LTC3616_15 Datasheet, PDF (20/28 Pages) Linear Technology – 6A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

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
V_OUT1
V_OUT2
TIME
(6a) Coincident Tracking
V_OUT1
V_OUT2
TIME
3616 F06
(6b) Ratiometric Tracking
Figure 6. Two Different Modes of Output Voltage Tracking
V_OUT2
V_OUT1
R4
V_FB2
R2
LTC3616
R4 R3
V_FB1
V_IN
R2 R2 _LTC3616
TRACK/SS2
R4 ≤ R3
TRACK/SS1
LTC3616 CHANNEL 2
SLAVE
LTC3616 CHANNEL 1
MASTER
3616 F07a
Figure 7a. Setup for Coincident Tracking
V_OUT2
V_OUT1
R1
V_FB2
R2
LTC3616
TRACK/SS2
LTC3616 CHANNEL 2
SLAVE
R5 R3 R1/R2 < R5/R6
V_FB1
R6 R4 _LTC3616
TRACK/SS1
V_IN
LTC3616 CHANNEL 1
MASTER
3616 F07b
Figure 7b. Setup for Ratiometric Tracking
3616fc
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