Datasheet

LTC3708
20
3708fb
APPLICATIONS INFORMATION
D1. D2 and D3 will therefore conduct the same current
and offer tight matching between V
FB2
and the internal
precision 0.6V reference. In the ratiometric mode, however,
TRACK2 equals 0.6V even at steady state. D1 will divert
part of the bias current and make V
FB2
slightly lower than
0.6V. Although this error is minimized by the exponential
I-V characteristic of the diodes, it does impose a fi nite
amount of output voltage deviation. Further, when channel
1’s output experiences dynamic excursions (under load
transient, for example), channel 2 will be affected as well.
For better output regulation, use the coincident tracking
mode instead of ratiometric.
The number of resistors in Figure 9a can be further reduced
with the scheme in Figure 11.
In a system that requires more than two tracked supplies,
multiple LTC3708s can be daisy-chained through the
TRACK1 pin. TRACK1 clamps channel 1’s reference in the
same manner TRACK2 clamps channel 2. To eliminate the
possibility of multiple LTC3708s coming on at different
times, only the master LTC3708’s RUN/SS pin should be
TIME
(8a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
TIME
3708 F08
(8b) Ratiometric Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
Figure 8. Two Different Modes of Output Voltage Tracking
R3 R1
R4 R2
R3
V
OUT2
R4
(9a) Coincident Tracking Setup
TO
V
FB1
PIN
TO
TRACK2
PIN
TO
V
FB2
PIN
V
OUT1
R1
R2
R3
V
OUT2
R4
3708 F09
(9b) Ratiometric Tracking Setup
TO
V
FB1
PIN
TO
TRACK2
PIN
TO
V
FB2
PIN
V
OUT1
Figure 9. Setup for Coincident and Ratiometric Tracking
R
R
V
R
R
V
OUT OUT
1
206
1
3
406
1
12
=−=−
⎛
⎝
⎜
⎞
⎠
⎟
.
,
.
–
+
II
D1
TRACK2
0.6V
V
FB2
D2
D3
3708 F10
EA2
Figure 10. Equivalent Input Circuit of
Error Amplifi er of Channel 2