LT1308A/LT1308B
U
W U U
APPLICATIONS INFORMATION
SHDN PIN
A cross plot of the low-battery detector is shown in
Figure 12. The LBI pin is swept with an input which varies
from 195mV to 205mV, and LBO (with a 100k pull-up
resistor) is displayed.
The LT1308A/LT1308B SHDN pin is improved over the
LT1308. The pin does not require tying to VIN 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 VIN.
Further, floatingthispinhasthesameeffectasgrounding,
which is to shut the device down, reducing current drain
to 1μA or less.
VLBO
1V/DIV
LOW-BATTERY DETECTOR
The low-battery detector on the LT1308A/LT1308B fea-
tures improved accuracy and drive capability compared to
theLT1308.The200mVreferencehasanaccuracyof±2%
andtheopen-collectoroutputcansink50μA.TheLT1308A/
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 refer-
ence can also be accessed as shown in Figure 11.
195
200
205
VLBI (mV)
1308 F12
Figure 12. Low-Battery Detector
Input/Output Characteristic
START-UP
The LT1308A/LT1308B can start up into heavy loads,
unlike many CMOS DC/DC converters that derive operat-
ing voltage from the output (a technique known as
“bootstrapping”). Figure13 detailsstart-upwaveformsof
Figure1’scircuitwitha20ΩloadandVIN of1.5V. 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 5Ω 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 10Ω load. The output
reaches 5V in about 1ms after the device is enabled.
5V
R1
V
LT1308A
LT1308B
IN
100k
LBI
+
–
LBO
TO PROCESSOR
R2
100k
200mV
V
– 200mV
2μA
LB
VOUT
2V/DIV
R1 =
INTERNAL
V
BAT
REFERENCE
GND
1308 F10
IL1
1A/DIV
Figure 10. Setting Low-Battery Detector Trip Point
VSHDN
5V/DIV
1ms/DIV
1308 F13
200k
V
IN
2N3906
REF
LBO
LBI
Figure 13. 5V Boost Converter of Figure 1.
Start-Up from 1.5V Input into 20Ω Load
V
LT1308A
LT1308B
BAT
V
200mV
+
GND
10k
10μF
1308 F11
Figure 11. Accessing 200mV Reference
1308abfa
11
Linear [ Linear ]
