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LTC2925 View Datasheet(PDF) - Linear Technology

Part Name
Description
MFG CO.
'LTC2925' PDF : 26 Pages View PDF
LTC2925
APPLICATIONS INFORMATION
3-Step Design Procedure
The following 3-step design procedure allows one to choose
the TRACK resistors, RTAx and RTBx, and the gate capacitor,
CGATE, that give any of the tracking or sequencing profiles
shown in Figures 1 to 4. A basic four supply application
circuit is shown in Figure 10.
1. Set the ramp rate of the master signal.
Solve for the value of CGATE, the capacitor on the GATE
pin, based on the desired ramp rate (V/s) of the master
supply, SM:
CGATE
=
IGATE
SM
where IGATE
10μA
(1)
If the external FET has a gate capacitance comparable to
CGATE, then the external capacitor’s value should be reduced
to compensate for the FET’s gate capacitance.
If no external FET is used, tie the GATE and RAMP pins
together, connect SENSEN and SENSEP to VCC, and con-
nect SCTMR to GND.
2. Solve for the pair of resistors that provide the desired
ramp rate of the slave supply, assuming no delay.
RSENSE
Q1
VIN
0.1μF
10Ω
CGATE
RONB
138k
RONA
100k
VIN
VCC SENSEP SENSEN GATE
ON
REMOTE
RAMP
PGI
SD1
FB1
MASTER
SUPPLY
MONITOR
RST
VIN
RUN/SS IN
DC/DC
FB
OUT
10k
VIN
10k
RTB1
RTA1
RTB2
RTA2 RTB3
RTA3
STATUS
FAULT
RAMPBUF
TRACK1
TRACK2
TRACK3
GND SCTMR
LTC2925
SDTMR
SD2
FB2
SD3
FB3
PGTMR
RFA1
RFB1
VIN
RUN/SS IN
DC/DC
FB
OUT
RFA2
RFB2
VIN
RUN/SS IN
DC/DC
FB
OUT
RFB3
RFA3
2925 F10
SLAVE1
SLAVE2
SLAVE3
Figure 10. Four Supply Application
Choose a ramp rate for the slave supply, SS. If the slave
supply ramps up coincident with the master supply or
with a fixed voltage offset, then the ramp rate equals the
master supply’s ramp rate. Be sure to use a fast enough
ramp rate for the slave supply so that it will finish ramping
before the master supply has reached its final supply value.
If not, the slave supply will be held below the intended
regulation value by the master supply. Use the following
formulas to determine the resistor values for the desired
ramp rate, where RFB and RFA are the feedback resistors in
the slave supply and VFB is the feedback reference voltage
of the slave supply:
RTB
=
RFB
SM
SS
(2)
RTA ′ =
VFB
+
VTRACK
VFB VTRACK
(3)
RFB RFA RTB
where VTRACK ≈ 0.8V.
Note that large ratios of slave ramp rate to master ramp
rate, SS/SM, may result in negative values for RTA´. If a
sufficiently large delay is used in step 3, RTA will be posi-
tive, otherwise SS/SM must be reduced.
3. Choose RTA to obtain the desired delay.
If no delay is required, such as in coincident and ratio-
metric tracking, then simply set RTA = RTA´. If a delay
is desired, as in offset tracking and supply sequencing,
calculate RTA´´ to determine the value of RTA where tD is
the desired delay.
RTA ′′
=
VTRACK • RTB
tD • SM
(4)
RTA = RTA ||RTA ′′
(5)
the parallel combination of RTA´ and RTA´´.
As noted in step 2, small delays and large ratios of slave
ramp rate to master ramp rate (usually only seen in se-
quencing) may result in solutions with negative values for
RTA. In such cases, either the delay must be increased or
the ratio of slave ramp rate to master ramp rate must be
reduced.
2925fc
13
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