LT3740_15 Datasheet, PDF (7/20 Pages) Linear Technology – Wide Operating Range, Valley Mode, Synchronous Step-Down Controller

LT3740
OPERATION
The LT3740 is a constant-frequency, valley current mode
controller for DC/DC step-down converters. At the start
of each oscillator cycle, the switch logic is set, which
turns on the bottom MOSFET. After a 500ns blanking
time, the bottom MOSFET current is sensed and added
to a stabilizing ramp, and the resulting sum is fed into the
PWM comparator A1. When this voltage goes below the
voltage at V_C pin, the switch logic is reset, which turns off
the bottom MOSFET, and turns on the top MOSFET. The
top MOSFET remains on until the next oscillator cycle. The
bottom MOSFET current can be determined by sensing
the voltage between the drain and source of the MOSFET
using the bottom MOSFET on-resistance, or by sensing
the voltage drop across a resistor between the source of
the bottom MOSFET and ground. The two current sensing
pins are SN⁺ and SN^–. The g_m error amplifier adjusts the
voltage on the V_C pin by comparing the feedback signal
V_FB with the reference, which is determined by the lower
of the internal 0.8V reference and the voltage at the XREF
pin. If the error amplifier’s output increases, more current
is delivered to the output; if it decreases, less current is
delivered.
The LT3740 features an open collector PGOOD signal. When
the voltage at FB pin is less than 720mV, the PGOOD output
is pulled low by a NPN transistor. The 720mV threshold is
independent of the voltage on XREF pin.
The small internal step-up converter provides a BIAS
voltage about 7V higher than the input voltage V_IN for the
drive of the top MOSFET. This enables the LT3740 to work
from an input voltage as low as 2.2V. The controller starts
operation when the BIAS pin is about 7V higher than V_IN
pin. The voltage supply for the bottom MOSFET drive is
provided through the BGDP pin. For V_IN lower than 7V,
BGDP should be connected to BIAS to get enough drive
bias. For V_IN higher than 7V, BGDP can be connected
directly to V_IN to reduce power loss.
Grounding the SHDN pin turns both the internal step-up
converter and the controller off. The SHDN pin can also
be used to implement an optional soft-start function.
Start-Up and Shutdown
During normal operation, when the feedback voltage is
above 720mV, the LT3740 operates in forced continuous
mode. When the feedback voltage is below 720mV, either
during the start-up or because an external reference is
applied, a zero current detect comparator is enabled to
monitor the on-state bottom MOSFET current. When the
current reaches zero, both the top and bottom MOSFETs
are turned off, resulting in discontinuous operation. During
the time that both top and bottom MOSFETs are off, no
current signal is fed into the LT3740. Only the stabilizing
ramp is fed into the PWM comparator to decide the next
turn on of the top MOSFET.
The LT3740 uses the SHDN pin to implement one of the
two different startup schemes. As shown in the block
diagram, the V_C pin is clamped to SHDN pin through a
PNP transistor. If the SHDN pin is slowly ramped up, the
V_C pin will track it up proportionally. As the V_C pin voltage
is compared to the current signal at comparator A1, this
will, in turn, slowly ramp up the switching current.
The tracking capability built into XREF can be used to
implement another startup scheme. If less than 0.8V is
applied to XREF, the LT3740 will use this voltage as the
reference for regulation. Slowly ramping up the voltage
at XREF forces the output to increase slowly, which limits
the start-up current, as shown in Typical Performance
Characteristics.
A sharp SHDN signal is recommended to shut down the
LT3740. If SHDN slowly ramps down, the V_C signal will
be dragged low for a considerable period of time before
SHDN reaches its turn-off threshold. During this period
of time, the output voltage could still be in regulation and
the circuit operates in forced continuous mode. A low
V_C voltage will result in large bottom MOSFET on-time,
which may cause a reverse inductor current that pumps
the energy from the output to the input. If there is another
supply at the output or the output has a big capacitor, the
input voltage could overshoot, and may cause overvoltage
damage to certain devices.
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