ZXCT1081 ZXCT1080/ZXCT1081 ZXCT1080/1081 ZXCT1080 ZXTN2038F TS16949 D-81541 - Datasheet Archive

 

DN87 Maintaining a short term supply rail to the ZXCT1081 during a short-circuit load or overload event John McKiness, Field

 

DN87 DN87 Maintaining a short term supply rail to the ZXCT1081 ZXCT1081 during a short-circuit load or overload event John McKiness, Field Applications Engineer, Zetex Semiconductors Introduction Zetex ZXCT1080/ZXCT1081 ZXCT1080/ZXCT1081 high-side current monitors are capable of operating during short circuit current conditions. This design note shows how this can be achieved successfully and describes circuit considerstions with design examples. Powering the device To make sure that these devices continue to provide a valid output during a short circuit event, the ZXCT1080/1081 ZXCT1080/1081 is equipped with a dedicated supply pin, VCC, which is normally independent of the rail that is being monitored. There may however be times when VCC can not be made totally independent of the monitored supply. For example, VCC can be derived from the monitored supply as shown in Figure 1A. In this case, if a short circuit event causes the monitored rail to collapse, so will VCC. This makes it impossible for the current monitor to provide a valid output during the short circuit conditions. The scheme in Figure 1A (and Figure 2A) can only be used if VSUPPLY lies between 4.5V and 12V. Figure 1B (or Figure 2B) will have to be used if VSUPPLY is greater than 12V. In both Figure 1B and Figure2B, Z1 is used to clamp VCC to an acceptable value whilst allowing VSUPPLY to range up to its design maximum. RS V SUPP LY RS VSUPPLY R1 S+ V CC GND S+ VCC GND SV OU T SVOUT Z1 V OUT 10V ZXCT1080 ZXCT1080 ZXCT1081 ZXCT1081 A VOUT ZXCT1080 ZXCT1080 ZXCT1081 ZXCT1081 B Figure 1 Issue 1 - March 2008 © Zetex Semiconductors plc 2008 - Powering Vcc from the monitored supply 1 www.zetex.com DN87 Solution In most cases, all that will be needed is for VOUT to remain valid long enough, following a short circuit or overload event, for action to be taken, e.g. for a microprocessor to read the output and either shut down the supply or raise a flag. The required hold-up time in such cases only needs to be of the order of 1ms to 100ms. This can be achieved by providing a hold-up time on the VCC pin as illustrated in Figure 2 below. In both circuits, Q1 is configured as a diode with relatively high reverse breakdown. Capacitor C1 is rapidly charged on application of VSUPPLY. If VSUPPLY collapses due to an overload, Q1 goes into blocking mode and C1 maintains supply to the VCC pin for the time it takes it to discharge below the limit of operation of the current monitor. The only task is to choose an appropriate value of C1, the hold-up capacitor. ICC S+ VCC GND C1 220nF Q1 S- ZXTN2038F ZXTN2038F ZXTN2038F ZXTN2038F Q1 RS VSUPPLY RS V SU PP LY R1 S+ VCC GND IR1 VOUT C1 1µF V OUT ZX C T 1080 ZX C T 1081 A SVOUT Z1 VOUT 10V ZXCT1080 ZXCT1080 ZXCT1081 ZXCT1081 B Figure 2 - Implementing hold-up time Specifying C1 Specifying C1 is reasonably straightforward. Although C1 discharges exponentially, a worst case scenario can be considered by assuming that it discharges with a constant current. Example 1. Assume the following conditions: Vsupply 10V ICC(max) 120 µA Vcc(min) 4.5V thold-up 10ms From C = I t V www.zetex.com 2 Issue 1 - March 2008 © Zetex Semiconductors plc 2008 DN87 C1 = 120 µA 10ms = 218nF 5.5V Hence a 220nF will give a hold-up time well in excess of the required 10ms. Example 2 (Figure 2B) The calculation is identical for Figure 2B except that IR1 is used instead of ICC. Since IR1 will be an order of magnitude higher than ICC, it can be expected that C1 will be correspondingly higher. Also, VSUPPLY in this case must be appropriately higher than 10V to maintain VCC at 10V. For example, assuming that IR1 = 1.2 mA, the computed value of C1 becomes 2.18 µF. Therefore, a 2.2 µF capacitor would be used. Again, the achieved hold-up time will be much longer than 10 ms because the discharge current will drop down to ICC once the capacitor voltage drops below VZ1. Reasons for choosing Q1 A normal diode could have been used and the circuit would still work. However the ZXTN2038F ZXTN2038F is used for a number of reasons: 1. It can handle relatively high peak currents, up to 2A compared to 500mA, or less for most signal diodes. This is important when it comes to charging C1 quickly without degrading either the diode (Q1 in this case) or start-up time. 2. ZXTN2038F ZXTN2038F has a ICES(max) of only 100nA. It therefore offers a guaranteed low reverse leakage diode which aids achieving maximum hold-up time from a given capacitance value. 3.In place of a diode (or Q1), a resistor might be used instead. However, as well as a reduced holdup time, there will also be a start-up delay which may or may not be acceptable. Conclusion The ZXCT1080 ZXCT1080 and ZXCT1081 ZXCT1081 can be used to monitor short circuit events. By making use of the independent VCC pin in a controlled way, the short circuit signal can be maintained even though the monitored supply has collapsed. Improved performance can be achieved by using a bipolar transistor to charge the hold-up capacitor Recommended further reading · AN39 - Current Measurement Applications Handbook Issue 1 - March 2008 © Zetex Semiconductors plc 2008 3 www.zetex.com DN87 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 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. Zetex sales offices Europe Americas Asia Pacific Corporate Headquarters Zetex GmbH Kustermann-park Balanstraße 59 D-81541 D-81541 München Germany Telefon: (49) 89 45 49 49 0 Fax: (49) 89 45 49 49 49 europe.sales@zetex.com Zetex Inc 700 Veterans Memorial Highway Hauppauge, NY 11788 USA Zetex (Asia Ltd) 3701-04 Metroplaza Tower 1 Hing Fong Road, Kwai Fong Hong Kong Zetex Semiconductors plc Zetex Technology Park, Chadderton Oldham, OL9 9LL United Kingdom Telephone: (1) 631 360 2222 Fax: (1) 631 360 8222 usa.sales@zetex.com Telephone: (852) 26100 611 Fax: (852) 24250 494 asia.sales@zetex.com Telephone: (44) 161 622 4444 Fax: (44) 161 622 4446 hq@zetex.com © 2008 Published by Zetex Semiconductors plc www.zetex.com 4 Issue 1 - March 2008 © Zetex Semiconductors plc 2008