6

LT1308

LOW-BATTERY DETECTOR

The LT1308’s low-battery detector is a simple PNP input

gain stage with an open collector NPN output. The nega-

Q3

SHUTDOWN

CURRENT

R2

400k

400k

Q2

1308 F03

Q1

START-UP

CURRENT

SHDN

Figure 3. Shutdown Circuit

tive 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 4

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 5.

distance from the circuit, an additional input capacitor may

be required. A 220

µFaluminumelectrolyticunitworkswell

in these cases. This capacitor need not have low ESR.

OPERATION FROM A LABORATORY POWER SUPPLY

If a lab supply is used, the leads used to connect the circuit

to the supply can have significant inductance at the

LT1308’s switching frequency. As in the previous situa-

tion, an electrolytic capacitor may be required at the circuit

in order to reduce the AC impedance of the input suffi-

ciently. An alternative solution is to attach the circuit

directly to the power supply at the supply terminals,

without the use of leads. The power supply’s output

capacitance will then provide the bulk capacitance the

LT1308 circuit requires.

SHUTDOWN PIN

The LT1308 has a shutdown pin (SHDN) that must be

grounded to shut the device down or tied to a voltage equal

shown in Figure 3.

Note that allowing SHDN to float turns on both the start-

lowed. This merely reverse-biases Q3’s base emitter junc-

tion, a benign condition.

GSM PHONES

The LT1308 is suitable for converting a single Li-Ion cell

to 5V for powering GSM RF power stages. Figure 6 details

a Li-Ion to 5V converter circuit using frequency compen-

sation optimized for a typical GSM pulsed load. Figure 7

details transient response of Figure 6’s circuit with a

100mA to 1A pulsed load. A slower time sweep is used to

additional output capacitance is recommended to help

voltage of 2.7V. Figure 10 expands the horizontal sweep

speed to 500

µs/division to show detail of one transmit

pulse.

Figure 5. Accessing 200mV Reference

Figure 4. Setting Low-Battery Detector Trip Point

LT1308

LBI

LBO

200k

10

µF

GND

10k

1308 F05

2N3906

200mV

+

LBO

LBI

TO PROCESSOR

R1

1M

R2

100k

LT1308

1308 F04

3.3V

GND

200mV

INTERNAL

REFERENCE

–

+

R1 =

2

µA