### 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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AN62
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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AN62
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.
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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AN62
Definitions
Product change
Diodes Incorporated reserves the right to alter, without notice, specifications, design, price or conditions of supply of any product or
service. Customers are solely responsible for obtaining the latest relevant information before placing orders.
Applications disclaimer
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
the user’s application and meets with the user’s requirements. No representation or warranty is given and no liability whatsoever is
assumed by Diodes Inc. with respect to the accuracy or use of such information, or infringement of patents or other intellectual property
rights arising from such use or otherwise. Diodes Inc. does not assume any legal responsibility or will not be held legally liable (whether
in contract, tort (including negligence), breach of statutory duty, restriction or otherwise) for any damages, loss of profit, business,
contract, opportunity or consequential loss in the use of these circuit applications, under any circumstances.
Life support
Diodes Zetex products are specifically not authorized for use as critical components in life support devices or systems without the express
written approval of the Chief Executive Officer of Diodes Incorporated. As used herein:
A. Life support devices or systems are devices or systems which:
1. are intended to implant into the body
or
2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the
labeling can be reasonably expected to result in significant injury to the user.
B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to
cause the failure of the life support device or to affect its safety or effectiveness.
Reproduction
The product specifications contained in this publication are issued to provide outline information only which (unless agreed by the
company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contract or be regarded as a
representation relating to the products or services concerned.
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two when the terms of the contract shall prevail) according to region, supplied at the time of order acknowledgement.
Quality of product
Diodes Zetex Semconductors Limited is an ISO 9001 and TS16949 certified semiconductor manufacturer.
To ensure quality of service and products we strongly advise the purchase of parts directly from Diodes Inc. or one of our regionally
authorized distributors. For a complete listing of authorized distributors please visit: www.zetex.com or www.diodes.com
Diodes Inc. does not warrant or accept any liability whatsoever in respect of any parts purchased through unauthorized sales channels.
ESD (Electrostatic discharge)
Semiconductor devices are susceptible to damage by ESD. Suitable precautions should be taken when handling and transporting devices.
The possible damage to devices depends on the circumstances of the handling and transporting, and the nature of the device. The extent
of damage can vary from immediate functional or parametric malfunction to degradation of function or performance in use over time.
Devices suspected of being affected should be replaced.
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Diodes Inc. is committed to environmental excellence in all aspects of its operations which includes meeting or exceeding regulatory
requirements with respect to the use of hazardous substances. Numerous successful programs have been implemented to reduce the use
of hazardous substances and/or emissions.
All Diodes Zetex components are compliant with the RoHS directive, and through this it is supporting its customers in their compliance
with WEEE and ELV directives.
Product status key:
“Preview”
Future device intended for production at some point. Samples may be available
“Active”
Product status recommended for new designs
Device will be discontinued and last time buy period and delivery is in effect
“Not recommended for new designs” Device is still in production to support existing designs and production
“Obsolete”
Production has been discontinued
Datasheet status key:
“Draft version”
This term denotes a very early datasheet version and contains highly provisional information, which
may change in any manner without notice.
“Provisional version”
This term denotes a pre-release datasheet. It provides a clear indication of anticipated performance.
However, changes to the test conditions and specifications may occur, at any time and without notice.
“Issue”
This term denotes an issued datasheet containing finalized specifications. However, changes to
specifications may occur, at any time and without notice.
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