Datasheet

ADP1828
Rev. C | Page 25 of 36
SOFT START
The ADP1828 uses an adjustable soft start to limit the output
voltage ramp-up period, limiting the input inrush current. The
soft start is selected by setting the capacitor, C
SS
, from SS to
GND. The ADP1828 charges C
SS
to 0.8 V through an internal
90 k resistor. The voltage on the soft start capacitor while it is
charging is
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−=
SS
C
t
CSS
eV
k90
1V8.0 (45)
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 soft
start time t
SS
:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−=
SS
C
t
e
k90
1V8.0V6.0 (46)
SSSS
RCt 386.1=
(47)
Because R = 90 k:
secF/8×=
SSSS
tC (48)
where
t
SS
is the desired soft start time in seconds.
SWITCHING NOISE AND OVERSHOOT REDUCTION
In any high speed step-down regulator, high frequency noise
(generally in the range of 50 MHz to 100 MHz) and voltage
overshoot are always present at the gate, the switch node (SW),
and the drains of the external MOSFETs. The high frequency
noise and overshoot are caused by the parasitic capacitance,
C
gd
, of the external MOSFET and the parasitic inductance of
the gate trace and the packages of the MOSFETs. When the
high current is switched, electromagnetic interference (EMI)
is generated, which can affect the operation of the surrounding
circuits. To reduce voltage ringing at the drain of the MOSFET,
an RC snubber can be added between SW and PGND, as illu-
strated in Figure 41. In most applications, R
SNUB
is about 2 Ω,
and C
SNUB
about 1.2 nF. R
SNUB
and C
SNUB
can be calculated using
the following equations:
OSS
SNUB
fC
R
π
=
2
1
(49)
OSSSNUB
CC =
(50)
where:
f is the high frequency ringing measured at the SW node.
C
OSS
is the total output capacitance of the top-side and low-side
MOSFETs, given in the MOSFET data sheet.
The size of the RC snubber components need to be chosen
correctly to handle the power dissipation. The power dissipated
in R
SNUB
is:
SWSNUB
IN
SNUB
fCVP
2
=
In most applications, a size 0805 component is sufficient. The
use of the RC snubber reduces the overall efficiency, generally
by an amount in the range of 0.1% to 0.5%. However, the RC
snubber cannot reduce the voltage overshoot. A resistor, shown
as R
RISE
in Figure 41, at the BST pin could help to reduce
overshoot and is generally between 1 Ω and 5 Ω.
DH
SW
CSL
DL
PGND
M2
M1
L
V
OUT
C
OUT
R
SNUB
C
SNUB
V
IN
R
CL
ADP1828
BST
PV
R
RISE
06865-042
Figure 41. Application Circuit with a Snubber
The ADP1828 includes a feature that tracks a master voltage.
portant when multiple ADP1828s
s the ADP1829) are powering sepa-
rate power supply voltages, 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 ADP1828 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, the SS voltage, 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: coin-
cident 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 final value of 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
e to be imposed on the slave voltage.
slave voltage is longer, the slave
acking relationship is not
l should still have a soft
start capacitor to give a small but reasonable soft start time to
protect the part in case of restart after a current-limit event.
VOLTAGE TRACKING
This feature is especially im
(or other controllers such a
rise time of the master voltag
If the soft start setting of the
comes up more slowly and the tr
seen at the output. The slave channe