Datasheet

LT1073
8
microampere range, a leaky capacitor will noticeably de-
crease efficiency. In this type application tantalum capaci-
tors are the best choice, with typical leakage currents in the
1µA to 5µA range.
Diode Selection
Speed, forward drop and leakage current are the three
main considerations in selecting a catch diode for LT1073
converters. “General-purpose” rectifiers such as the
1N4001 are unsuitable for use in any switching regulator
application. Although they are rated at 1A, the switching
time of a 1N4001 is in the 10µs to 50µs range. At best,
efficiency will be severely compromised when these
diodes are used and at worst, the circuit may not work at
all. Most LT1073 circuits will be well served by a 1N5818
Schottky diode. The combination of 500mV forward drop
at 1A current, fast turn-on and turn-off time and 4µA to
10µA leakage current fit nicely with LT1073 requirements.
At peak switch currents of 100mA or less, a 1N4148 signal
diode may be used. This diode has leakage current in the
1nA to 5nA range at 25°C and lower cost than a 1N5818.
(You can also use them to get your circuit up and running,
but beware of destroying the diode at 1A switch currents.)
In situations where the load is intermittent and the LT1073
is idling most of the time, battery life can sometimes be
extended by using a silicon diode such as the 1N4933,
which can handle 1A but has leakage current of less than
1µA. Efficiency will decrease somewhat compared to a
1N5818 while delivering power, but the lower idle current
may be more important.
Step-Up (Boost Mode) Operation
A step-up DC/DC converter delivers an output voltage
higher than the input voltage. Step-up converters are not
short-circuit protected since there is a DC path from input
to output.
The usual step-up configuration for the LT1073 is shown
in Figure 6. The LT1073 first pulls SW1 low causing V
IN
-
V
CESAT
to appear across L1. A current then builds up in L1.
At the end of the switch ON time the current in L1 is
2
:
i
V
L
t
PEAK
IN
ON
=
APPLICATIO S I FOR ATIO
WUUU
Figure 3. Aluminum Figure 4. Tantalum Figure 5. OS-CON
NOTE 2: This simple expression neglects the effect of switch and coil resistance. These are taken
into account in the “Inductor Selection” section.
Figure 6. Step-Up Mode Hookup.
(Refer to Table 1 for Component Values)
50mV/DIV
20µs/DIV
50mV/DIV
20µs/DIV
50mV/DIV
20µs/DIV
Immediately after switch turn-off, the SW1 voltage pin
starts to rise because current cannot instantaneously stop
flowing in L1. When the voltage reaches V
OUT
+ V
D
, the
inductor current flows through D1 into C1, increasing
V
OUT
. This action is repeated as needed by the LT1073 to
keep V
FB
at the internal reference voltage of 212mV. R1
and R2 set the output voltage according to the formula:
V
IN
+
*= OPTIONAL
R3*
L1
D1
V
OUT
R1
C1
R2
1073 F06
LT1073
I
LIM
V
IN
SW1
FB
SW2GND