LTC2925 Linear Technology, LTC2925 Datasheet - Page 16

• Manufacturer Part Number: LTC2925
• Description: Multiple Power Supply Tracking Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC2925
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:
From Equation 3:
16
R
R
TB
TA
16 5
1 235
16 5
.
.
1V/DIV
k
.
k
V
1000
100
U
1 235
0 8
35 7
V s
.
.
V s
.
V
U
k
V
1 65
–
.
1 65
0 8
.
.
10ms/DIV
k
W
V
k
Figure 17. Supply Sequencing from Figure 18
– .
2 13
U
k
2925 F17a
MASTER
SLAVE2
SLAVE1
SLAVE3
3.3V V
3. Choose RTA to obtain the desired delay.
From Equation 4:
From Equation 5:
R
1.65k
R
3.48k
4.87k
R
TB1
TA1
R
R
IN
TA2
TA
TA
V
V
IN
IN
10k
10k
R
8.87k
R
154k
R
100k
8.66k
3.74k
R
R
TB2
ONB
ONA
TB3
TA3
0.1 F
– .
10
2 13
Figure 18. Supply Sequencing Example
0 8
ON
REMOTE
STATUS
FAULT
V
RAMPBUF
TRACK1
TRACK2
TRACK3
GND
CC
ms
.
k
SENSEP SENSEN
V
SCTMR
10ms/DIV
|| .
•
• .
0.015
C
0.41 F
100
1 32
SCTMR
1 65
LTC2925
V s
SDTMR
k
k
10
Si4412ADY
C
0.082 F
GATE
SDTMR
Q1
3 48
.
1 32
C
0.1 F
GATE
PGTMR
.
k
www.DataSheet4U.com
C
0.82 F
2925 F17b
RAMP
PGTMR
k
SD1
SD3
SD2
FB1
FB2
FB3
PGI
1V/DIV
MASTER
R
35.7k
R
41.2k
R
100k
2925 F18
RST
RUN/SS
FB = 1.235V
RUN/SS
FB = 0.8V
RUN/SS
FB = 0.8V
FA1
FA2
FA3
MONITOR
SUPPLY
DC/DC
DC/DC
DC/DC
16.5k
86.6k
R
88.7k
R
R
FB1
FB3
FB2
3.3V
OUT
3.3V
OUT
3.3V
OUT
IN
IN
IN
1.8V
SLAVE1
2.5V
SLAVE2
1.5V
SLAVE3
2925f