LTC2927 Linear Technology, LTC2927 Datasheet - Page 10

• Manufacturer Part Number: LTC2927
• Description: Single Power Supply Tracking Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC2927
Ratiometric Tracking Example
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
to ramp-up at 50V/s it would only reach 1.65V because
the RAMPBUF signal would reach its fi nal value of V
3.3V before the slave supply reached 1.8V.
2. Solve for the pair of resistors that provide the desired
Step 3 is unnecessary because there is no delay, so
R
10
TA
slave supply behavior, assuming no delay.
From Equation 2:
From Equation 3:
R
R
= R
TB
TA
′ =
TA
=
’
16 5
16 5
1 235
.
.
1V/DIV
.
k
Ω
k
Ω
V
•
U
100
+
60
35 7
1 235
V s
.
V s
0 8
.
/
.
/
U
k
V
Ω
V
=
CC
−
27 4
. If the 1.8V supply were
27 4
.
10ms/DIV
0 8
k
W
.
.
Ω
Figure 11. Ratiometric Tracking (from Figure 12)
k
V
Ω
≈
10
k
U
Ω
CC
=
MASTER
SLAVE2
SLAVE1
EARLY V
EARLY
3.3V
3.3V
IN
R
R
27.4k
138k
100k
R
R
383k
R
R
ONB
ONA
10k
TB1
TA1
TB2
1M
TA2
Figure 12. Ratiometric Tracking Example
ON
RAMPBUF
TRACK
ON
RAMPBUF
TRACK
10ms/DIV
LTC2927
LTC2927
GND
GND
V
V
CC
CC
RAMP
RAMP
SDO
SDO
2927 F12
FB
FB
www.DataSheet4U.com
R
35.7k
R
412k
FA1
FA2
0.1μF
0.1μF
0.1μF
RUN/SS
FB = 1.235V
RUN/SS
FB = 0.8V
2927 F11
DC/DC
DC/DC
16.5k
887k
1V/DIV
3.3V
R
3.3V
R
IN
IN
FB1
FB2
OUT
OUT
MASTER
3.3V
SLAVE1
1.8V
SLAVE2
2.5V
2927fb