6
LT1305
OPERATION
U
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 3Ω 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 2µs. 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
V
IN
V
IN
GND
LBO
LBI
LT1305
+
100k
R2
R1
2.2µF
2N3906
V
REF
OUTPUT
V
REF
= 1.24V 1 +
V
IN
≥ V
REF
+ 200mV
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
0.05Ω 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 10µH inductor is recom-
mended (such as Coilcraft DO3316-103). For inputs above
4V to 5V use a 22µH 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 1Ω, even for relatively large values
such as 100µF, 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 0.05Ω to 0.1Ω for
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
V
BAT
5V
R1 = (V
TRIP
–1.24V) (43.5k)
HYSTERESIS ≈ 30mV
LT1305 • F03
