APPLICATIONS INFORMATION
LTC2925
1V/DIV
SLAVE2
SLAVE1
SLAVE3
1V/DIV
10ms/DIV
10ms/DIV
2925 F13
Figure 13. Ratiometric Tracking from Figure 14
Ratiometric Tracking Example
This example converts the coincident tracking example to
the ratiometric tracking profile shown in Figure 13, using
three supplies without an external FET. The ramp rate of
the master signal remains unchanged (step 1) and there
is no delay in ratiometric tracking (step 3), so only the
result of step 2 in the 3-step design procedure needs to
be considered. In this example, the ramp rate of the 1.8V
slave 1 supply ramps up at 60V/s, the 2.5V slave 2 supply
ramps up at 85V/s, and the 1.5V slave 3 supply ramps up
at 50V/s. Always verify that the chosen ramp rate will al-
low the supplies to ramp-up completely before RAMPBUF
reaches VCC. If the 1.8V supply were to ramp-up at 50V/s
it would only reach 1.65V because the RAMPBUF signal
would reach its final value of VCC = 3.3V before the slave
supply reached 1.8V.
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
60V/s
≈
27.4k
From Equation 3:
RTA ′
=
1.235V
+
0.8V
1.235V
–
0.8V
≈ 10k
16.5k 35.7k 27.5k
Step 3 is unnecessary because there is no delay, so
RTA = RTA´.
3.3V VIN
0.1μF
CGATE
0.1μF
RONB
138k
RONA
100k
VIN
10k
VIN
10k
RTB1
27.4k
RTA1
10k
RTA2
38.3k
RTB2
100k
RTB3
174k
RTA3
63.4k
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
0.41μF
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 F14
Figure 14. Ratiometric Tracking Example
2925fc
15