6

LT1305

If switch current reaches 2A, causing C2 to trip, switch on

time is reduced and off time increases slightly. This allows

continuous operation during bursts. C2 monitors the

voltage across 3

Ω resistor R1 which is directly related to

the switch current. Q2’s collector current is set by the

emitter-area ratio to 0.6% of Q1’s collector current. When

R1’s voltage drop exceeds 36mV, corresponding to 2A

switch current, C2’s output goes high, truncating the on

time portion of the oscillator cycle and increasing off time

to about 2

µs. Response time of C2, which determines

minimum on time, is approximately 300ns.

Low-Battery Detector

The low-battery detector is enabled when SHDN is low and

disabled when SHDN is high. The comparator has no

hysteresis built in, but hysteresis can be added by

connecting a high-value resistor from LBI to LBO as

shown in Figure 3. The internal reference can be accessed

via the comparator as shown in Figure 4.

Figure 4. Accessing Internal Reference

GND

LBO

LBI

LT1305

+

100k

R2

R1

2.2

µF

2N3906

OUTPUT

R1 + R2

≈ 33k

R2

R1

()

LT1305 • F04

Figure 3. R3 Adds Hysteresis to Low-Battery Detector

Inductor Selection

Inductors used with the LT1305 must fulfill two require-

ments. First, the inductor must be able to handle current

of 2A to 2.5A without runaway saturation. Rod or drum

core units usually saturate gradually and it is acceptable to

exceed manufacturer’s published saturation current by

20% or so. Second, the unit must have low DCR, under

0.05

Ω so that copper loss is kept low and excess heating

is avoided. Inductance value is not critical. Generally, for

low voltage inputs below 3V a 10

µH inductor is recom-

mended (such as Coilcraft DO3316-103). For inputs above

4V to 5V use a 22

µH unit (such as Coilcraft DO3316-223).

Switching frequency can reach up to 300kHz so the core

material should be able to operate at high frequency

without excessive core loss. Ferrite or molypermalloy

cores are a better choice than powdered iron. If EMI is a

concern, a toroidal inductor is suggested, such as

Coiltronics CTX20-4.

Capacitor Selection

Output and input capacitors should have low ESR for best

performance. Inexpensive aluminum electrolytics some-

times have ESR above 1

Ω, even for relatively large values

such as 100

µF, 16V units. Since the LT1305 has a 2A

current limit, 2V of ripple voltage would result with such a

capacitor at the output. Keep ESR below 0.05

Ωto 0.1Ωfor

reasonable ripple voltage. Tantalum capacitors such as

AVX TPS series or Sprague 593D have low ESR and are

surface mount components. For lowest ESR, use Sanyo

OS-CON units (OS-CON is also available from Vishay).

These capacitors have superior ESR, small size and per-

form well at cold temperatures.

Diode Selection

A 2A Schottky diode such as Motorola MBRS130LT3 is a

good choice for the rectifier diode. A 1N5821 or

MBRS130T3 are suitable as well. Do not use “general

purpose” diodes such as 1N4001. They are much too slow

for use in switching regulator applications.

–

+

1.24V

LT1305

R4

47k

R1

R3

2M

R2

49.9k

1%

LBO

LBI

5V

HYSTERESIS

≈ 30mV

LT1305 • F03