Datasheet

LT3800
13
3800fc
APPLICATIONS INFORMATION
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 LT3800
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
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 LT3800 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 a LT3800 converter can provide full power regardless
of required duty cycle.
Shutdown
The LT3800 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 LT3800 operational status is primarily controlled by
a UVLO circuit on the V
CC
regulator pin. When the IC is
enabled via the SHDN pin, only the V
CC
regulator is enabled.
Switching remains disabled until the UVLO threshold is
achieved at the V
CC
pin, when the remainder of the IC is
enabled and switching commences.
Because an LT3800 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
IN
supply to ground.
Setting the divider output to 1.35V when that supply is at
an adequate voltage prevents an LT3800 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.
Current Limit Foldback Circuit for
Applications That Use Soft-Start
Alternative Current Limit Foldback Circuit for Applications That
Have Soft-Start Disabled
V
C
V
OUT
1N4148
3800 AI09
47k
V
C
V
OUT
1N4148
1N4148
3800 AI10
39k
27k