lt3742 Linear Technology Corporation, lt3742 Datasheet - Page 8

• Manufacturer Part Number: lt3742
• Description: Dual, 2-phase Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

The LT3742 is a dual, constant frequency, current mode
DC/DC step-down controller. The two controllers in each
device share some common circuitry including protec-
tion circuitry, the internal bias supply, voltage reference,
master oscillator and the gate drive boost regulator. The
Block Diagram shows the shared common circuitry and
the independent circuitry for both DC/DC controllers.
Important protection features included in the LT3742 are
undervoltage lockout and thermal shutdown. When either
of these conditions exist, the gate drive bias regulator and
both DC/DC controllers are disabled and both RUN/SS pins
are discharged to ~0.7V to get ready for a new soft-start
cycle. Undervoltage lockout (UVLO) is programmed using
two external resistors. When the UVLO pin drops below
1.25V, a 3μA current sink is activated to provide program-
mable hysteresis for the UVLO function. A separate, less
accurate, internal undervoltage lockout will disable the
LT3742 when V
The gate drive boost regulator is enabled when all internal
fault conditions have been cleared. This regulator uses
both an internal NPN power switch and Schottky diode to
generate a voltage at the BIAS pin that is 7V higher than
the input voltage. Both DC/DC controllers are disabled until
the BIAS voltage has reached ~90% of its fi nal regulation
voltage. This ensures that suffi cient gate drive to fully
enhance the external MOSFETs is present before the driver
is allowed to turn on.
The master oscillator runs at 1MHz and clocks the gate
drive boost regulator at this frequency. The master oscilla-
tor also generates two 500kHz clocks, 180° out of phase,
for the DC/DC controllers.
LT3742
OPERATION
8
IN
is less than 3V.
A power good comparator pulls the PG pin low whenever
the FB pin is not within ±7.5% of the 800mV internal
reference voltage. PG is the open-collector output of an
NPN that is off when the FB pin is in regulation, allowing
an external resistor to pull the PG pin high. This power
good indication is valid only when the device is enabled
(RUN/SS is high) and V
The LT3742 enables each controller independently when its
RUN/SS pin is above ~0.5V and each controller generates
its own soft-start ramp. During start-up, the error amplifi er
compares the FB pin to the soft-start ramp instead of the
precision 800mV reference, which slowly raises the output
voltage until it reaches its resistor programmed regulation
point. Control of the inductor current is strictly maintained
until the output voltage is reached. The LT3742 is ideal
for applications where both DC/DC controllers need to
operate separately.
A pulse from the 500kHz oscillator sets the RS fl ip-fl op
and turns on the external N-channel MOSFET. Current in
the switch and the external inductor begins to increase.
When this current reaches a level determined by the control
voltage (V
turning off the MOSFET. The current in the inductor then
fl ows through the external Schottky diode and begins to
decrease. This cycle begins again at the next set pulse from
the slave oscillator. In this way, the voltage at the V
controls the current through the inductor to the output.
The internal error amplifi er regulates the output voltage
by continually adjusting the V
of the peak inductor current on a cycle-by-cycle basis is
managed by the current sense amplifi er. Because the induc-
tor current is constantly monitored, the devices inherently
provide excellent output short-circuit protection.
C
), the PWM comparator resets the fl ip-fl op,
IN
is 4V or greater.
C
pin voltage. Direct control
C
3742f
pin