Datasheet

LT1950
7
1950fa
BLANK (Pin 9): The BLANK pin is used to adjust the
leading edge blanking period of the current sense amplifier
during FET turn-on. Shorting the BLANK pin to ground
provides a default blanking period of approximately 110ns.
A resistor from the BLANK pin to ground increases the
blanking period up to 290ns for R
BLANK
= 75k.
I
SENSE
(Pin 10): The I
SENSE
pin is the current sense input
for the control loop. Connect this pin to the sense resistor
in the source of the external power MOSFET.
V
IN2
(Pin 11): The V
IN2
pin is the supply pin for the
MOSFET gate drive circuit. Power can be supplied to this
pin by an external supply such as V
IN
, and must exceed 8V
(the undervoltage lockout threshold for the gate driver
supply). For low V
IN
supply voltages an internal boost
regulator can be used to generate as much as 11V at the
V
IN2
pin. This allows the LT1950 to run with V
IN
supply
voltages down to 3V while still supplying enough gate
drive for standard level MOSFETs.
GATE (PIN 12): The GATE pin is the output of a high current
gate drive circuit used to drive an external MOSFET. The
output is actively clamped to a max voltage of 13V if V
IN2
is supplied by a high voltage.
PGND (Pin 13): This is the ground connection for the high
current gate driver stage. See the Applications Informa-
tion section for recommendations on ground connec-
tions.
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BOOST (Pin 14): The BOOST pin is the NPN collector
output of the internal boost converter which can be used
to generate an 11V supply for the MOSFET gate driver
circuit. The boost converter runs with a fixed off-time of
0.5µs and a current limit of 125mA. The converter runs
until the V
IN2
voltage exceeds 11V and then turns off until
the V
IN2
voltage drops below 10V. If the V
IN2
voltage is
supplied externally, the BOOST pin should be shorted to
ground or left open.
V
IN
(Pin 15): The V
IN
pin is the main supply pin for the
LT1950. This pin must be closely bypassed to ground. If
V
IN2
is generated using the BOOST pin then the LT1950
will be fully functional, internal V
REF
will be active and the
gate output will be enabled with a V
IN
voltage as low as 3V.
An internal undervoltage lockout threshold exists at ap-
proximately 2.6V on the V
IN
pin. Undervoltage lockout
voltages greater than 3V can be programmed using a
voltage divider on the SHDN pin.
V
SEC
(Pin 16): The V
SEC
pin is used to program the
maximum duty cycle of the gate driver circuit. The maxi-
mum duty cycle will be equal to (105/V
SEC
)% for V
SEC
between 1.4V and 2.8V. This is a useful function to limit the
flyback voltage in a forward converter. If the maximum
duty cycle function is not used then the V
SEC
pin should be
tied to ground.