lt3845ife-trpbf Linear Technology Corporation, lt3845ife-trpbf Datasheet - Page 12

• Manufacturer Part Number: lt3845ife-trpbf
• Description: High Voltage Synchronous Current Mode Step-down Controller With Adjustable Operating Frequency
• Manufacturer: Linear Technology Corporation

LT3845
at start-up the soft-start circuit will discharge the V
voltage below the DC control voltage equivalent to zero
inductor current. This will reduce the input supply inrush
current. The soft-start circuit is disabled once the C
voltage has been charged to 200mV above the internal
reference of 1.231V.
During a V
C
In normal operation the C
diode above the V
C
retrigger a soft-start. If any of the above UVLO conditions
occur, the C
current source. There is a diode worth of voltage headroom
to ride through the fault before the C
its active region and the soft-start function is enabled.
Also, since the C
the V
age is pulled low because the V
the short has been removed the V
recover. The soft-start circuit takes control of the output
voltage slew rate once the V
the slowly ramping C
voltage overshoot during a short circuit recovery.
Adaptive Nonoverlap (NOL) Output Stage
The FET driver output stages implement adaptive nonover-
lap control. This feature maintains a constant dead time,
preventing shoot-through switch currents, independent
of the type, size or operating conditions of the external
switch elements.
Each of the two switch drivers contains a NOL control
circuit, which monitors the output gate drive signal of the
other switch driver. The NOL control circuits interrupt the
“turn on” command to their associated switch driver until
the other switch gate is fully discharged.
Antislope Compensation
Most current mode switching controllers use slope com-
pensation to prevent current mode instability. The LT3845
is no exception. A slope-compensation circuit imposes an
artifi cial ramp on the sensed current to increase the rising
slope as duty cycle increases. Unfortunately, this additional
APPLICATIONS INFORMATION
12
SS
SS
pin voltage is discharged with a 50μA current source.
capacitor is relevant to how long of a fault event will
FB
pin voltage, during a short circuit the C
IN
SS
UVLO, V
pin voltage will be discharged with a 50μA
SS
FB
pin voltage is clamped to a diode above
pin voltage. Therefore, the value of the
SS
CC
UVLO or SHDN UVLO event, the
pin voltage, reducing the output
SS
FB
pin voltage is clamped to a
FB
pin voltage has exceeded
pin voltage is low. Once
FB
SS
pin voltage starts to
pin voltage enters
SS
pin volt-
SS
C
pin
pin
ramp corrupts the sensed current value, reducing the
achievable current limit value by the same amount as the
added ramp represents. As such, current limit is typically
reduced as duty cycles increase. The LT3845 contains
circuitry to eliminate the current limit reduction typically
associated with slope compensation. As the slope-com-
pensation ramp is added to the sensed current, a similar
ramp is added to the current limit threshold reference.
The end result is that current limit is not compromised,
so an LT3845 converter can provide full power regardless
of required duty cycle.
Shutdown
The LT3845 SHDN pin uses a bandgap generated reference
threshold of 1.35V. This precision threshold allows use of
the SHDN pin for both logic-level controlled applications
and analog monitoring applications such as power supply
sequencing.
The LT3845 operational status is primarily controlled by
a UVLO circuit on the V
enabled via the SHDN pin, only the V
Switching remains disabled until the UVLO threshold is
achieved at the V
enabled and switching commences.
Because an LT3845 controlled converter is a power
transfer device, a voltage that is lower than expected on
the input supply could require currents that exceed the
sourcing capabilities of that supply, causing the system
to lock up in an undervoltage state. Input supply start-up
protection can be achieved by enabling the SHDN pin
using a resistive divider from the V
Setting the divider output to 1.35V when that supply is at
an adequate voltage prevents an LT3845 converter from
drawing large currents until the input supply is able to
provide the required power. 120mV of input hysteresis
on the SHDN pin allows for almost 10% of input supply
droop before disabling the converter.
R
The current sense resistor, R
current of the supply (See Typical Application on front
page). Its value is chosen based on the maximum required
output load current. The LT3845 current sense amplifi er
SENSE
Selection
CC
pin, when the remainder of the IC is
CC
regulator pin. When the IC is
SENSE
, monitors the inductor
CC
IN
regulator is enabled.
supply to ground.
3845fc