LTC3708 LINER [Linear Technology], LTC3708 Datasheet - Page 20

• Manufacturer Part Number: LTC3708
• Description: Fast 2-Phase, No RSENSE Buck Controller with Output Tracking
• Manufacturer: LINER [Linear Technology]

LTC3708
APPLICATIONS INFORMATION
D1. D2 and D3 will therefore conduct the same current
and offer tight matching between V
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
0.6V. Although this error is minimized by the exponential
20
TRACK2
0.6V
V
FB2
Figure 10. Equivalent Input Circuit of
Error Amplifi er of Channel 2
D1
TRACK2
V
OUT1
PIN
TO
(8a) Coincident Tracking
I
D2
(9a) Coincident Tracking Setup
R3
R4
I
D3
TIME
+
–
R1
R2
EA2
Figure 8. Two Different Modes of Output Voltage Tracking
Figure 9. Setup for Coincident and Ratiometric Tracking
FB2
TO
V
PIN
FB1
FB2
slightly lower than
and the internal
V
PIN
FB2
TO
⎛
⎝ ⎜
3708 F10
R
R
1
2
V
V
OUT1
OUT2
=
R3
R4
V
OUT
0 6
.
V
OUT2
1
−
1
,
R
R
3
4
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
=
V
TRACK2
V
OUT
0 6
OUT1
.
PIN
(9b) Ratiometric Tracking Setup
TO
2
−
1
(8b) Ratiometric Tracking
⎞
⎠ ⎟
R1
R2
TO
V
PIN
FB1
TIME
V
PIN
FB2
TO
R3
R4
3708 F09
3708 F08
V
OUT2
V
V
OUT1
OUT2
3708fb