LT1308A/LT1308B
11
1308abfb
APPLICATIONS INFORMATION
SHDN PIN
The LT1308A/LT1308B SHDN pin is improved over the
LT1308. The pin does not require tying to V
IN
to enable
the device, but needs only a logic level signal. The voltage
on the SHDN pin can vary from 1V to 10V independent
of V
IN
. Further, fl oating this pin has the same effect as
grounding, which is to shut the device down, reducing
current drain to 1µA or less.
LOW-BATTERY DETECTOR
The low-battery detector on the LT1308A/LT1308B fea-
tures improved accuracy and drive capability compared
to the LT1308. The 200mV reference has an accuracy of
±2% and the open-collector output can sink 50µA. The
LT1308A/LT1308B low-battery detector is a simple PNP
input gain stage with an open-collector NPN output. The
negative input of the gain stage is tied internally to a 200mV
reference. The positive input is the LBI pin. Arrangement as
a low-battery detector is straightforward. Figure 10 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 reference
can also be accessed as shown in Figure 11.
A cross plot of the low-battery detector is shown in
Figure 12. The LBI pin is swept with an input which var-
ies from 195mV to 205mV, and LBO (with a 100k pull-up
resistor) is displayed.
START-UP
The LT1308A/LT1308B can start up into heavy loads, unlike
many CMOS DC/DC converters that derive operating voltage
from the output (a technique known as “bootstrapping”).
Figure 13 details start-up waveforms of Figure 1’s circuit
with a 20 load and V
IN
of 1.5V. Inductor current rises to
3.5A as the output capacitor is charged. After the output
reaches 5V, inductor current is about 1A. In Figure 14, the
load is 5 and input voltage is 3V. Output voltage reaches
5V in 500µs after the device is enabled. Figure 15 shows
start-up behavior of Figure 5’s SEPIC circuit, driven from a
9V input with a 10 load. The output reaches 5V in about
1ms after the device is enabled.
Figure 11. Accessing 200mV Reference
Figure 10. Setting Low-Battery Detector Trip Point
Figure 12. Low-Battery Detector
Input/Output Characteristic
Figure 13. 5V Boost Converter of Figure 1.
Start-Up from 1.5V Input into 20 Load
LBO
LBI
TO PROCESSOR
R1
100k
R2
100k
V
IN
V
BAT
LT1308A
LT1308B
1308 F10
5V
GND
200mV
INTERNAL
REFERENCE
–
+
R1 =
V
LB
– 200mV
2µA
V
IN
V
BAT
LT1308A
LT1308B
LBI
LBO
200k
10µF
GND
10k
1308 F11
2N3906
V
REF
200mV
+
1308 F12
V
LBO
1V/DIV
200
V
LBI
(mV)
195 205
1308 F13
V
OUT
2V/DIV
I
L1
1A/DIV
V
SHDN
5V/DIV
1ms/DIV
