Datasheet

LT1941
14
1941fb
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
IN
and one of the RUN/SS pins are allowed
to fl oat, 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 shutdown
mode and the SW pin current drops to ~50mA. However, if
the V
IN
pin is grounded while the output is held high, then
parasitic diodes inside the LT1941 can pull large currents
from the output through the SW pin and the V
IN
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:
RR
V
V
OUT
34
125
1=
⎛
⎝
⎜
⎞
⎠
⎟
.
–
R4 should be 10k or less to avoid bias current errors.
NFB should be tied to FB3.
Figure 4. Diode D4 Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output
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
OUT
. Choose the resistors
according to:
R
RV
V
OUT
4
3
124
=
•
.
V
IN
V
IN
V
OUT
SW
LT1941
D4
PARASITIC DIODE
1941 F04
FB3
R3
R4
1941 AI01
V
OUT
NFB
FB3
R4
R3
1941 AI02
–V
OUT
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 application
using the inverting topology is:
DC
V
VV
OUT
IN OUT
=
+
The duty cycle for a given application using the boost
topology is:
DC
VV
V
OUT IN
OUT
=
–
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
APPLICATIONS INFORMATION