lt1303-5 Linear Technology Corporation, lt1303-5 Datasheet - Page 7

• Manufacturer Part Number: lt1303-5
• Description: Micropower High Efficiency Dc/dc Converters With Low-battery Detector Adjustable And Fixed 5v
• Manufacturer: Linear Technology Corporation

OPERATION
APPLICATIONS
Inductor Section
Inductors suitable for use with the LT1303 usually fall in
the 5 H to 50 H range. The inductor must: (1) handle
current of 1.25A without saturating, (2) have enough
inductance to provide a di/dt lower than 400mA/ s, 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.
Table 1. Recommended Inductors
to about 2 s. 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
VENDOR
Coilcraft
Coiltronics
Sumida
Gowanda
Figure 5. R3 Adds Hysteresis to Low-Battery Detector
V
R1 = (V
HYSTERESIS 30mV
BAT
R1
R2
49.9k
1%
SERIES
D03316
D01608
OCTAPAK
CTX20-1
CTX20-2
CTX33-4
CD54
GA10
TRIP
–1.24V) (43.5k)
U
U
1.24V
APPROPRIATE VALUES NUMBERS
10 H to 47 H
10 H
20 H
20 H
33 H
10 H to 33 H
10 H to 33 H
INFORMATION
U
–
+
LT1303
2M
R3
W
PHONE
[phone]
[phone]
[phone]
[phone]
U
5V
LT1303 F05
R4
47k
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
500mA/DIV
V
V
R1 + R2 33k
OUTPUT
Figure 6. Accessing Internal Reference
REF
IN
I
L
V
2N3906
= 1.24V 1 +
REF
V
REF
+
+ 200mV
2.2 F
R2
R2
R1
V
IN
100k
LT1303/LT1303-5
5 s/DIV
R1
LBO
LBI
GND
V
IN
LT1303
LT1303 F07
LT1303 F06
7