LTC3546 Datasheet, PDF (19/28 Pages) Linear Technology – Dual Synchronous, 3A/1A or 2A/2A Confi gurable Step-Down DC/DC Regulator

APPLICATIONS INFORMATION
The LTC3546 can also track an external voltage during
startup by using an external voltage divider to the TRACK/SS
pin, and also insuring that the ramp rate on the TRACK/SS
pin is slower than the internal 1.2ms ramp rate.
As indicated in Figure 3, a resistor divider from an external
voltage can be connected to the TRACK/SS pin to allow
the start-up of V_OUT to ratiometrically track an external
voltage V_X.
For V_TRACKSS < 0.6V
V_OUT
=
V_X
•
R2X
R1X + R2X
•
R1+ R2
R2
V_X
V_OUT
R1X
R1
LTC3546
V_TRACK/SS V_FB
R2X
R2 3546 F03
LTC3546
V_X
V_OUT
TIME
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Figure 4. Coincident Tracking
V_X
V_OUT
Figure 3. Tracking External Voltages
Coincident tracking is where V_OUT = V_X during startup. To
implement coincident tracking R1X in Figure 3 is set to the
same value as R1 and R2X to the value of R2. Coincident
tracking is illustrated in Figure 4. The voltage at TRACK/SS
when V_X is at its final value should be ≥0.8V (sufficient
margin above the 0.6V reference voltage).
Ratiometric tracking is where V_OUT ≠ V_X during startup
but rather, it is set to some fractional value of V_X. To
implement ratiometric tracking (as illustrated in Figure
5), set R1X in Figure 3 to the same value as R1 and R2X
to the value of R2 + ΔR. The ΔR added to R2 should be
sufficient so that the TRACK/SS voltage is ≥ 0.8V when
V_X is at its final value.
The internal 1.15μA pull up current on TRACK/SS can cause
a tracking error at V_OUT when using a resistor divider on
TRACK/SS. For example, if a 59k resistor is chosen for
R2X, the R2X current will be about 10μA (0.6V/59k). In this
case, the 1.15μA internal current source will cause about
11% (1.15μA/10μA) tracking error, which is about 66mV
referred to V_FB. This is acceptable for most applications.
If a better tracking accuracy is required, the value of R2X
can be reduced or the 1.15μA current can be taken into
account in the equations.
TIME
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Figure 5. Ratiometric Tracking
Table 2 summarizes the different states in which the
TRACK/SS can be used.
Table 2. The States of the TRACK/SS Pin
TRACK/SS PIN
RESULT
Capacitor to Ground
External Soft-Start
Floating
Internal Soft-Start
Resistor Divider
V_OUT Tracking an External Voltage V_X
Regardless of the mode implemented, the TRACK/SS pin
should never be pulled high externally as this will result
in excessive current during shutdown.
The LTC3546 can smoothly handle starting up into a pre-
biased output. The tracking function will pick up from the
pre-biased voltage and ramp the output up from there.
Mode Selection
The SYNC/MODE pin is a multipurpose pin which provides
mode selection and frequency synchronization. Connecting
this pin to V_IN enables Burst Mode operation for both regula-
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