1308AI Linear Technology Corporation, 1308AI Datasheet - Page 11

• Manufacturer Part Number: 1308AI
• Description: Lt1308b - Single Cell High Currentmicropower 600khz Boost Dc/dc Converter
• Manufacturer: Linear Technology Corporation

APPLICATIONS
SHDN PIN
The LT1308A/LT1308B SHDN pin is improved over the
LT1308. The pin does not require tying to V
device, but needs only a logic level signal. The voltage on
the SHDN pin can vary from 1V to 10V independent of V
Further, floating 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 refer-
ence can also be accessed as shown in Figure 11.
V
BAT
Figure 10. Setting Low-Battery Detector Trip Point
V
BAT
R1
R2
100k
Figure 11. Accessing 200mV Reference
200mV
V
LBI
REF
2N3906
GND
U
200mV
INTERNAL
REFERENCE
–
+
10k
INFORMATION
V
U
IN
+
200k
LT1308A
LT1308B
10μF
LBO
1308 F10
W
LBO
LBI
5V
LT1308A
LT1308B
GND
V
100k
R1 =
IN
IN
1308 F11
to enable the
V
TO PROCESSOR
LB
U
– 200mV
2μA
IN
.
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.
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”). Figure 13 details start-up waveforms of
Figure 1’s circuit with a 20Ω load and V
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.
2V/DIV
1A/DIV
5V/DIV
1V/DIV
V
V
SHDN
V
OUT
I
LBO
L1
Figure 13. 5V Boost Converter of Figure 1.
Start-Up from 1.5V Input into 20Ω Load
195
Figure 12. Low-Battery Detector
Input/Output Characteristic
LT1308A/LT1308B
1ms/DIV
V
LBI
200
(mV)
IN
of 1.5V. Inductor
1308 F13
1308 F12
205
11
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