LTC3708_15 Datasheet, PDF (20/32 Pages) Linear Technology – Fast 2-Phase, No RSENSE Buck Controller with Output Tracking

LTC3708
APPLICATIONS INFORMATION
V_OUT1
V_OUT1
V_OUT2
V_OUT2
TIME
(8a) Coincident Tracking
TIME
3708 F08
(8b) Ratiometric Tracking
Figure 8. Two Different Modes of Output Voltage Tracking
V_OUT1
TO
TRACK2
PIN
R3 R1
TO
TO
V_FB1 V_FB2
PIN
PIN
R4 R2
V_OUT2
R3
R4
V_OUT1
TO
TRACK2
PIN
R1
TO
TO
V_FB1 V_FB2
PIN
PIN
R2
V_OUT2
R3
R4
(9a) Coincident Tracking Setup
3708 F09
(9b) Ratiometric Tracking Setup
Figure 9. Setup for Coincident and Ratiometric Tracking
⎛ R1
⎝⎜ R2
=
V_OUT1
0.6
− 1, ^R3
R4
=
V_OUT2
0.6
⎞
− 1_⎠⎟
I
I
D1
TRACK2
0.6V
V_FB2
+
D2
EA2
–
D3
3708 F10
Figure 10. Equivalent Input Circuit of
Error Amplifier of Channel 2
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 finite
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
3708fb
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