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. 1.5GΩ of leakage between a DNC pin and a 5V
supply will conduct 2.5nA which induces a 0.2% error in
V
OUT
. Board leakage can be minimized by encircling the
DNC pins with a guard ring operated at a potential of V
OUT
.
By tying the guard ring to V
OUT
as shown in Figure 2,
leakage paths are eliminated.
–
+
LT1495
LT1389-1.25
V
IN
≥ 10.5V
R1
249k TO 8.66M
R2
1.24M
1389 F01
V
OUT
1.5V TO 10V
R
B
=
V
OUT
– 1.25V
0.8µA
Figure 1. Adjustable Output Voltage Reference
1
2
3
4
8
7
6
5
DNC
DNC
DNC
GND
DNC
DNC
V
OUT
GND
BOARD METAL TRACE
LT1389
1389 F02
Figure 2. Guard Ring to Reduce Board Leakage
APPLICATIO S I FOR ATIO
WUUU
