )( )
COMPCOMP
LC
ZC
RC
f
f
π2
1
2
==
(48)
Solving for C
COMP
,
( )( )
COMPLC
COMP
Rf
C
π2
2
=
(49)
Choose the feed-forward capacitor, C
FF
, to set the feed-forward
zero at 1/7 of the crossover frequency
7
CO
ZFF
f
f =
(50)
or
( ) ( )
FF
TOP
CO
CR
f
π2
7
=
(51)
Simplifying and solving for C
FF
,
( ) ( )
SW
TOP
FF
fR
C
14.11
=
(52)
Choose the feed-forward resistor, R
FF
, to set the condition
( )
( )( )
FFFF
CO
CR
f
π27
1
=
(53)
Simplifying and solving for R
FF
,
( )
( )
FF
SW
FF
Cf
R
227.0
=
(54)
SETTING THE SOFT START PERIOD
The ADP1822 uses an adjustable soft start to limit the output
voltage ramp-up period, limiting the input inrush current. The
soft start is set by selecting the capacitor, C
SS
, from SS to GND.
The ADP1822 charges C
SS
to 0.8 V through an internal resistor.
The voltage on C
SS
while it is charging is








−=
SS
RC
CSS
eV
1
1V8.0
(55)
where R is the internal 100 kΩ resistor. The soft start period, t
SS
,
is achieved when V
CSS
= 0.6 V, or








−=
)kΩ100
1V8.0V6.0
SS
SS
C
t
e
(56)