Philip Simpson, Maxim Integrated
Application Note 1186
When a processor-controlled device must be guaranteed to operate, designers often choose to reset the processor periodically rather than rely on a watchdog circuit. In low-power systems the periodic reset generator can consume a large part of the system current budget or may not be guaranteed to operate at low voltages.
This application note describes a low-power reset generator that generates a low going reset pulse of 100 µS duration once per second, consumes less than 1 µA and will operate from 1.8 V to 5 V with only slight variation of the output period (Figure 1).
The circuit is an adaptation of a normal relaxation oscillator with a differentiator and diode clamp on the output to generate the 100 µS low going pulse. The pulse width can be adjusted by varying CP or RP and the polarity changed by repositioning D1. The period can be adjusted by varying R1 or C1.
The 350 nA supply current, 1.8 V-5.5 V supply voltage range and SOT23 package make the MAX919 ideal for this application. Measurements for the complete circuit give operating currents of less than 1 µA at 25 °C, which would allow the circuit to operate from a single AA lithium cell for 250 years!
With careful component choice this circuit is able to generate periods from mS to minutes. To ensure good temperature stability R1 and RP should be metal film and C1 and CP should be NP0 type capacitors. Assuming a CMOS type input, high impedance and with a logic threshold of 30% of the supply rail, then the following formulas can be used to adjust the output pulse width and period:
Pulse width ≡ 0.36 RPCP
Measured pulse width:
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