Application Note AN0023 Device: ICS1708 Description: Variabl
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Application Note AN0023
Device: ICS1708
Description: Variable Pulse Width 525mA Switchmode Charger cell NiCd/NiMH Theory Operation
Purpose schematic provided illustrates variable pulse width (hysteretic), switch-mode current source using cost, input offset current, LM311 comparator ICS1708 provides precision charge control. shown, circuit charges from cell NiCd/NiMH battery packs without modification when powered from 12VDC automotive regulated 12VDC input. charger also powered from unregulated 12VDC wall unit rated greater. Depending unregulated 12VDC wall unit used, 33uF input capacitor shown have increased factor unregulated wall units tend minimum amount filter capacitance resulting much ripple voltage content. Charger Operation LM311 comparator senses charge current using resistor divider network each side high side current sense resistor. LM311 open collector output switches high current typically ramps between 440mA 610mA, producing 525mA average charge current. divider action load side results LM311 output switches between 5.7V ground. 5.7V produced when LM311 output pull-up resistor input voltage forward bias 1N4148/BAS16 diode above output produced ICS1708 When ICS1708 output 2N7000/2 NFET switched LM311pin LM311 goes switch PFET 2N7000/2 NFET long ICS1708 output high (5V). LM311 controls time PFET into 82uH inductor regulating charge current 525mA typical. 68.1K resistor from LM311 input output, along with 30.1K resistor from LM311 input 2.5V created 332/332 divider across creates about 42.5 hysteresis. result 170mA tooth current 440mA level. 440mA level plus 170mA triangular current produces typical 525mA average charge current. Typical Performance When powered from 12VDC automotive, regulated, unregulated wall unit input with proper filtering, circuit produces 525mA typical operating 16KHz higher short output 10.2V minimum (1.70V/cell cell pack). between these extremes, switching frequency typically 60KHz 100KHz range.
3/22/02 Dave Whitmer Galaxy Power, Inc. Manager, Product Applications (whitmer@galaxypower.com)
Rev:04/04/02
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Rev:04/04/02
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