Datasheet

LTC3616
20
3616fc
For more information www.linear.com/LTC3616
applicaTions inForMaTion
Output Voltage Tracking Input
If the DDR pin is not tied to SV
IN
, once V
TRACK/SS
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 for track
-
ing down/up the output voltage of another supply. If the
V
TRACK/SS
drops below 0.6V, the LTC3616 enters the
down tracking state and V
OUT
is referenced to the TRACK/
SS voltage. If the TRACK/SS pin drops below 0.2V, the
switching frequency is reduced to ensure that the mini
-
mum duty cycle limit does not prevent the output from
following the TRACK/SS pin. The run state will resume if
V
TRACK/SS
again exceeds 0.6V and V
OUT
is referenced to
the internal precision reference (see Figure 8).
Through the TRACK/SS pin, the output voltage can be set
up for either coincident or ratiometric tracking, as shown
in Figure 6.
To implement the coincident tracking behavior in Fig-
ure
6a, 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 to be the same as that of the
slave channel’s feedback divider (Figure 7a).
In this track-
ing mode, the master channel’s output must be set higher
than
slave channel’s output. To implement the ratiometric
tracking behavior in Figure 6b, different resistor divider
values must be used as specified in Figure 7b.
For coincident start-up, the voltage value at the TRACK/SS
pin for the slave channel needs to reach the final reference
value after the internal soft-start time (around 1ms). The
master start-up time needs to be adjusted with an external
capacitor and resistor to ensure this.
External Reference Input (DDR Mode)
If the DDR pin is tied to SV
IN
(DDR mode), the run state is
entered when V
TRACK/SS
exceeds 0.3V and tracking down
behavior is possible if the V
TRACK/SS
voltage is below 0.6V.
This allows TRACK/SS to be used as an external reference
between 0.3V and 0.6V if desired. During the run state in
DDR mode, the power good window moves in relation
Figure 7a. Setup for Coincident Tracking
Figure 7b. Setup for Ratiometric Tracking
V
FB2
R4
R2
R4
R2
R3
R2
R4 ≤ R3
V
OUT2
V
OUT1
LTC3616
TRACK/SS2
V
FB1
V
IN
LTC3616
LTC3616 CHANNEL 2
SLAVE
LTC3616 CHANNEL 1
MASTER
TRACK/SS1
3616 F07a
V
FB2
R1
R2
R5
R6
R3 R1/R2 < R5/R6
R4
V
OUT2
V
OUT1
LTC3616
TRACK/SS2
V
FB1
V
IN
3616 F07b
LTC3616
LTC3616 CHANNEL 2
SLAVE
LTC3616 CHANNEL 1
MASTER
TRACK/SS1
Figure 6. Two Different Modes of Output Voltage Tracking
TIME
(6b) Ratiometric Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
TIME
3616 F06
(6a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE