LTC2927
10
2927fb
This example converts the coincident tracking example to
the ratiometric tracking profi le shown in Figure 11. 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 and the 2.5V slave 2
supply ramps up at 85V/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 50V/s it would only reach 1.65V because
the RAMPBUF signal would reach its fi nal 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
=• =16 5
100
60
27 4.
/
/
.ΩΩ
From Equation 3:
R
V
V
k
V
k
V
k
k
TA
′
=
+−
≈
08
1 235
16 5
1 235
35 7
08
27 4
10
.
.
.
.
.
.
.ΩΩΩ
Ω
Step 3 is unnecessary because there is no delay, so
R
TA
= R
TA
’
Figure 11. Ratiometric Tracking (from Figure 12)
1V/DIV
10ms/DIV
SLAVE1
MASTER
SLAVE2
2927 F11
10ms/DIV
1V/DIV
MASTER
3.3V
R
ONB
138k
R
ONA
100k
ON RAMP
SLAVE1
1.8V
V
CC
3.3V
EARLY V
IN
3.3V
LTC2927
GND
SDO RUN/SS
FB
IN
DC/DC
FB = 1.235V OUT
RAMPBUF
TRACK
0.1μF
R
TB1
27.4k
R
FB1
16.5k
R
FA1
35.7k
R
TA1
10k
ON RAMP
SLAVE2
2.5V
2927 F12
V
CC
3.3V
EARLY
3.3V
LTC2927
GND
SDO RUN/SS
FB
IN
DC/DC
FB = 0.8V OUT
RAMPBUF
TRACK
0.1μF
0.1μF
R
TB2
1M
R
FB2
887k
R
FA2
412k
R
TA2
383k
Ratiometric Tracking Example
APPLICATIO S I FOR ATIO
WUU
U
Figure 12. Ratiometric Tracking Example
