11

LT1308A/LT1308B

1308abfa

SHDN PIN

The LT1308A/LT1308B SHDN pin is improved over the

device, but needs only a logic level signal. The voltage on

Further, floating this pin has the same effect as grounding,

which is to shut the device down, reducing current drain

to 1

μA 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 50

μA.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.

LBO

LBI

TO PROCESSOR

R1

100k

R2

100k

LT1308A

LT1308B

1308 F10

5V

GND

200mV

INTERNAL

REFERENCE

–

+

R1 =

2

μA

LT1308A

LT1308B

LBI

LBO

200k

10

μF

GND

10k

1308 F11

2N3906

200mV

+

195

200

205

1308 F12

1V/DIV

Figure 12. Low-Battery Detector

Input/Output Characteristic

Figure 10. Setting Low-Battery Detector Trip Point

Figure 11. Accessing 200mV Reference

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 20

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

Ω and input voltage is 3V. Output

voltage reaches 5V in 500

μs 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

Ω load. The output

reaches 5V in about 1ms after the device is enabled.

2V/DIV

1A/DIV

5V/DIV

1ms/DIV

1308 F13

Figure 13. 5V Boost Converter of Figure 1.

Start-Up from 1.5V Input into 20

Ω Load