LT1308A/ LT1308B
U
W U U
APPLICATIONS INFORMATION
SHDN PIN
START-UP
The LT1308A/LT1308B SHDN pin is improved over the
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”). Figure10 details start-upwaveforms of
LT1308. The pin does not require tying to V to enable the
IN
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, floatingthis pinhas thesameeffectas grounding,
which is to shut the device down, reducing current drain
to 1µA or less.
Figure1’s circuitwitha20ΩloadandV of1.5V. Inductor
IN
current rises to 3.5A as the output capacitor is charged.
After the output reaches 5V, inductor current is about 1A.
In Figure 11, the load is 5Ω and input voltage is 3V. Output
voltage reaches 5V 500µs after the device is enabled.
Figure 12 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.
LOW-BATTERY DETECTOR
The low-battery detector on the LT1308A/LT1308B fea-
tures improved accuracy and drive capability compared to
theLT1308.The200mVreferencehas anaccuracyof±2%
andtheopen-collectoroutputcansink50µA.TheLT1308A/
LT1308B low-battery detector is a simple PNP input gain
stage with an open-collector NPN output. The negative
GSM AND CDMA PHONES
TheLT1308A/LT1308Baresuitableforconvertingasingle
input of the gain stage is tied internally to a 200mV Li-Ion cell to 5V for powering RF power stages in GSM or
reference. The positive input is the LBI pin. Arrangement
as a low-battery detector is straightforward. Figure 7
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 8.
CDMA phones. Improvements in the LT1308A/LT1308B
error amplifiers allow external compensation values to be
reduced, resulting in faster transient response compared
to the LT1308. The circuit of Figure 13 (same as Figure 1,
printed again for convenience) provides a 5V, 1A output
from a Li-Ion cell. Figure 14 details transient response at
the LT1308A operating at a V of 4.2V, 3.6V and 3V.
IN
A cross plot of the low-battery detector is shown in
Figure 9. The LBI pin is swept with an input which varies
from 195mV to 205mV, and LBO with a 100k pull-up
resistor, is displayed.
Ripple voltage in Burst Mode operation can be seen at
10mA load. Figure 15 shows transient response of the
LT1308B under the same conditions. Note the lack of
Burst Mode ripple at 10mA load.
5V
R1
V
IN
LT1308A
LT1308B
100k
LBI
+
–
200k
V
IN
LBO
TO PROCESSOR
R2
100k
2N3906
LBO
LBI
V
BAT
LT1308A
LT1308B
V
REF
200mV
+
GND
200mV
10k
10µF
V
– 200mV
LB
R1 =
INTERNAL
REFERENCE
V
BAT
2µA
1308 F08
GND
1308 F07
Figure 8. Accessing 200mV Reference
Figure 7. Setting Low-Battery Detector Trip Point
9
Linear [ Linear ]
