DN77

DN77
Transient and noise protection for Zetex ZXCT series
current monitors
by Peter Abiodun Bode, Snr. Applications Engineer
Introduction
Transients are changes in current or voltage which occur in short duration.
Tranients can be both internal and external and may not only damage components but also cause
a complete system failure if the circuit doesn't have sufficient protection. Transients can either
cause catastrophic failures, partial failures or progressive damage to components over a period
of time. Internally generated transients may often be energy stored in inductive or capacitive
elements which is then released if operating conditions change. Such a transient is created every
time an inductive element is switched within a circuit, for example a motor, contactor or relay.
Typical internal transient sources are load dumps, inductive switching or ignition pulses.
The interference source is generally referred to as noise when it is not severe enough to cause
catastrophic failures but may interfere, or has the potential to interfere, with normal workings of
the system.
The Zetex range of current monitors may need to be protected where such transient noise exists.
A number of methods are available to do this and these are explored in this document.
Methods of protection
Prevention
Prevention is always better than cure. Hence preventing the transient in the first place, if this is
possible, is better than trying to cope with it. Figure 1 shows the ZXCT1008/9 monitoring current
through an inductive load. Without the freewheel diode D1, the inductance of the load would have
generated a large voltage spike when the transistor switches off. The inclusion of the diode to
clamp this voltage and the way it is connected ensures that the current through the coil and
current monitor would decay to zero in a controlled manner and the current monitor never sees
any noise.
Output series resistance
One method that is particularly suitable for the Zetex 3-terminal devices is again shown in Figure 1.
Since these are current output devices, it means that the output transfer function is not adversely
affected by the addition of a protective resistance, RLIM, in series with the output pin provided
certain circuit parameters are taken into consideration.
For low to moderate transient voltages this resistance, RLIM, is all that is needed for protection.
This needs to be carefully specified as it needs to be high enough to be functional but not too high
to compromise normal operation of the device. At the lower end of its value, it is limited by the
maximum current that can be supplied by the device. Whilst at the higher end, the limiting factor
is the available circuit compliance. The two limits are determined as follows.
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DN77
D1
RS
L1
ILOAD
VSUPPLY
VSENSE
2
3
S+
Z1
S-
ZXCT1008 & 9
OUT
1
IOUT
RLIM
VOUT
Controller
Q1
RG
Figure 1
Monitoring current through an inductive load
VPK − VMAX
I OUT (max)
Equation 1
RG (VSUPPLY (min) − (VDO + VOUT (max) ) )
Equation 2
RLIM (min) =
RLIM (max) =
VOUT (max)
where,
VPK = Peak transient voltage to be withstood
VMAX = Maximum operating voltage (20V in most cases)
IOUT(max) = Max continuous output current (25mA for ZXCT1008/9)
VSUPPLY(min) = Minimum supply operating voltage,
VDO = Drop-out voltage
VOUT(min) = Maximum required output voltage
For practical determination of RLIM, since the value has to lie between these two limits, pick a
value nearest mid-point between them. The mid-point value for RLIM is calculated from,
RLIM =
RLIM (min) + RLIM (max)
Equation 3
2
Adding a zener diode across the current monitor as shown provides better protection by ensuring
that the voltage across it is clamped to a known safe value regardless of the amplitude of the
transient voltage or RLIM.
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RC filtering
If the noise is of a relatively low level, a simple RC filter could be used for protection as shown in
Figure 2 and Figure 3 below. This method could be used for any of the current monitors and these
two examples specifically demonstrate this for the ZXCT1050 and generically for all others.
VSUPPLY
ILOAD
RS
RG
GT
C1
VCC
ILOAD
R1
C1
S+
SVCC ZXCT1050
GND
OUT
S+
SZXCT10xx
GND
IOUT
OUT
IOUT
VOUT
VOUT
RG
Figure 2
RS
VSUPPLY
RG
RC noise suppression for
ZXCT1050
Figure 3 RC noise suppression for a
generic current monitor
Notice in Figure 2 that the transconductance resistor is also used as the snubber resistor whilst in
most other cases this would be an additional component.
Design example
Consider an application where a 3 Ampere current needs to be measured. 130V transient spikes
(Figure 4) are present in the system and a 5V output is required. The supply voltage ranges from
12V to 16V. The device is ZXCT1009 and an output resistance of 10k is required.
Figure 4
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±130V interference
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DN77
Solution:
The solution uses the ZXCT1009 as shown in Figure 5 below.
•
First determine RG as follows
Set VSENSE to 100mV (at 3A this would mean a loss of 300mW in RS)
Hence,
RS =
VSENSE 0.1
=
= 33mΩ
I LOAD
3
Therefore,
I OUT = GT ⋅ RS ⋅ I LOAD = 0.001 ⋅ 0.033 ⋅ 3 = 0.99mA
Giving,
RG =
•
VOUT (max)
I OUT
Next determine RLIM(MIN) and RLIM(MAX):
RLIM (min) =
RLIM (max) =
VPK − VMAX 130V − 20V
=
= 4.4kΩ
I OUT (max)
25mA
RG (VSUPPLY (min) − (VDO + VOUT (max) ) )
=
VOUT (max)
5.05(12 − (2.5 + 5))
= 4.545kΩ
5
As the two values are very close, either value can be used. Hence RLIM = 4.4k - 4.5k.
RS
VSUPPLY
ILOAD
VSENSE
2
3
S+
SZXCT1009
OUT
1
IOUT
RLIM
VOUT
RG
Figure 5
Solution to worked example
Note, that the two values can converge which is also acceptable. If, as can sometimes happen,
RLIM(min) is larger than RLIM(max), this would indicate that the maximum output voltage, VOUT(max),
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DN77
required from the circuit is too high for the set of circumstances. This can be corrected by doing
any or all of the following.
•
Reduce VOUT(max) this means lowering the value of RG.
•
Increase minimum supply voltage VSUPPLY(min).
•
Reduce IOUT (note that, in cases where the transconductance is internally fixed, this can only
be done by reducing RS which will result in an increase in error which may not be desirable).
Conclusion
Protection of current monitors from transients and external noise can be easily achieved with the
addition of a few components.
Recommended further reading
1. AN39 - Current Measurement Applications Handbook
2. AN45 - High voltage current monitoring using the ZXCT series in power supplies
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DN77
Definitions
Product change
Zetex Semiconductors 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 Zetex 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. Zetex 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
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 Zetex Semiconductors plc. 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
labelling 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.
Terms and Conditions
All products are sold subjects to Zetex’ terms and conditions of sale, and this disclaimer (save in the event of a conflict between the two
when the terms of the contract shall prevail) according to region, supplied at the time of order acknowledgement.
For the latest information on technology, delivery terms and conditions and prices, please contact your nearest Zetex sales office .
Quality of product
Zetex 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 Zetex Semiconductors or one of our
regionally authorized distributors. For a complete listing of authorized distributors please visit: www.zetex.com/salesnetwork
Zetex Semiconductors 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.
Green compliance
Zetex Semiconductors 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 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
“Last time buy (LTB)”
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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