RESET_GENERATOR

Published:2009/7/14 2:57:00 Author:May | From:SeekIC

This VCC monitor circuit draws less than 10μA,yet it can generate reliable RESET signals. Very low quiescent current makes the reset generator shown suitable for use with microcontrollers that spend most of their time in the “sleep” mode .The system’s power Source,a charged 0.047-μF “Supercap” capacitor(C1),can go without a recharge for intervals exceeding24 hours .The allowable supply-voltage range for the reset circuit is 2 to 11 V. R1 and R2 make it possible to set the switching threshold as low as 1.2 V (the values shown provide a 4.10-V threshold). Hysteresis introduced by R3 minimizes the effect of reference noise and external EMI . Using the values gtven for R2 and R3 (1 MΩ and 10 MΩ),the switching threshold (Vt) is obtained by adjusting the value of R1 alone∶R1 (in megohms)=0.770Vt-0.910. External connections tie the internal reference voltage tothe inverting input of each comparator. Comparator A monitors the supply voltage .When this voltage dips below the threshold established by R1 and R2,the comparator output initiates a RESET pulse via D3 and discharges the timing capacitor （C3）via diode D1. Comparator B determines the pulse duration with help from the time-delay components R4 and C3.Because the duration （approximately 250 ms） is well above the 5-ms mlnlmum required in this case,the effect of supply voltage on pulse width call be ignored . D2 allows the circuit to respond to short outages by providing another quick-discharge path for the timing capacitor. Comparator C is a spare that supplies a complementary output or low-line warnlng. Comparator D acts as an active AND gate for logic-high signals from comparators A and B . The pull-up resistor in many such circuits is 100 kΩ or less (to avoid errors caused by the comparator’s input leakage). In this particular case,it would provide a path for excessive battery discharge during reset.



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