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LT1389BCS8-5

Nanopower Precision Shunt Voltage Reference

Linear Technology 

LT1389

APPLICATIO S I FOR ATIO

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. 1.5GΩ of leakage between a DNC pin and a 5V

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

VOUT. Board leakage can be minimized by encircling the

DNC pins with a guard ring operated at a potential of VOUT.

By tying the guard ring to VOUT as shown in Figure 2,

leakage paths are eliminated.

RB

=

VOUT – 1.25V

0.8µA

VIN ≥ 10.5V

LT1389-1.25

+

LT1495

–

VOUT

1.5V TO 10V

R1

249k TO 8.66M

R2

1.24M

1389 F01

Figure 1. Adjustable Output Voltage Reference

BOARD METAL TRACE

DNC 1

DNC 2

DNC 3

GND 4

LT1389

8 DNC

7 DNC

6 VOUT

5 GND

1389 F02

Figure 2. Guard Ring to Reduce Board Leakage

9

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