LTC3546 Datasheet, PDF (13/28 Pages) Linear Technology – Dual Synchronous, 3A/1A or 2A/2A Confi gurable Step-Down DC/DC Regulator

LTC3546
OPERATION
The output capacitor and the inductor supply the power
to the load until I_TH exceeds 0.31V, turning on the switch
and the main control loop which starts another cycle.
The Burst Mode peak inductor current can be set externally
via the BMC pin. When this pin is set somewhere between
0V to 0.6V, the voltage on this pin controls the Burst Mode
clamp level. When the BMC pin is pulled to V_IN, an internal
Burst Mode clamp level is used.
For lower output voltage ripple at low currents, pulse-
skipping mode can be used. In this mode, the LTC3546
continues to switch at constant frequency down to very low
currents, where it will eventually begin skipping pulses.
Finally, in forced continuous mode, the inductor current
is constantly cycled which creates a fixed output voltage
ripple at all output current levels. This feature is desirable in
telecommunications since the noise is a constant frequency
and is thus easy to filter out. Another advantage of this mode
is that the regulator is capable of both sourcing current into
a load and sinking some current from the output. In forced
continuous operation, an overvoltage comparator monitors
the V_FB pin and decreases the current limit whenever an
overvoltage condition is detected (V_FB > 0.63V).
The SYNC/MODE pin selects what mode the LTC3546 is in.
The SYNC/MODE pin sets the mode for both regulators.
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases to 100% which
is the dropout condition. In the dropout condition, the
PMOS switch is turned on continuously with the output
voltage being equal to the input voltage minus the volt-
age drops across the internal P-channel MOSFETs and
inductors.
Low Supply Operation
The LTC3546 incorporates an undervoltage lockout circuit
which shuts down the part when the input voltage drops
below about 2.14V to prevent unstable operation.
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