7

LT1020

S

APPLICATI

I FOR ATIO

The LT1020 is especially suited for micropower system

applications. For example, the comparator section of the

LT1020 may be used as a battery checker to provide an

indication of low battery. The dropout detector can shut

down the system when the battery voltage becomes too

low to regulate. Another type of system application for the

LT1020 would be to generate the equivalent of split

supplies from a single power input. The regulator section

provides regulated output voltage and the reference, which

can both source and sink current, is then an artificial

system ground providing a split supply for the system.

For many applications the comparator can be frequency

compensated to operate as an amplifier. Compensation

values for various gains are given in the data sheet. The

comparator gain is purposely low to make it easier to

frequency compensate as an amplifier. Two outputs are

available on the comparator, the NPN output is capable of

sinking 10mA and can drive loads connected to voltages

in excess of the positive power supply. This is useful for

driving switches or linear regulators from a higher input

voltage. The PNP output, which is capable of sourcing

100

µA can drive loads below ground. It can be used to

make negative regulators with the addition of an external

pass transistor. Both outputs can be tied together to

provide an output that swings from rail-to-rail for com-

parator or amplifier applications. Although it is not speci-

fied, the gain for the PNP output is about 500 to 1000.

If the PNP output is being used, to maximize the gain a 1

µA

to 5

µA load should be placed upon the NPN output

collector. This is easily done by connecting a resistor

between the NPN collector and the reference output.

(Providing this operating current to the NPN side in-

creases the internal emitter base voltages and maximizes

the gain of the PNP stage.) Without this loading on the

NPN collector, at temperatures in excess of 75

°C, the gain

of the PNP collector can decrease by a factor of 2 or 3.

Reference

Internal to the LT1020 is a 2.5V trimmed class B output

reference. The reference was designed to be able to source

or sink current so it could be used in supply splitting

applications as well as a general purpose reference for

external circuitry. The design of the reference allows it to

source typically 4mA or 5mA and sink 2mA. The available

source and sink current decreases as temperature in-

creases. It is sometimes desirable to decrease the AC

output impedance by placing an output capacitor on them.

The reference in the LT1020 becomes unstable with large

capacitive loads placed directly on it. When using an

output capacitor, about 20

Ω should be used to isolate the

capacitor from the reference pin. This 20

Ω resistor can be

placed directly in series with the capacitor or alternatively

the reference line can have 20

Ω placed in series with it and

then a capacitor to ground. This is shown in Figure 1. Other

than placing large capacitive loads on the reference, no

I

DAGRA

BLOCK

5

6

7

8

3

13

11

2

9

REF

2.5V

REF OUT

CURRENT

LIMIT

DROPOUT

DETECTOR

FB

GND

INVERTING

NON-

INVERTING

NPN OUT

PNP OUT

10

4

1020 BD