lt3740 Linear Technology Corporation, lt3740 Datasheet - Page 7

• Manufacturer Part Number: lt3740
• Description: Wide Operating Range, Valley Mode, No Rsense Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

OPERATIO
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 stabiliz-
ing ramp, and the resulting sum is fed into the PWM
comparator A1. When this voltage goes below the voltage
at V
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
voltage on the V
V
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
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
pin. The voltage supply for the bottom MOSFET drive is
provided through the BGDP pin. For V
BGDP should be connected to BIAS to get enough drive
bias. For V
directly to V
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.
FB
C
with the reference, which is determined by the lower
pin, the switch logic is reset, which turns off the
IN
+
IN
and SN
higher than 7V, BGDP can be connected
to reduce power loss.
C
U
pin by comparing the feedback signal
–
. The g
m
error amplifier adjusts the
IN
lower than 7V,
IN
for the
IN
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. Dur-
ing 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
transistor. If the SHDN pin is slowly ramped up, the V
will track it up proportionally. As the V
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
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
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.
C
pin is clamped to SHDN pin through a PNP
C
C
pin voltage is
LT3740
signal will be
C
3740fa
7
pin
C