LT1173

8

S

APPLICATI

I FOR ATIO

Figure 2. Aluminum

Figure 3. Tantalum

Figure 4. OS-CON

Note 1: This simple expression neglects the effect of switch and coil

resistance. This is taken into account in the “Inductor Selection” section.

In very low power applications where every microampere

is important, leakage current of the capacitor must be

considered. The OS-CON units do have leakage current in

the 5

µA to 10µA range. If the load is also in the microam-

pere range, a leaky capacitor will noticeably decrease

efficiency. In this type application tantalum capacitors are

the best choice, with typical leakage currents in the 1

µA to

5

µA range.

Diode Selection

Speed, forward drop, and leakage current are the three

main considerations in selecting a catch diode for LT1173

converters. General purpose rectifiers such as the 1N4001

are

unsuitable for use in any switching regulator applica-

tion. Although they are rated at 1A, the switching time of

a 1N4001 is in the 10

µs-50µsrange.Atbest,efficiencywill

be severely compromised when these diodes are used; at

worst, the circuit may not work at all. Most LT1173 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 10µA leakage

current fit nicely with LT1173 requirements. At peak

switch currents of 100mA or less, a 1N4148 signal diode

may be used. This diode has leakage current in the 1nA-

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 LT1173 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 LT1173 is shown

i

V

L

t

PEAK

IN

ON

=

()

13

L1

LT1173 • TA10

GND

SW2

SW1

LIM

I

IN

V

D1

R3*

LT1173

+

R2

R1

C1

* = OPTIONAL

FB

Figure 5. Step-Up Mode Hookup.

Refer to Table 1 for Component Values

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

5 s/DIV

LT1173 • TA09

µ

5 s/DIV

LT1173 • TA07

µ

5 s/DIV

LT1173 • TA08

µ