Datasheet

LTC2923
13
2923fa
APPLICATIO S I FOR ATIO
WUUU
Ratiometric Tracking Example
Figure 13. Ratiometric Tracking Example
This example converts the coincident tracking example to
the ratiometric tracking profile shown in Figure 12, using
two 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 consid-
ered. In this example, the ramp rate of the 1.8V slave 1
supply ramps up at 600V/s and the 2.5V slave 2 supply
ramps up at 850V/s. Always verify that the chosen ramp
rate will allow the supplies to ramp-up completely before
RAMPBUF reaches V
CC
. If the 1.8V supply were to ramp-
up at 500V/s it would only reach 1.65V because the
RAMPBUF signal would reach its final value of V
CC
= 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:
Rk
Vs
Vs
k
TB
= Ω≈Ω16 5
1000
600
27 4.•
/
/
.
From Equation 3:
R
V
V
k
V
k
V
k
k
TA
′
=
Ω
+
ΩΩ
= Ω
08
1 235
16 5
1 235
35 7
08
27 5
10
.
.
.
.
.
–
.
.
Step 3 is unnecessary because there is no delay, so
R
TA
= R
TA
′.
1ms/DIV 2923 F12b
1V/DIV
1ms/DIV
2923 F12a
1V/DIV
Figure 12. Ratiometric Tracking (from Figure 13)
SLAVE2
SLAVE1
C
GATE
10nF
0.1µF
V
CC
R
ONB
138k
3.3V
R
TB1
27.4k
R
TB2
1M
R
FA1
35.7k
1.8V
SLAVE1
R
FB1
16.5k
R
TA2
383k
R
TA1
10k
R
ONA
100k
ON FB1
GATE
LTC2923
GND
2923 F13
RAMP
R
FA2
412k
2.5V
SLAVE2
R
FB2
887k
RAMPBUF
TRACK1
TRACK2
FB2
DC/DC
IN
3.3V
3.3V
FB = 1.235V OUT
DC/DC
IN
FB = 0.8V OUT