SWITCHES MULTIPLEXERS Keywords: analog switch, analogue switch, r
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Maxim Notes BATTERY MANAGEMENT
SWITCHES MULTIPLEXERS
Keywords: analog switch, analogue switch, relay power dissapation, boost pull-in pick-up voltage, dropout voltage
2002
APPLICATION NOTE
Analog Switch Lowers Relay Power Consumption
Abstract: Design idea outlining method analog switch discrete components reduce power dissipated relay actuation. Relays often used electrically controlled switches. Unlike transistors, their switch contacts electrically isolated from control input. other hand, power dissipation relay coil unattractive battery-operated applications. lower this dissipation adding analog switch that allows relay operate lower voltage (Figure
Figure Analog switch lowers relay power dissipation. Power consumed relay coil equals circuit lowers this dissipation (after actuation) applying less than normal operating voltage Note that voltage required turn relay (pick-up voltage) greater than that required keep (dropout voltage). relay shown 3.5V pick-up voltage 1.5V dropout voltage, circuit allows operate from intermediate supply voltage 2.5V. Table compares relay's power dissipation with fixed operating voltages across with Figure circuit place. Table Power Dissipated Relay Voltage (normal operating voltage) 3.5V (pick-up voltage) 2.5V (circuit Figure Current 90mA 63mA 45mA Total Power Dissipation 450mW 221mW 112mW
When close SW1, current flows relay coil, begin charge. relay remains inactive because supply voltage less than pick-up voltage. time constants such that charges almost completely before voltage across reaches logic threshold analog switch. When reaches that threshold, analog switch connects series with 2.5V supply relay coil. This action turns relay boosting voltage across coil (twice supply voltage). discharges through coil, coil voltage drops back 2.5V minus drop across relay remains because that voltage above relay's dropout voltage (1.5V). Component values this circuit depend relay characteristics supply voltage. value which protects analog switch from initial current surge through should sufficiently small allow charge rapidly, large enough prevent surge current from exceeding peak current specified analog switch. U1's peak current 400mA, peak surge current IPEAK (VIN VD1)/(R1 RON), where on-resistance analog switch (typically 1.2). value depends relay characteristics difference between relay's pick-up voltage. Relays that need more turn-on energy require larger values. values selected allow charge almost completely before C2's voltage reaches logic threshold analog switch. this case, time constant C2R2 about seven times C1(R1 RON). Larger C2R2 values increase delay between switch closure relay activation. similar version this article appeared December 2001 issue magazine.
Application Note 956: http://www.maxim-ic.com/an956 More Information technical questions support: http://www.maxim-ic.com/support samples: http://www.maxim-ic.com/samples Other questions comments: http://www.maxim-ic.com/contact Related Parts MAX4624: QuickView Full (PDF) Data Sheet Free Samples
AN956, 956, APP956, Appnote956, Appnote Copyright Maxim Integrated Products Additional legal notices: http://www.maxim-ic.com/legal
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