LT1241 Series
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Undervoltage Lockout
The LT1241 series devices incorporate an undervoltage
lockout comparator which prevents the internal reference
circuitry and the output from starting up until the supply
voltage reaches the start-up threshold voltage. The quies-
cent current, below the start-up threshold, has been
reduced to less than 250µA (170µA typ.) to minimize the
power loss due to the bleed resistor used for start-up in
off-line converters. In undervoltage lockout both V
REF
(Pin 8) and the output (Pin 6) are actively pulled low by
Darlington connected PNP transistors. They are designed
to sink a few milliamps of current and will pull down to
about 1V. The pull-down transistor at the reference pin can
be used to reset the external soft start capacitor. The pull-
down transistor at the output eliminates the external pull-
down resistor required, with earlier devices, to hold the
external MOSFET gate low during undervoltage lockout.
Output
The LT1241 series devices incorporate a single high
current totem pole output stage. This output stage is
capable of driving up to ±1A of output current. Cross-
conduction current spikes in the output totem pole have
been eliminated. This device is primarily intended for
driving MOSFET switches. Rise time is typically 40ns and
fall time is typically 30ns when driving a 1.0nF load. A
clamp is built into the device to prevent the output from
rising above 18V in order to protect the gate of the
MOSFET switch.
The output is actively pulled low during undervoltage
lockout by a Darlington PNP. This PNP is designed to sink
several milliamps and will pull the output down to approxi-
mately 1V. This active pull-down eliminates the need for an
external resistor which was required in older designs. The
output pin of the device connects directly to the emitter of
the upper NPN drive transistor and the collector of the
lower NPN drive transistor in the totem pole. The collector
of the lower transistor, which is n-type silicon, forms a
p-n junction with the substrate of the device. This junction
is reverse biased during normal operation.
In some applications the parasitic LC of the external
MOSFET gate can ring and pull the OUTPUT pin below
ground. If the OUTPUT pin is pulled negative by more than
a diode drop the parasitic diode formed by the collector of
the output NPN and the substrate will turn on. This can
cause erratic operation of the device. In these cases a
Schottky clamp diode is recommended from the output to
ground.
Reference
The internal reference of the LT1241 series devices is a 5V
bandgap reference, trimmed to within ±1% initial toler-
ance. The reference is used to power the majority of
internal logic and the oscillator circuitry. The oscillator
charging current is supplied from the reference. The
feedback pin voltage and the clamp level for the current
sense comparator are derived from the reference voltage.
The reference can supply up to 20mA of current to power
external circuitry. Note that using the reference in this
manner, as a voltage regulator, will significantly increase
power dissipation in the device which will reduce the
useful operating ambient temperature range.
Design/Layout Considerations
LT1241 series devices are high speed circuits capable of
generating pulsed output drive currents of up to 1A peak.
The rise and fall time for the output drive current is in the
range of 10ns to 20ns. High speed circuit techniques must
be used to insure proper operation of the device. Do not
attempt to use Proto-boards or wire-wrap techniques to
breadboard high speed switching regulator circuits.
They
will not
work properly.
Printed circuit layouts should include separate ground
paths for the voltage feedback network, oscillator capaci-
tor, and switch drive current. These ground paths should
be connected together directly at the ground pin (Pin 5) of
the LT124X. This will minimize noise problems due to
pulsed ground pin currents. V
CC
should be bypassed, with
a minimum of 0.1µF, as close to the device as possible.
High current paths should be kept short and they should
be separated from the feedback voltage network with
shield traces if possible.
