AD420

Rev. H | Page 13 of 16

EXTERNAL BOOST FUNCTION

The external boost transistor reduces the power dissipated in

the AD420 by reducing the current flowing in the on-chip

output transistor (dividing it by the current gain of the external

circuit). A discrete NPN transistor with a breakdown voltage,

AD420

MJD31C

OR

2N3053

19

BOOST

0.022µF

1kΩ

18

Figure 13. External Boost Configuration

The external boost capability has been developed for those

users who may wish to use the AD420, in the SOIC package, at

the extremes of the supply voltage, load current, and

temperature range. The PDIP package (because of its lower

thermal resistance) will operate safely over the entire specified

voltage, temperature, and load current ranges without the boost

transistor. The plot in Figure 14 shows the safe operating region

for both package types. The boost transistor can also be used to

reduce the amount of temperature induced drift in the part.

This will minimize the temperature induced drift of the on-chip

voltage reference, which improves drift and linearity.

28V

20V

12V

4V

–60

–40

–20

0

20

40

60

80

100

TEMPERATURE (°C)

25V

AD420 OR AD420-32

32V

WHEN USING SOIC PACKAGED DEVICES,

AN EXTERNAL BOOST TRANSISTOR IS

REQUIRED FOR OPERATION IN THIS AREA.

Figure 14. Safe Operating Region