Datasheet

LTC2923
14
2923fa
Offset Tracking Example
APPLICATIO S I FOR ATIO
WUUU
Figure 15. Offset Tracking Example
Converting the circuit in the coincident tracking example
to the offset tracking shown in Figure 14 is relatively
simple. Here the 1.8V slave 1 supply ramps up 1V below
the master. The ramp rate remains the same (1000V/s), so
there are no changes necessary to steps 1 and 2 of the
3-step design procedure. Only step 3 must be considered.
Be sure to verify that the chosen voltage offsets will allow
the slave supplies to ramp up completely. In this example,
if the voltage offset were 2V, the slave supply would only
ramp up to 3.3V – 2V = 1.3V.
3. Choose R
TA
to obtain the desired delay.
First, convert the desired voltage offset, V
OS
, to a delay,
t
D
, using the ramp rate:
t
V
S
V
Vs
ms
D
OS
S
== =
1
1000
1
/
(6)
From Equation 4:
R
Vk
ms V s
k
TA
′′
=
Ω
= Ω
0 8 16 5
1 1000
13 2
.•.
•/
.
From Equation 5:
R
TA
= 13.1kΩ||13.2kΩ ≈ 6.65kΩ
1ms/DIV 2923 F14b
1V/DIV
1ms/DIV
2923 F14a
1V/DIV
Figure 14. Offset Tracking (from Figure 15)
SLAVE2
SLAVE1
MASTER
Q1
C
GATE
10nF
V
CC
R
ONB
138k
3.3V
R
TB1
16.5k
R
TB2
887k
R
FA1
35.7k
1.8V
SLAVE1
3.3V
MASTER
R
FB1
16.5k
R
TA2
316k
R
TA1
6.65k
R
ONA
100k
ON FB1
FB2
GATE
LTC2923
GND
2923 F15
RAMP
R
FA2
412k
2.5V
SLAVE2
R
FB2
887k
RAMPBUF
TRACK1
TRACK2
DC/DC
IN
3.3V
3.3V
FB = 1.235V OUT
DC/DC
IN
FB = 0.8V OUT
0.1µF
10Ω