ADP1823 Datasheet, PDF (22/32 Pages) Analog Devices – Dual, Interleaved, Step-Down DC-to-DC Controller with Tracking

ADP1823
Calculate the capacitor value for the high frequency pole:
C_HF =
1
2π R_Z f_P
(58)
Check that the calculated component values are reasonable. For
instance, capacitors smaller than about 10 pF should be avoided.
In addition, the ADP1823 error amplifier has finite output
current drive, so RZ values less than a few kΩ and CI values
greater than 10 nF should be avoided. If necessary, recalculate
the compensation network with a different starting value of
RTOP. If CHF is too small, start with a smaller value RTOP. If RZ is
too small and CI is too big, start with a larger value of RTOP.
This compensation technique should yield a good working
solution. 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., application
support team.
SOFT START
The ADP1823 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, CSS, from SS1 and
SS2 to GND. The ADP1823 charges CSS to 0.8 V through an
internal 90 kΩ resistor. The voltage on the soft start capacitor
while it is charging is
V_CSS
=
0.8
V _{⎜⎜⎛}1
−
t
e ^RCSS
⎟⎞
⎟
(59)
⎝
⎠
The soft start period ends when the voltage on the soft start pin
reaches 0.6 V. Substituting 0.6 V for VSS and solving for the
number of RC time constants:
0.6V
=
0.8
V _{⎜⎜⎛}1
−
t _SS
e ^{90 kΩ( CSS)}
⎟⎞
⎟
(60)
⎝
⎠
t _SS = 1.386 RC_SS
(61)
Because R = 90 kΩ :
C_SS = t _SS × 8 μF/sec
(62)
where tSS is the desired soft start time in seconds.
VOLTAGE TRACKING
The ADP1823 includes a tracking feature that prevents an
output voltage from exceeding a master voltage. This is
especially important when the ADP1823 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 ADP1823 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.
V_OUT
COMP
FB
R_TOP
R_BOT
ERROR
AMPLIFIER
TRK
0.6V
SS
MASTER
VOLTAGE
R_TRKT
DETAIL VIEW OF
ADP1823
R_TRKB
Figure 29. Voltage Tracking
COINCIDENT TRACKING
The most common application is coincident tracking, used in
core vs. I/O voltage sequencing and similar applications.
Coincident tracking limits the slave output voltage to be the
same as the master voltage until it reaches regulation. Connect
the slave TRK input to a resistor divider from the master voltage
that is the same as the divider used on the slave FB pin. This
forces the slave voltage to be the same as the master voltage.
For coincident tracking, use RTRKT = RTOP and RTRKB = RBOT,
where RTOP and RBOT are the values chosen in the Compensating
the Voltage Mode Buck Regulator section.
MASTER VOLTAGE
SLAVE VOLTAGE
TIME
Figure 30. Coincident Tracking
As the master voltage rises, the slave voltage rises identically.
Eventually, the slave voltage reaches its regulation voltage,
where the internal reference takes over the regulation while the
TRK input continues to increase and thus removes itself from
influencing the output voltage.
Rev. A | Page 22 of 32