AN27

Application Note 27
Issue 1 June 1996
ZR431 Application Note
David Bradbury
The ZR431 is an enhanced version of the
industry standard 431. It is a three
terminal shunt regulator giving excellent
temperature stability and the capability
of operating at currents from 50µA up
to100mA. Its output voltage can be set to
any voltage in the range of Vref (2.5V) to
20V by the addition of two external
divider resistors. The reference input
current is typically only 100nA, so high
value resistors can be used without
error. Its minimum operating current
and reference input current are ten times
lower than industry standard 431 parts.
(Please refer to Appendix A)
Following are a few example
applications of the ZR431 which show
how the parts can be used.
Shunt Regulator
The ZR431 is ideal for providing low
voltage stabilised supplies or
references. Simple low output voltage
supplies can be difficult to build as the
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
2.5V or 5V with no intermediate values).
+ve
R3
1k8
+ve
Input
(3.2 - 60V)
IC1
ZR431
R2
24k
C1 Output
100nF (3.0V)
R1
120k
-ve
-ve
Figure 1
ZR431 used as a 3V Shunt Regulator.
The circuit in Figure 1 shows a 3V shunt
regulator utilising the ZR431. The output
of this supply can be set to any voltage
in the range 2.5V to 20V by adjusting the
ra ti o of R1 and R2, following the
relationship:Vout =Vref
(R1+R2)
R1
Where Vref is the reference voltage of the
ZR431.
Note that a small correction to this
formula is required if R1 and R2 are
giv en ve ry high values, since the
reference input current of the ZR431
AN 27 - 1
Applications Note 27
Issue1 June 1996
flows through R2 but not R1 thereby
causing a small error. For the Zetex
ZR431, this reference current is 200nA
maximum so as long as a current of at
least 20µA is passed through R1, setting
its maximum 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
input current into account:Vout = Iref x R2 + Vref
(R1+R2)
R1
Where Iref is the reference input current
of the ZR431.
R3 is selected so as to support the
maximum load current at minimum
input voltage, yet still maintain adequate
operating current for the ZR431. Since
the minimum cathode current of the
ZR431 is only 50µA, it rarely has an effect
on the choice of R3.
R3 =
Vin(min)−Vout
Iout(max)+Iz(min)
Where Iz(min) is the minimum cathode
current of the ZR431.
The low series resistance of the ZR431
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 frequencies and ensures stability
should the power supply’s load be
capacitive. For the Zetex ZR431, a
capacitor greater than 1.5nF will ensure
stability for any load. (Competitors’
versions of the ’431 generally require
values of 3.3µF or greater).
Switch-Mode Power Supply
Controller
The circuit shown in Figure 2 is
commonly used in the control loop of
sw itch-mode pow er suppl ies. The
output voltage is sensed via R3/R4 by the
ZR431, and the IC controls the current
passed through an opto-coupler and
hence feeds back output voltage status
to the switching regulator. The
minimum supply voltage on which this
popular circuit can be used is set by the
minimum cathode voltage of the
regulator IC, plus the forward voltage
drop of the opto-coupler LED. This limits
the minimum operating voltage of the
circuit to around 4V.
The components C1 and R2 are not
required for correct operation of the
ZR431. They have been included as they
are frequently needed to stabilise the
overall control loop of the switch-mode
power supply of which Figure 2 is only a
p a r t . The ZR431 is unconditionally
stable without additional components.
Note 1:
Resistor R5 is only necessary if the
minimum operating current of the ZR431
causes excess opto-coupler current
before the reference functions correctly.
The minimum operating current of the
ZR431 is only 50µA so R5 is rarely
required when using the Zetex part.
(Competitors versions can pass in
excess of 1mA before operating
correctly, hence the occasional need for
R5).
AN 27 - 2
Applications Note 27
Issue 1 June 1996
D1
+ve
+ve
OPT1
R5
* Note 1
Input
(From Converter
Transformer)
R1
220
R3
120k
C1
22nF
IC1
ZR431
Output
C2
220uF (5.0V)
R2
10k
R4
120k
-ve
-ve
Figure 2
Control Loop of a Switched Mode Power Supply :
C1, R2 and R5 are Optional Components.
Series Regulator Power Supply
Although shunt regulators such as the
circuit given in Figure 1 provide very
simple and low cost power supplies,
they are often inefficient because the
pass resistor R3 must be selected to
supply the maximum load expected at
the minimum input voltage. When the
actual load is lower or the input voltage
is higher than for these worst-case
conditions, excess current must be
shunted away by the ZR431. This is not
usually a problem for low output current
supplies or when efficiency is not a
major concern, but for loads greater
than a few milliamps losses can be
significant.
H o w e v e r , a s e r i e s r e g u l a to r c a n
significantly reduce power losses and
hence improve efficiencies. Series
regulators are usually much more
complex than shunt circuits, requiring a
reference, error amplifier, driver and
series pass elements. However, it is
possible to construct a very simple
serie s regulator using a ZR431 to
perform all but the pass transistor
functi on. F igure 3 shows a series
regulator for an automotive application.
The circuit has been designed to provide
30mA at 5V to a microcontroller, to
operate with a normal input supply
range of 7V-15V, to withstand
load-dump supply transients of 60V and
a sustained overvoltage input of 24V,
and tolerate reverse battery connection.
All parts used are surface-mount so the
supply can be constructed using little
PCB area.
AN 27 - 3
Applications Note 27
Issue1 June 1996
flows through R2 but not R1 thereby
causing a small error. For the Zetex
ZR431, this reference current is 200nA
maximum so as long as a current of at
least 20µA is passed through R1, setting
its maximum 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
input current into account:Vout = Iref x R2 + Vref
(R1+R2)
R1
Where Iref is the reference input current
of the ZR431.
R3 is selected so as to support the
maximum load current at minimum
input voltage, yet still maintain adequate
operating current for the ZR431. Since
the minimum cathode current of the
ZR431 is only 50µA, it rarely has an effect
on the choice of R3.
R3 =
Vin(min)−Vout
Iout(max)+Iz(min)
Where Iz(min) is the minimum cathode
current of the ZR431.
The low series resistance of the ZR431
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 frequencies and ensures stability
should the power supply’s load be
capacitive. For the Zetex ZR431, a
capacitor greater than 1.5nF will ensure
stability for any load. (Competitors’
versions of the ’431 generally require
values of 3.3µF or greater).
Switch-Mode Power Supply
Controller
The circuit shown in Figure 2 is
commonly used in the control loop of
sw itch-mode pow er suppl ies. The
output voltage is sensed via R3/R4 by the
ZR431, and the IC controls the current
passed through an opto-coupler and
hence feeds back output voltage status
to the switching regulator. The
minimum supply voltage on which this
popular circuit can be used is set by the
minimum cathode voltage of the
regulator IC, plus the forward voltage
drop of the opto-coupler LED. This limits
the minimum operating voltage of the
circuit to around 4V.
The components C1 and R2 are not
required for correct operation of the
ZR431. They have been included as they
are frequently needed to stabilise the
overall control loop of the switch-mode
power supply of which Figure 2 is only a
p a r t . The ZR431 is unconditionally
stable without additional components.
Note 1:
Resistor R5 is only necessary if the
minimum operating current of the ZR431
causes excess opto-coupler current
before the reference functions correctly.
The minimum operating current of the
ZR431 is only 50µA so R5 is rarely
required when using the Zetex part.
(Competitors versions can pass in
excess of 1mA before operating
correctly, hence the occasional need for
R5).
AN 27 - 2
Applications Note 27
Issue 1 June 1996
D1
+ve
+ve
OPT1
R5
* Note 1
Input
(From Converter
Transformer)
R1
220
R3
120k
C1
22nF
IC1
ZR431
Output
C2
220uF (5.0V)
R2
10k
R4
120k
-ve
-ve
Figure 2
Control Loop of a Switched Mode Power Supply :
C1, R2 and R5 are Optional Components.
Series Regulator Power Supply
Although shunt regulators such as the
circuit given in Figure 1 provide very
simple and low cost power supplies,
they are often inefficient because the
pass resistor R3 must be selected to
supply the maximum load expected at
the minimum input voltage. When the
actual load is lower or the input voltage
is higher than for these worst-case
conditions, excess current must be
shunted away by the ZR431. This is not
usually a problem for low output current
supplies or when efficiency is not a
major concern, but for loads greater
than a few milliamps losses can be
significant.
H o w e v e r , a s e r i e s r e g u l a to r c a n
significantly reduce power losses and
hence improve efficiencies. Series
regulators are usually much more
complex than shunt circuits, requiring a
reference, error amplifier, driver and
series pass elements. However, it is
possible to construct a very simple
serie s regulator using a ZR431 to
perform all but the pass transistor
functi on. F igure 3 shows a series
regulator for an automotive application.
The circuit has been designed to provide
30mA at 5V to a microcontroller, to
operate with a normal input supply
range of 7V-15V, to withstand
load-dump supply transients of 60V and
a sustained overvoltage input of 24V,
and tolerate reverse battery connection.
All parts used are surface-mount so the
supply can be constructed using little
PCB area.
AN 27 - 3
Applications Note 27
Issue1 June 1996
D1
Vout=Vref
+ve
BAS21
R3
1k8
C2
47pF
IC1
ZR431
-ve
(R1+R2)
R1
Vbe. (The ratio will require modification
should a nother transistor type be
substituted). The ratio of R1/R4 has been
adjusted to set the final output voltage
to 6.9V at 25°C. The exceptionally low
bias current of the ZR431 means that no
a l l o w a n c e n e e d b e t a k e n o f th e
regulators reference input current in this
circuit. Thanks to the low quiescent
current of the ZR431, the shunt reference
circuit will operate at currents down to
l e s s t h a n 2 0 0µA . N o t e , f o r b e s t
performance, the reference circuit
should be situated close to the battery
pack under charge to ensure
temperature tracking.
+Ve
Where Vref is the reference voltage of
the ZR431.
Q1
FMMT493
Input
(7 - 60V)
Applications Note 27
Issue 1 June 1996
+ve
R2
120k
C1 Output
10uF (5V)
R1
120k
-ve
Figure 3
S er i e s R e gula t or for A utomotive
Applications.
In this circuit, R3 provides base drive for
the series pass transistor Q1. The ZR431
senses the output voltage of the supply
via R1/R2, compares this with its internal
reference and shunts excess base drive
from Q1 so as to maintain the required
supply output. Reverse polarity
protection is provided by D1. The output
resistance of the supply is around 20mΩ.
Capacitor C1 helps maintain this very
low output impedance at high
frequencies. The stabilising capacitor C2
has been kept small so as not to degrade
the excellent high frequency
performance of the ZR431. The circuit
will supply a 30mA load at minimum
input and for higher inputs it can source
much more. Note that the circuit does
not include a current limit, and so care
must be taken not to short the output for
sustained periods.
The output of this type of supply can be
set to any voltage in the range 2.5V to
20V by adjusting the ratio of R1 and R2,
following the relationship:-
It may be noticed, that this is the same
formula give for the shunt regulator
circuit in Figure 1. The comments
concerning reference input current
made for the shunt regulator also apply
with this circuit.
R1
15k
R2
220k
R3 is selected so as to adequate base
drive to Q1 at the minimum input
voltage.
Temperature Coefficient
Compensated Regulator
There are often occasions when a
reference with a large and closely
defined temperature coefficient (T.C.)
are required. A common example of this
is in the management of rechargeable
batteries. Certain cell technologies such
a s L i th iu m a n d L ea d - a c id re q u i r e
charging to a set voltage to ensure they
are fully charged. Failure to do this
adequately can result in a reduction of
battery life and in extreme cases, cell
rupture. Unfortunately, this end of
charge voltage varies with temperature
and so the voltage reference controlling
charge termination must be temperature
compensated. Following is an example
of how an accurate, low temperature
coefficient regulator can be modified to
produce a temperature compensated
reference for lead-acid battery charging
applications.
By adding a transistor with a known
temperature coefficient to the reference
input circuit of the ZR431, a reference
AN 27 - 4
ZTX
108B
R3
56k
ZR431
R4
24k
-Ve
Figure 4
Temperature Coefficient Compensated
Regulator.
with the same T.C. as the lead-acid
batteries can be produced. Figure 4
shows a 6.9V shunt reference which has
a T.C. of -11.7mV/°C, matching a three
cell lead-acid battery pack terminal
voltage and temperature coefficient.
In this T.C. corrected reference circuit,
the ratio of R2/R3 sets the overall
temperature coefficient by amplifying
the well characterised negative
temperature coefficient of the ZTX108B
AN 27 - 5
Applications Note 27
Issue1 June 1996
D1
Vout=Vref
+ve
BAS21
R3
1k8
C2
47pF
IC1
ZR431
-ve
(R1+R2)
R1
Vbe. (The ratio will require modification
should a nother transistor type be
substituted). The ratio of R1/R4 has been
adjusted to set the final output voltage
to 6.9V at 25°C. The exceptionally low
bias current of the ZR431 means that no
a l l o w a n c e n e e d b e t a k e n o f th e
regulators reference input current in this
circuit. Thanks to the low quiescent
current of the ZR431, the shunt reference
circuit will operate at currents down to
l e s s t h a n 2 0 0µA . N o t e , f o r b e s t
performance, the reference circuit
should be situated close to the battery
pack under charge to ensure
temperature tracking.
+Ve
Where Vref is the reference voltage of
the ZR431.
Q1
FMMT493
Input
(7 - 60V)
Applications Note 27
Issue 1 June 1996
+ve
R2
120k
C1 Output
10uF (5V)
R1
120k
-ve
Figure 3
S er i e s R e gula t or for A utomotive
Applications.
In this circuit, R3 provides base drive for
the series pass transistor Q1. The ZR431
senses the output voltage of the supply
via R1/R2, compares this with its internal
reference and shunts excess base drive
from Q1 so as to maintain the required
supply output. Reverse polarity
protection is provided by D1. The output
resistance of the supply is around 20mΩ.
Capacitor C1 helps maintain this very
low output impedance at high
frequencies. The stabilising capacitor C2
has been kept small so as not to degrade
the excellent high frequency
performance of the ZR431. The circuit
will supply a 30mA load at minimum
input and for higher inputs it can source
much more. Note that the circuit does
not include a current limit, and so care
must be taken not to short the output for
sustained periods.
The output of this type of supply can be
set to any voltage in the range 2.5V to
20V by adjusting the ratio of R1 and R2,
following the relationship:-
It may be noticed, that this is the same
formula give for the shunt regulator
circuit in Figure 1. The comments
concerning reference input current
made for the shunt regulator also apply
with this circuit.
R1
15k
R2
220k
R3 is selected so as to adequate base
drive to Q1 at the minimum input
voltage.
Temperature Coefficient
Compensated Regulator
There are often occasions when a
reference with a large and closely
defined temperature coefficient (T.C.)
are required. A common example of this
is in the management of rechargeable
batteries. Certain cell technologies such
a s L i th iu m a n d L ea d - a c id re q u i r e
charging to a set voltage to ensure they
are fully charged. Failure to do this
adequately can result in a reduction of
battery life and in extreme cases, cell
rupture. Unfortunately, this end of
charge voltage varies with temperature
and so the voltage reference controlling
charge termination must be temperature
compensated. Following is an example
of how an accurate, low temperature
coefficient regulator can be modified to
produce a temperature compensated
reference for lead-acid battery charging
applications.
By adding a transistor with a known
temperature coefficient to the reference
input circuit of the ZR431, a reference
AN 27 - 4
ZTX
108B
R3
56k
ZR431
R4
24k
-Ve
Figure 4
Temperature Coefficient Compensated
Regulator.
with the same T.C. as the lead-acid
batteries can be produced. Figure 4
shows a 6.9V shunt reference which has
a T.C. of -11.7mV/°C, matching a three
cell lead-acid battery pack terminal
voltage and temperature coefficient.
In this T.C. corrected reference circuit,
the ratio of R2/R3 sets the overall
temperature coefficient by amplifying
the well characterised negative
temperature coefficient of the ZTX108B
AN 27 - 5
Applications Note 27
Issue1 June 1996
Appendix A
Partial Characterisation of ZR431. Full Characterisation available within the “High
Performance Linear Bipolar Integrated Circuits Data Book”.
ABSOLUTE MAXIMUM RATING
Cathode Voltage (VZ)
20V
Cathode Current
150mA
Operating Temperature
-40 to 85°C
Storage Temperature
-55 to 125°C
Recommended Operating Conditions
Min
Max
Cathode Voltage
Vref
20V
Cathode Current
50µA
100mA
Power Dissipation (Tamb=25°C)
SOT23
330mW
TO92
780mW
SOT223
2W
SO8
780mW
ELECTRICAL CHARACTERISTICS TEST CONDITIONS (Unless otherwise stated):Tamb=25°C
VALUE
PARAMETER
SYMBOL
Reference Voltage
2%
1%
V ref
UNITS
CONDITIONS
2.55
2.525
V
IL=10mA , VZ=V ref
MIN
TYP
MAX
2.45
2.475
2.50
2.50
Deviation of Reference
Input Voltage over
Temperature
V dev
8.0
17
mV
IL=10mA, VZ=Vref
Ta=full range
Ratio of the change in
Reference Voltage to the
Change in Cathode Voltage
∆ V ref
-1.85
-2.7
mV/V
VZ from Vref to 10V
IZ=10mA
-1.0
-2.0
mV/V
VZ from 10V to 20V
IZ=10mA
Reference Input Current
Iref
0.12
1.0
µA
R1=10K, R2=O/C,
lL=10mA
Deviation of Reference
Input Current over
Temperature
∆ Iref
0.04
0.2
µA
R1=10K, R2=O/C,
IL=10mA Ta=full range
Minimum Cathode Current
for Regulation
IZmin
35
50
µA
V Z=V ref
Off-state Current
IZoff
0.1
µA
V Z=20V, Vref =0V
0.75
Ω
V Z=V ref , f=0Hz
∆VZ
Dynamic Output Impedance RZ
AN 27 - 6
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