LTC3709 Datasheet, PDF (18/24 Pages) Linear Technology – Fast 2-Phase, No RSENSE Synchronous DC/DC Controller with Tracking/Sequencing

LTC3709
APPLICATIO S I FOR ATIO
V_CC
3.3V OR 5V
D1
V_IN
RUN/SS
R_SS*
C_SS
(5a)
R_SS*
D2* ^RUN/SS
2N7002
C_SS
3709 F05
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
(5b)
Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated
Output Voltage Tracking
The feedback voltage, V_FB, will follow the TRACK pin
voltage when the TRACK pin voltage is less than the
reference voltage, V_REF (0.6V). When the TRACK pin
voltage is greater than V_REF, the feedback voltage will
servo to V_REF. When selecting components for the TRACK
pin, ensure the final steady-state voltage on the TRACK pin
is greater than V_REF at the end of the tracking interval.
The LTC3709 allows the user to set up start-up sequenc-
ing among different supplies in either coincident tracking
or ratiometric tracking as shown in Figure 6. To implement
the coincident tracking, connect an extra resistor divider
to the output of supply 1. This resistor divider is selected
to be the same as the divider across supply 2’s output. The
TRACK pin of supply 2 is connected to this extra resistor
divider. For the ratiometric tracking, simply connect the
TRACK pin of supply 2 to the V_FB pin of supply 1. Figure 7
shows this implementation. Note that in the coincident
tracking, output voltage of supply 1 has to be set higher
than output voltage of supply 2.
Note that since the shutdown trip point varies from part to
part, the “slave” part’s RUN/SS pin will need to be con-
nected to V_CC. This eliminates the possibility that different
LTC3709s may shut down at different times.
If output sequencing is not needed, connect the TRACK
pins to V_CC. Do Not Float these pins.
SUPPLY 1
V_FB
SUPPLY 2
V_{OUT1 LTC3709}
R1 R3
V_FB
TRACK
R2 R4
V_OUT2
R5
R6
3709 F07
R3
R4
=
R5
R6
V_OUT2 COINCIDENTLY TRACKS V_OUT1
R3
R4
=
R1
R2
RATIOMETRIC POWER UP
BETWEEN V_OUT1 AND V_OUT2
Figure 7. Setup for Coincident and Ratiometric Tracking
V_OUT1
V_OUT2
V_OUT1
V_OUT2
TIME
(6a) Coincident Tracking
TIME
3709 F06
(6b) Ratiometric Tracking
Figure 6. Two Different Forms of Output Voltage Sequencing
3709f
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