### DN37

```Design Note 37
Issue 1 June 1996
The ZR2431 Low Reference Voltage Shunt
Regulator
Shunt Regulator
+ve
The ZR2431 is ideal for providing low
voltage stabilised supplies or
references. Low output voltage supplies
c a n b e d i f fi c u l t t o d e s i g n a s t h e
components normally used, such as
zener diodes or band gap references, are
either too poor in performance or not
available at the desired voltage
(band-gap devices are usually fixed at
1.2V or 2.5V with no intermediate
values). The circuit in Figure 1 shows a
1. 5 V s hunt r e gulator utilising the
ZR2431. The output of this supply can be
set to any voltage in the range 1.23V to
20V by adjusting the ratio of R1 and R2,
following the relationship:Vout =Vref
(R1+R2)
R1
Where Vref is the reference voltage of the
ZR2431.
Note that a small correction to this
formula is required if R1 and R2 are
given ve ry high values, since the
reference input current of the ZR2431
flows through R2 but not R1 causing a
small error. This reference current is
only 200nA maximum for the ZR2431 so
as long as a current of at least 20µA is
passed through R1, setting its maximum
R3
1k8
+ve
Input
(1.7 - 60V)
IC1
ZR2431
R2
24k
C1 Output
2.2uF (1.5V)
R1
120k
-ve
-ve
Figure 1
1.5V Shunt Regulator.
value at 60k ohms, the reference current
can be ignored. For very low current
applications where it is advantageous to
set R1 and R2 at a level where the
reference current must be considered,
the following formula takes this input
current into account:Vout = Iref x R2 + Vref
(R1+R2)
R1
Where Iref is the reference input current
of the ZR2431.
R3 is selected so as to support the
input voltage, yet still maintain adequate
DN 37- 1
Design Note 37
Issue 1 June 1996
Applications Note 1
Issue 2 October 1995
operating current for the ZR2431. Since
the minimum cathode current of the
ZR2431 is only 100µA, it rarely has an
effect on the choice of R3.
R3 =
Vin(min)âˆ’ Vout
Iout(max)+ Iz(min)
Where Iz(min) is the minimum cathode
current of the ZR2431.
The low series resistance of the ZR2431
not only gives the shunt regulator good
load regulation but also a line rejection
of over 60dB. In this circuit, capacitor C1
both maintains this rejection figure at
high fre quencies and ens ures the
ZR2431 is stable should the load of the
regulator be reactive. The value of C1
necessary to ensure stability varies with
the regulatorâ€™s output voltage (set by R1
and R2). The value shown is sufficient for
all voltages but the size of this capacitor
can be r educed for higher output
voltages e.g. only 47nF is necessary at
the highest working output voltage of
20V.
DN 37 - 2
AN 1 - 4
```