LTC3879_15 Datasheet, PDF (17/28 Pages) Linear Technology – Fast, Wide Operating Range No RSENSE Step-Down Controller

APPLICATIONS INFORMATION
The LTC3879 is designed to start up into pre-biased outputs
with no reverse current. Both TG and BG outputs remain
low until the applied TRACK/SS voltage plus internal offset
exceeds the voltage on the V_FB pin. Once this condition
is exceeded, the switcher will start up normally and track
the TRACK/SS voltage until it exceeds 0.6V. Once switch-
ing starts, the mode of operation is determined by the
externally programmed MODE pin input. When TRACK/SS
exceeds 0.6V, the output regulates to the internal reference
value of 0.6V.
When the regulator is configured to track another supply,
the feedback voltage of the other supply is duplicated
by a resistor divider and applied to the TRACK/SS pin.
Therefore, the voltage ramp rate on this pin is determined
by the ramp rate of the other supply’s voltage. Note that
the small soft-start capacitor charging current is always
flowing, producing a small offset error. To minimize this
error, one can select the tracking resistive divider value to
be small enough to make this error negligible.
In order to track down another supply after the soft-start
phase expires, the LTC3879 must be configured for forced
continuous operation.
Output Voltage Tracking
The LTC3879 allows the user to program how its output
ramps up and down by means of the TRACK/SS pin.
Through this pin, the output can be set up to either co-
incidentally or ratiometrically track with another supply’s
output, as shown in Figure 7. In the following discussions,
V_MASTER refers to a master supply and V_OUT refers to the
LTC3879’s output as a slave supply. To implement the
coincident tracking in Figure 7, connect a resistor divider
to the V_MASTER, and connect its midpoint to the TRACK/SS
pin of the LTC3879. The track divider should be the same
as the LTC3879’s feedback divider. In this tracking mode,
V_{OUT_MASTER} must be higher than V_OUT. To implement
ratiometric tracking, the ratio of the resistor divider con-
nected to V_{OUT_MASTER} is determined by:
V_{OUT−MASTER = R3 + R4}
0.6V
R4
LTC3879
V_MASTER
V_OUT
TIME
3879 F07a
(7a) Coincident Tracking
V_MASTER
V_OUT
TIME
3879 F07b
(7b) Ratiometric Tracking
Figure 7. Two Different Modes of Output Voltage Tracking
V_{OUT_ MASTER}
TO
TRACK/SS
PIN
R1
TO
V_FB
PIN
R2
V_OUT
R1
R2
3879 F08a
(8a) Coincident Tracking Setup
V_{OUT_MASTER}
TO
TRACK/SS
PIN
R3
TO
V_FB
PIN
R4
V_OUT
R1
R2
3879 F08b
(8b) Ratiometric Tracking Setup
Figure 8. Setup for Coincident and Ratiometric Tracking
3879f
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