lt1941 Linear Technology Corporation, lt1941 Datasheet - Page 14

• Manufacturer Part Number: lt1941
• Description: Triple Monolithic Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT1941
Converter with Backup Output Regulator
There is another situation to consider in systems where
the output will be held high when the input to the LT1941
is absent. If the V
to float, then the LT1941’s internal circuitry will pull its
quiescent current through its SW pin. This is acceptable if
the system can tolerate a few mA of load in this state. With
both RUN/SS pins grounded, the LT1941 enters shut-
down mode and the SW pin current drops to ~50mA.
However, if the V
high, then parasitic diodes inside the LT1941 can pull large
currents from the output through the SW pin and the V
pin. A Schottky diode in series with the input to the
LT1941, as shown in Figure 4, will protect the LT1941 and
the system from a shorted or reversed input.
INVERTER/BOOST CONSIDERATIONS
Regulating Positive Output Voltages
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistors
according to:
R4 should be 10k or less to avoid bias current errors.
NFB should be tied to FB3.
14
R
3
=
Figure 4. Diode D4 Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output
R
V
4
IN
⎛
⎜
⎝
PARASITIC DIODE
1 25
V
D4
.
OUT
IN
IN
V
and one of the RUN/SS pins are allowed
pin is grounded while the output is held
U
–
V
IN
1
⎞
⎟
⎠
V
OUT
U
LT1941
R3
R4
1941 AI01
FB3
SW
W
U
1941 F04
V
OUT
IN
Regulating Negative Output Voltages
The LT1941 contains an inverting op-amp with its nonin-
verting terminal tied to ground and its output connected to
the FB3 pin. Use this op-amp to generate a voltage at FB3
that is proportional to V
ing to:
Use 10k or larger, up to 20k for R3.
Duty Cycle Range
The maximum duty cycle (DC) of the LT1941 inverter/
boost regulator is 77%. The duty cycle for a given applica-
tion using the inverting topology is:
The duty cycle for a given application using the boost
topology is:
The LT1941 can still be used in applications where the DC,
as calculated above, is above 77%; however, the part must
be operated in discontinuous mode so that the actual duty
cycle is reduced.
Inductor Selection
Several inductors that work well with the LT1941 inverter/
boost regulator are listed in Table 4. Besides these, many
other inductors will work. Consult each manufacturer for
detailed information and for their entire selection of related
parts. Use ferrite core inductors to obtain the best effi-
ciency. When using coupled inductors, choose one that
R
DC
DC
4
=
=
=
R
V
V
3
IN
OUT
1 24
V
•
.
V
+
OUT
OUT
V
–
OUT
V
V
OUT
V
IN
–V
OUT
OUT
R4
R3
. Choose the resistors accord-
NFB
FB3
1941 AI02
1941fa