AN62

A Product Line of
Diodes Incorporated
AN62
Designing with Shunt Regulators - Other applications
Peter Abiodun A. Bode, Snr. Applications Engineer, Diodes Zetex Ltd
Introduction
Shunt regulators or voltage references can be applied to other applications beyond the obvious
PSU ones. Some of these are shown below.
A simple voltage comparator
V
R3
V+
Flag
IS H
Vin
R1
VREF
V+
Flag
VIN
TLV431
R2
VTH
GND
VKA(min)
t
0
R1 ⎞
⎛
VTH = VREF ⎜1 +
⎟
R
2⎠
⎝
R3 =
V + − VKA(min)
0.1mA ≤ I SH ≤ 15mA
ISH
Figure 1 Using the TLV431 as a level detector
In its open loop state, the 3-terminal reference is analogous to a line-powered comparator with its
non-inverting input internally connected to a reference voltage. This means the remaining inverting
input can be used for comparator functions.
Figure 1 above shows the TLV431 being used as a level comparator. Its output (Flag) is normally
high and goes low when the input reaches or exceeds the threshold (VTH) determined by R1 and R2.
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A window comparator
V
V+
R1H
R3
R1L
IS H
VRE F
VREF
REF2
R2H
Flag
V HI
V a lid
l id
sup ply
ra n ge
ge
REF1
V IINN
V LO
R2L
F la g
V K A (m in )
GND
t
0
V + − K A(min)
R1H ⎞
⎛
VHI = VREF ⎜1 +
⎟ R3 =
R 2H ⎠
I SH
⎝
R1L ⎞
⎛
VLO = VREF ⎜1 +
⎟
⎝ R 2L ⎠
0.1mA ≤ I SH ≤ 15mA
Figure 2 Window comparator for PSU supervision or Power-On Reset
An extended variation of Figure 1 is the use of two references to implement a window
comparator. It is effectively two level comparators in series. It is a circuit that gives an output
only when the input is within a window defined by a lower (VLO) and a higher (VHI) limit. The
window comparator is used either in general PSU supervision, status indicator or as a power-on
reset (POR) in many types of applications.
Circuit explanation
The graph shows how it works. At input voltages below VLO, both devices are off and so the
output (Flag) simply follows the input. At input voltages above VLO, REF1 switches on taking Flag
low. The circuit remains in this state until the input voltage reaches or exceeds VHI. At this point,
REF2 switches on, inhibiting the input to REF1 which therefore switches off causing Flag to go
high again.
Thus the flag represents an indication of the input voltage lying in the range of an acceptable
window.
Simple current sources
Vin
Q1
ZXTN2038F
Rs
R1
VREF
I R1
IB
⎛ hFE ⎞
⎜⎜
⎟⎟ + I R1
⎝ hFE + 1⎠
By making I R1 << VREF and hFE > 100
RS
Iout
REF1
0V
IOUT =
VREF
RS
I OUT ≈
VREF
RS
R1 =
VIN (min) − (VOUT (max) + VREF + VBE )
I KA(min) + I B
GND
Figure 3 Constant current source
Constant current circuits are used in many applications, e.g. relaxation oscillators, biasing
circuits, active loads, battery chargers, test and measurement, etc.
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Figure 3 above shows a very simple constant current source. The output current expression
includes two sources of error, one of which is the effect of the finite current gain, hFE, of transistor
Q1. This error can be minimised by using a transistor with the highest possible gain. If necessary
a Darlington pair may be used which will practically remove this error.
A far more dominant error source is the IR1 term in the expression. This is largely influenced by
the requirement of the reference device that is used. A reference with very small IKA(min) such as
the TLV431 will help in keeping this error current down to a minimum. Ultimately, this error term
cannot be got rid of and it puts a lower limit on how effectively the circuit can be used as a
constant current source for very small currents. Significant errors can be expected for currents
below 10mA.
A constant current source that does not have this problem is shown in Figure 4 below. It is more
appropriately a constant "current sink" and has eliminated the IR1 error altogether. IR1 still flows
and has same requirements but it is not seen by the load which is connected between VIN and
Q1's collector. The circuit is good enough down to at least 10µA or less depending on the transistor used.
Vin
Iout
IOUT =
R1
I R1
REF1
IB
Q1
ZXTN2038F
VREF
RS
⎛ hFE ⎞
⎟⎟
⎜⎜
⎝ hFE + 1⎠
By making
I OUT ≈
VREF
VREF
RS
VIN (max) − (VOUT (min) + VREF + VBE )
Rs
I KA(max) + I B
≤ R1 ≤
VIN (min) − (VOUT (max) + VREF + VBE )
I KA(min) + I B
GND
Figure 4 Constant current sink
Conclusion
The preceding examples illustrate the flexibility of 3-terminal voltage references beyond the
obvious and intended applications. These examples can either be used on their own or as
building blocks for more complex applications.
Recommended further reading
AN58 - Designing with Shunt Regulators - Shunt Regulation
AN59 - Designing with Shunt Regulators - Series Regulation
AN60 - Designing with Shunt Regulaors - Fixed Regulators and Opto-Isolation
AN61 - Designing with Shunt Regulators - Extending the operating voltage range
AN63 - Designing with Shunt Regulators - ZXRE060 Low Voltage Regulator
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The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit for
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or
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