Datasheet

ADP1829 Data Sheet
Rev. C | Page 22 of 28
Because of the finite output current drive of the error amplifier,
C
I
needs to be less than 10 nF. If it is larger than 10 nF, choose a
larger R
TOP
and recalculate R
Z
and C
I
until C
I
is less than 10 nF.
Because C
HF
<< C
I
, combining Equation 29 and Equation 39
yields
Z
SW
HF
Rf
C
π
1
 (44)
Next, calculate the feedforward capacitor, C
FF.
. Assume R
FF
<<
R
TOP
, then Equation 28 is simplified to
TOP
FF
Z2
RC
f
π2
1
 (45)
Solving C
FF
in Equation 45 yields
Z2TOP
FF
fR
C
π2
1
 (46)
where
f
Z2
is obtained from Equation 40 or Equation 41.
The feedforward resistor, R
FF
, can be calculated by combining
Equation 30 and Equation 39
SW
FF
FF
fC
R
π
1
 (47)
Check that the calculated component values are reasonable. For
instance, capacitors smaller than about 10 pF should be avoided.
In addition, the ADP1829 error amplifier has finite output
current drive, so R
Z
values less than 3 kΩ and C
I
values greater
than 10 nF should be avoided. If necessary, recalculate the
compensation network with a different starting value of R
TOP
.
If R
Z
is too small and C
I
is too big, start with a larger value of R
TOP
.
This compensation technique should yield a good working
solution.
In general, aluminum electrolytic capacitors have high ESR, and
a Type II compensation is adequate. However, if several aluminum
electrolytic capacitors are connected in parallel, producing a low
effective ESR, then Type III compensation is needed. In addition,
ceramic capacitors have very low ESR, in the order of a few
milliohms. Type III compensation is needed for ceramic output
capacitors. Type III compensation offers better performance
than Type II in terms of more low frequency gain and more
phase margin and less high frequency gain at the cross-over
frequency.
For a more exact method or to optimize for other system
characteristics, a number of references and tools are available
from your Analog Devices, Inc. applications support team.
SOFT START
The ADP1829 uses an adjustable soft start to limit the output
voltage ramp-up period, thus limiting the input inrush current.
The soft start is set by selecting the capacitor, C
SS
, from SS1 and
SS2 to GND. The ADP1829 charges C
SS
to 0.8 V through an
internal 90 kΩ resistor. The voltage on the soft start capacitor
while it is charging is









SS
RC
t
CSS
eV 1V8.0
(48)
The soft start period ends when the voltage on the soft start pin
reaches 0.6 V. Substituting 0.6 V for V
SS
and solving for the
number of RC time constants,










SS
SS
C
t
e
kΩ90
1V8.0V6.0
(49)
SSSS
RCt 386.1

(50)
Because R = 90 kΩ,
secμF/8


SSSS
tC (51)
where t
SS
is the desired soft start time in seconds.
VOLTAGE TRACKING
The ADP1829 includes a tracking feature that prevents an
output voltage from exceeding a master voltage. This is
especially important when the ADP1829 is powering separate
power supply voltages on a single integrated circuit, such as the
core and I/O voltages of a DSP or microcontroller. In these
cases, improper sequencing can cause damage to the load.
The ADP1829 tracking input is an additional positive input to
the error amplifier. The feedback voltage is regulated to the lower of
the 0.6 V reference or the voltage at TRK, so a lower voltage on
TRK limits the output voltage. This feature allows implementation
of two different types of tracking: coincident tracking, where
the output voltage is the same as the master voltage until the
master voltage reaches regulation, or ratiometric tracking, where
the output voltage is limited to a fraction of the master voltage.
In all tracking configurations, the master voltage should be
higher than the slave voltage.
Note that the soft start time setting of the master voltage should
be longer than the soft start of the slave voltage. This forces the
rise time of the master voltage to be imposed on the slave voltage.
If the soft start setting of the slave voltage is longer, the slave
comes up more slowly and the tracking relationship is not seen
at the output. The slave channel should still have a soft start
capacitor to give a small but reasonable soft start time to protect
in case of restart after a current-limit event.
COMP
FB
TRK
SS
MASTER
VOLTAGE
R
TRKT
R
TRKB
0.6V
DETAIL VIEW OF
ADP1829
ERROR
AMPLIFIER
V
OUT
R
TOP
R
BOT
06784-029
Figure 30. Voltage Tracking