LTC3615 Datasheet, PDF (22/32 Pages) Linear Technology – Dual 4MHz, 3A Synchronous Step-Down DC/DC Converter

LTC3615
Applications Information
Output Voltage Tracking Input
If SRLIM is low, once V_TRACK/SS reaches or exceeds 0.6V
the run state is entered, and the MODE selection, power
good and current foldback circuits are enabled.
In the run state, the TRACK/SS pin can be used to track
down/up the output voltage of another supply. If the
V_TRACK/SS again drops below 0.6V, the LTC3615 enters
the down-tracking state and the V_OUT is referenced to
the TRACK/SS voltage. If V_TRACK/SS reaches 0.1V value
the switching frequency is reduced by 4x to ensure that
the minimum duty cycle limit does not prevent the out-
put from following the TRACK/SS pin. The run state will
resume if the V_TRACK/SS again exceeds 0.6V and the V_OUT
is referenced to the internal reference.
Through the TRACK/SS pin, the output voltage can be set
up to either coincidental or ratiometric tracking, as shown
in Figures 8 and 9.
To implement the coincidental tracking waveform in
Figure 8, connect an extra resistive divider to the output
of the master channel and connect its midpoint to the
TRACK/SS pin for the slave channel. The ratio of this
divider should be selected the same as that of the slave
channel’s feedback divider (Figure 10).
In this tracking mode, the master channel’s output must
be set higher than slave channel’s output. To implement
the ratiometric start-up in Figure 9, no extra divider is
needed; simply connect the TRACK/SS pin to the other
channel’s V_FB pin (Figure 12).
V_OUT1
V_OUT1
V_OUT2
V_OUT2
TIME
3615 F08
Figure 8. Coincident Start-Up Tracking
TIME
3615 F09
Figure 9. Ratiometric Start-Up Tracking
V_OUT1
R3
R1 _LTC3615
FB1
R4
R2
V_OUT2
TRACK/SS2
R5
FB2
R6
3615 F10
Figure 10. Set for Coincidentally
Tracking (R3 = R5, R4 = R6)
22
V_OUT1
R1
R2 _LTC3615
FB1
R3
V_OUT2
TRACK/SS2
R4
FB2
R5
3615 F11
Figure 11. Alternative Set-Up for Coincident
Start-Up Tracking (R1 = R3, R2 = R3 = R5)
V_OUT1
R1
R2
LTC3615
FB1
V_OUT2
R3
R4
TRACK/SS2
FB2
3615 F12
Figure 12. Set-Up for
Ratiometric Tracking
3615f