LT1940 Linear Technology, LT1940 Datasheet - Page 13

• Manufacturer Part Number: LT1940
• Description: Dual Monolithic 1.4A/ 1.1MHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

Soft-Start and Shutdown
The RUN/SS (Run/Soft-Start) pins are used to place the
individual switching regulators and the internal bias cir-
cuits in shutdown mode. 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. If both
RUN/SS pins are pulled to ground, the LT1940 enters its
shutdown mode with both regulators off and quiescent
current reduced to ~30 A. Internal 2 A current sources
pull up on each pin. If either pin reaches ~0.5V, the internal
bias circuits start and the quiescent current increases to
~3.5mA.
If a capacitor is tied from the RUN/SS pin to ground, then
the internal pull-up current will generate a voltage ramp on
this pin. This voltage clamps the V
switch current and therefore input current during start up.
A good value for the soft-start capacitor is C
where C
The RUN/SS pins can be left floating if the shutdown
feature is not used. They can also be tied together with a
single capacitor providing soft-start. The internal current
sources will charge these pins to ~2.5V.
Power Good Indicators and Output Sequence
The PG pin is the open collector output of an internal
comparator. PG remains low until the FB pin is within 10%
of the final regulation voltage. Tie the PG pin to any supply
with a pull-up resistor that will supply less than 250 A.
APPLICATIO S I FOR ATIO
Figure 6. The Power Good Comparator can be used to Sequence
the Two Regulators. Switcher 1 will Start First.
OFF
ON
OUT
is the value of the output capacitor.
U
RUN/SS1
RUN/SS2
U
GND
PG2
PG1
V
C2
C
W
pin, limiting the peak
V
OUT2
POWER GOOD
OUT
U
/10,000,
1940 F06
Note that this pin will be open when the LT1940 is placed
in shutdown mode (both RUN/SS pins at ground) regard-
less of the voltage at the FB pin. Power good is valid when
the LT1940 is enabled (either RUN/SS pin is high) and V
is greater than ~2.4V.
The PG pin can be used to sequence the two switching
regulators. The circuit in Figure 6 provides soft-start and
sequencing with the fewest components. PG1 is tied to
V
in regulation. A single capacitor provides the soft-start
ramp for both regulators.
Shorted Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, the LT1940 will tolerate a shorted output. There is
another situation to consider in systems where the output
will be held high when the input to the LT1940 is absent.
If the V
then the LT1940’s internal circuitry will pull its quiescent
current through its SW pin. This is fine if your system can
tolerate a few mA of load in this state. With both RUN/SS
pins grounded, the LT1940 enters shutdown mode and
the SW pin current drops to ~30 A. However, if the V
is grounded while the output is held high, then parasitic
diodes inside the LT1940 can pull large currents from the
output through the SW pin and the V
diode in series with the input to the LT1940 will protect the
LT1940 and the system from a shorted or reversed input.
C2
, preventing switcher 2 from operating until output 1 is
Figure 7.Diode D4 Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output.
V
IN
IN
and one of the RUN/SS pins are allowed to float,
PARASITIC DIODE
D4
V
IN
LT1940
SW
IN
pin. A Schottky
LT1940
1940 F07
V
OUT
13
IN
1940i
pin
IN