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LTC2925 View Datasheet(PDF) - Linear Technology

Part Name
Description
MFG CO.
'LTC2925' PDF : 26 Pages View PDF
LTC2925
APPLICATIONS INFORMATION
1V/DIV
MASTER
SLAVE2
SLAVE1
SLAVE3
1V/DIV
10ms/DIV
10ms/DIV
2925 F11
Figure 11. Coincident Tracking from Figure 12
Coincident Tracking Example
A typical four supply application is shown in Figure 12.
The master signal is a 3.3V module. The slave 1 supply
is a 1.8V switching power supply, the slave 2 supply is a
2.5V switching power supply, and the slave 3 supply is
a 1.5V supply. All three slave supplies track coincidently
with the 3.3V supply that is controlled with an external
FET. The ramp rate of the supplies is 100V/s. The 3-step
design procedure detailed previously can be used to
determine component values. Only the slave 1 supply
is considered here as the procedure is the same for the
other supplies.
1. Set the ramp rate of the master signal.
From Equation 1:
CGATE
=
10μA
100V/s
=
0.1μF
2. Solve for the pair of resistors that provide the desired
slave supply behavior, assuming no delay.
From Equation 2:
RTB
=
16.5k
100V/s
100V/s
=
16.5k
From Equation 3:
RTA
=
1.235V
+
0.8V
1.235V
0.8V
13k
16.5k 35.7k 16.5k
3. Choose RTA to obtain the desired delay.
Since no delay is desired, RTA = RTA´.
14
In this example, all supplies remain low while the ON pin
is held below 1.23V. When the ON pin rises above 1.23V,
10μA pulls up CGATE and the gate of the FET at 100V/s.
As the gate of the FET rises, the source follows and pulls
up the output to 3.3V at 100V/s. This output serves as
the master signal and is buffered from the RAMP pin
to the RAMPBUF pin. As this output and the RAMPBUF
pin rise, the current from the TRACKx pins is reduced.
Consequently, the voltages at the slave supplies’ outputs
increase, and the slave supplies track the master supply.
When the ON pin is again pulled below 1.23V, 10μA will
pull down CGATE and the gate of the FET at 100V/s. If the
loads on the outputs are sufficient, all outputs will track
down coincidently at 100V/s.
3.3V VIN
0.1μF
0.015Ω
Q1
Si4412ADY
CGATE
10Ω 0.1μF
MASTER
RONB
138k
RONA
100k
VIN
10k
VIN
10k
RTB1
16.5k
RTA1
13k
RTA2
41.2k
RTB2
88.7k
RTB3
86.6k
RTA3
100k
VCC SENSEP SENSEN GATE
ON
REMOTE
RAMP
PGI
SD1
FB1
SUPPLY
MONITOR
RST
3.3V
RUN/SS
IN
DC/DC
FB = 1.235V OUT
STATUS
LTC2925
SD2
FAULT
FB2
RAMPBUF
TRACK1
TRACK2
SD3
FB3
TRACK3
GND SCTMR
SDTMR
PGTMR
CSCTMR
0.47μF
CSDTMR
0.082μF
CPGTMR
0.82μF
RFA1 RFB1
35.7k 16.5k
3.3V
RUN/SS
IN
DC/DC
FB = 0.8V OUT
RFA2 RFB2
41.2k 88.7k 3.3V
RUN/SS
IN
DC/DC
FB = 0.8V OUT
RFA3
100k
RFB3
86.6k
1.8V
SLAVE1
2.5V
SLAVE2
1.5V
SLAVE3
2925 F12
Figure 12. Coincident Tracking Example
2925fc
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