LT1940/LT1940L
5
1940fa
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PI FU CTIO S
BOOST1, BOOST2 (Pins 1, 8): The BOOST pins are used
to provide drive voltages, higher than the input voltage, to
the internal bipolar NPN power switches.Tie through a
diode from V
OUT
or from V
IN
.
SW1, SW2 (Pins 2, 7): The SW pins are the outputs of the
internal power switches. Connect these pins to the induc-
tors, catch diodes and boost capacitors.
V
IN
(Pins 3, 4, 5, 6): The V
IN
pins supply current to the
LT1940’s internal regulator and to the internal power
switches. These pins must be tied to the same source, and
must be locally bypassed.
FB1, FB2 (Pins 9, 16): The LT1940 regulates each feed-
back pin to 1.25V. Connect the feedback resistor divider
taps to these pins.
V
C1
,
V
C2
(Pins 10, 15): The V
C
pins are the outputs of the
internal error amps. The voltages on these pins control the
peak switch currents. These pins are normally used to
compensate the control loops, but can also be used to
override the loops. Pull these pins to ground with an open
drain to shut down each switching regulator.
PG1, PG2 (Pins 11, 14): The Power Good pins are the
open collector outputs of an internal comparator. PG
remains low until the FB pin is within 10% of the final
regulation voltage. As well as indicating output regulation,
the PG pins can be used to sequence the two switching
regulators. These pins can be left unconnected. The PG
outputs are valid when V
IN
is greater than 2.4V and either
of the RUN/SS pins is high. The PG comparators are
disabled in shutdown.
RUN/SS1, RUN/SS2 (Pins 12, 13): The RUN/SS pins are
use to shut down the individual switching regulators and
the internal bias circuits. They also provide a soft-start
function. To shut down either regulator, pull the RUN/SS
pin to ground with an open drain or collector. Tie a
capacitor from these pins to ground to limit switch current
during start-up. If neither feature is used, leave these pins
unconnected.
GND (Pin 17): The Exposed Pad of the package provides
both electrical contact to ground and good thermal con-
tact to the printed circuit board. The Exposed Pad must be
soldered to the circuit board for proper operation.
RUN/SS Thresholds vs
Temperature
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
RUNN/SS THRESHOLDS (V)
TEMPERATURE (°C)
–50 25 75
1940 G13
–25 0
50 100 125
TO SWITCH
TO RUN
LOAD CURRENT (mA)
1
MINIMUM INPUT VOLTAGE (V)
6.0
5.5
5.0
4.5
4.0
3.5
3.0
10 100 1000
1940 G14
V
IN
TO START
BOOST DIODE
TIED TO OUTPUT
BOOST DIODE
TIED TO INPUT
V
IN
TO RUN
T
A
= 25°C
D
BOOST
= BAT54
LOAD CURRENT (mA)
1
MINIMUM INPUT VOLTAGE (V)
7.5
7.0
6.5
6.0
5.5
5.0
4.5
10 100 1000
1940 G14
V
IN
TO START
BOOST DIODE
TIED TO OUTPUT
V
IN
TO RUN
T
A
= 25°C
D
BOOST
= BAT54
BOOST DIODE
TIED TO INPUT
Minimum Input Voltage,
V
OUT
= 3.3V
Minimum Input Voltage,
V
OUT
= 5V
TYPICAL PERFOR A CE CHARACTERISTICS
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