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

1

µA 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 10

µs 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 4

µA to

10

µA 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

1

µA. 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

i

V

L

t

PEAK

IN

ON

=

APPLICATIO S I FOR ATIO

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

20

µs/DIV

50mV/DIV

20

µs/DIV

50mV/DIV

20

µs/DIV

Immediately after switch turn-off, the SW1 voltage pin

starts to rise because current cannot instantaneously stop

inductor current flows through D1 into C1, increasing

and R2 set the output voltage according to the formula:

+

*= OPTIONAL

R3*

L1

D1

R1

C1

R2

1073 F06

LT1073

SW1

FB

SW2

GND