LTC2923 Linear Technology, LTC2923 Datasheet - Page 13

• Manufacturer Part Number: LTC2923
• Description: Power Supply Tracking Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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
up at 500V/s it would only reach 1.65V because the
RAMPBUF signal would reach its final value of V
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:
R
TB
=
16 5
.
1V/DIV
k
Ω
U
•
CC
1000
600
. If the 1.8V supply were to ramp-
U
V s
V s
/
/
≈
27 4
W
1ms/DIV
.
Figure 12. Ratiometric Tracking (from Figure 13)
k
Ω
U
CC
= 3.3V
2923 F12a
SLAVE2
SLAVE1
Step 3 is unnecessary because there is no delay, so
R
3.3V
27.4k
R
R
TA
10k
TB1
TA1
From Equation 3:
= R
R
R
138k
R
100k
TA
R
R
383k
TA
ONB
ONA
TB2
TA2
1M
′ =
′.
Figure 13. Ratiometric Tracking Example
0.1µF
16 5
1 235
ON
RAMPBUF
TRACK1
TRACK2
V
.
CC
.
1ms/DIV
k
GATE
Ω
V
LTC2923
GND
+
35 7
1 235
C
10nF
GATE
0 8
.
RAMP
.
.
FB1
FB2
k
V
Ω
V
www.DataSheet4U.com
–
2923 F12b
R
412k
R
35.7k
2923 F13
FB = 1.235V OUT
FB = 0.8V
27 5
FA2
FA1
0 8
.
.
1V/DIV
DC/DC
DC/DC
16.5k
R
R
887k
3.3V
3.3V
LTC2923
k
V
FB1
FB2
IN
IN
Ω
OUT
=
10
k
13
Ω
1.8V
SLAVE1
2.5V
SLAVE2
2923fa