Datasheet

7
LT1303/LT1303-5
Inductor Section
Inductors suitable for use with the LT1303 usually fall in
the 5µH to 50µH range. The inductor must: (1) handle
current of 1.25A without saturating, (2) have enough
inductance to provide a di/dt lower than 400mA/µs, and
(3) have low enough DC resistance to avoid excessive
heating or efficiency losses. Higher value inductors will
deliver more power but tend to be physically larger. Most
ferrite core drum or rod inductors such as those specified
in Table 1 are suitable for use. It is acceptable to bias open-
flux inductors (e.g. Sumida CD54) into saturation by 10 to
20% without adverse effects.
OPERATION
U
APPLICATIONS INFORMATION
WUU
U
hysteresis built in, but hysteresis can be added by
connecting a high-value resistor from LBI to LBO as
shown in Figure 5. The internal reference can be accessed
via the comparator as shown in Figure 6.
Figure 7 shows inductor current of a suitable inductor,
di/dt is controlled at all times. The rapid rise in current
shown in Figure 8 results from this inductor saturating at
approximately 1A. Saturation occurs when the inductor
cannot hold any more magnetic energy in the core. Current
then increases rapidly, limited only by the resistance of the
winding. Figure 9’s inductor has high DC resistance which
results in the exponential time constant shape of the
inductor current.
Figure 7. Properly Chosen Inductor Does Not Saturate
I
L
500mA/DIV
5µs/DIV
LT1303 F07
Figure 6. Accessing Internal Reference
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
Figure 5. R3 Adds Hysteresis to Low-Battery Detector

–
+
1.24V
LT1303
R4
47k
R1
R3
2M
R2
49.9k
1%
V
BAT
5V
R1 = (V
TRIP
–1.24V) (43.5k)
HYSTERESIS ≈ 30mV
LT1303 F05
PHONE
VENDOR SERIES APPROPRIATE VALUES NUMBERS
Coilcraft D03316 10µH to 47µH [phone]
D01608 10µH
Coiltronics OCTAPAK [phone]
CTX20-1 20µH
CTX20-2 20µH
CTX33-4 33µH
Sumida CD54 10µH to 33µH [phone]
Gowanda GA10 10µH to 33µH [phone]
Table 1. Recommended Inductors
V
IN
V
IN
GND
LBO
LBI
LT1303
+
100k
R2
R1
2.2µF
2N3906
V
REF
OUTPUT
V
REF
= 1.24V 1 + 
V
IN
≥ V
REF
+ 200mV
R1 + R2 ≈ 33k
R2
R1
()
LT1303 F06