LTC2925
APPLICATIONS INFORMATION
1V/DIV
MASTER
SLAVE2
SLAVE1
SLAVE3
1V/DIV
10ms/DIV
10ms/DIV
2925 F15
Figure 15. Offset Tracking from Figure 16
Offset Tracking Example
Converting the circuit in the coincident tracking example
to the offset tracking shown in Figure 15 is relatively
simple. Here the 1.8V slave 1 supply ramps up 1V below
the master. The ramp rate remains the same (100V/s), so
there are no changes necessary to steps 1 and 2 of the
3-step design procedure. Only step 3 must be considered.
Be sure to verify that the chosen voltage offsets will allow
the slave supplies to ramp up completely. In this example,
if the voltage offset were 2V, the slave supply would only
ramp up to 3.3V – 2V = 1.3V.
3. Choose RTA to obtain the desired delay.
First, convert the desired voltage offset, VOS, to a delay,
tD, using the ramp rate:
tD
=
VOS
SS
=
1V
100V/s
=
10ms
(6)
From Equation 4:
RTA″
=
0.8V • 16.5k
1ms • 100V/s
=
13.2k
From Equation 5:
RTA = 13.1k || 13.2k ≈ 6.65k
3.3V VIN
0.1μF
0.015Ω
Q1
Si4412ADY
CGATE
10Ω 0.1μF
MASTER
RONB
138k
RONA
100k
VIN
VCC SENSEP SENSEN GATE
ON
REMOTE
RAMP
PGI
SD1
FB1
SUPPLY
MONITOR
RST
3.3V
RUN/SS
IN
DC/DC
FB = 1.235V OUT
10k
VIN
10k
RTB1
16.5k
RTA1
6.65k
RTA2
31.6k
RTB2
88.7k
RTB3
86.6k
RTA3
31.6k
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 F16
Figure 16. Offset Tracking Example
2925fc
16