LTC2925 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2925' PDF : 26 Pages View PDF
APPLICATIONS INFORMATION
LTC2925
1V/DIV
MASTER
SLAVE2
SLAVE1
SLAVE3
1V/DIV
10ms/DIV
10ms/DIV
2925 F17
Figure 17. Supply Sequencing from Figure 18
Supply Sequencing Example
In Figure 17, the three slave supplies are sequenced in-
stead of tracking. As in the coincident tracking example,
the 3.3V master supply ramps up at 100V/s through an
external FET, so step 1 remains the same. The 1.8V slave
1 supply ramps up at 1000V/s beginning 10ms after the
master signal starts to ramp up. The 2.5V slave 2 supply
ramps up at 1000V/s beginning 20ms after the master
signal begins to ramp up. The 1.5V slave 3 supply ramps
up at 1000V/s beginning 25ms after the master signal
begins to ramp up. Note that not every combination of
ramp rates and delays is possible. Small delays and large
ratios of slave ramp rate to master ramp rate may result
in solutions that require negative resistors. In such cases,
either the delay must be increased or the ratio of slave
ramp rate to master ramp rate must be reduced. In this
example, solving for the slave 1 supply yields:
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
1000V/s
≈
1.65k
From Equation 3:
RTA ′
=
1.235V
+
0.8V
1.235V
–
0.8V
≈ –2.13k
16.5k 35.7k 1.65k
3. Choose RTA to obtain the desired delay.
From Equation 4:
RTA″
=
0.8V • 1.65k
10ms • 100V/s
=
1.32k
From Equation 5:
RTA = –2.13k || 1.32k ≈ 3.48k
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
1.65k
RTA1
3.48k
RTA2
4.87k
RTB2
8.87k
RTB3
8.66k
RTA3
3.74k
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 F18
Figure 18. Supply Sequencing Example
2925fc
17
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