13
LT1307/LT1307B
1307fa
tions where small size is not critical, Panasonic HFQ series
aluminum electrolytic capacitors have been found to per-
form well.
Table 2. Vendor Telephone Numbers
VENDOR COMPONENTS TELEPHONE
Coilcraft Inductors (708) 639-6400
Marcon Capacitors (708) 913-9980
Murata-Erie Inductors, Capacitors (404) 436-1300
Sumida Inductors (847) 956-0666
Tokin Capacitors (408) 432-8020
AVX Capacitors (207) 282-5111
Sprague Capacitors (603) 224-1961
Coiltronics Inductors (407) 241-7876
Diodes
Most of the application circuits on this data sheet specify
the Motorola MBR0520L surface mount Schottky diode.
This 0.5A, low drop diode complements the LT1307 quite
well. In lower current applications, a 1N4148 can be used,
although efficiency will suffer due to the higher forward
drop. This effect is particularly noticeable at low output
voltages. For higher voltage output applications, such as
LCD bias generators, the extra drop is a small percentage
of the output voltage so the efficiency penalty is small. The
low cost of the 1N4148 makes it attractive wherever it can
be used. In through hole applications the 1N5818 is the all
around best choice.
SHUTDOWN PIN
The LT1307 has a Shutdown pin (SHDN) that must be
grounded to shut the device down or tied to a voltage equal
or greater than V
IN
to operate. The shutdown circuit is
shown in Figure 13.
Note that allowing SHDN to float turns on both the start-
up current (Q2) and the shutdown current (Q3) for V
IN
>
2V
BE
. The LT1307 doesn’t know what to do in this situation
and behaves erratically. SHDN voltage above V
IN
is al-
lowed. This merely reverse-biases Q3’s base emitter junc-
tion, a benign condition.
Figure 13. Shutdown Circuit
V
IN
Q3
SHUTDOWN
CURRENT
R2
400k
200k
Q2
1307 F13
Q1
START-UP
CURRENT
SHDN
LOW-BATTERY DETECTOR
The LT1307’s low-battery detector is a simple PNP input
gain stage with an open collector NPN output. The nega-
tive input of the gain stage is tied internally to a 200mV
±5% reference. The positive input is the LBI pin. Arrange-
ment as a low-battery detector is straightforward. Figure
14 details hookup. R1 and R2 need only be low enough in
value so that the bias current of the LBI pin doesn’t cause
large errors. For R2, 100k is adequate. The 200mV refer-
ence can also be accessed as shown in Figure 15.
LBO
LBI
TO PROCESSOR
R1
1M
R2
100k
V
IN
LT1307
1307 F14
3.3V
GND
200mV
INTERNAL
REFERENCE
–
+
R1 =
V
LB
– 200mV
2µA
Figure 14. Setting Low-Battery Detector Trip Point
V
IN
LT1307
LBI
LBO
200k
10µF
GND
10k
1307 F15
2N3906
V
REF
200mV
+
Figure 15. Accessing 200mV Reference
APPLICATIO S I FOR ATIO
WUUU
