9

LT1389

The reverse characteristics of the LT1389 resembles a

simple resistor Zener diode parallel connection. This well

behaved characteristic is important to the proper opera-

tion of circuits like Figure 1. The adjustable output voltage

reference depends upon positive feedback from the

LT1495’s output to start-up and regulate the bias current

for the LT1389. The LT1389 has no negative resistance

regions that can interfere with the proper start-up of the

buffered reference.

Board leakage is a concern for a nanopower precision

shunt voltage reference. The LT1389 requires attention to

detail in board layout in order to maximize its perfor-

mance. 1.5G

Ω of leakage between a DNC pin and a 5V

supply will conduct 2.5nA which induces a 0.2% error in

leakage paths are eliminated.

–

+

LT1495

LT1389-1.25

R1

249k TO 8.66M

R2

1.24M

1389 F01

1.5V TO 10V

0.8

µA

Figure 1. Adjustable Output Voltage Reference

1

2

3

4

8

7

6

5

DNC

DNC

DNC

GND

DNC

DNC

GND

BOARD METAL TRACE

LT1389

1389 F02

Figure 2. Guard Ring to Reduce Board Leakage

APPLICATIO S I FOR ATIO