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MAX709MEPA Datasheet(PDF) 4 Page - Maxim Integrated Products |
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MAX709MEPA Datasheet(HTML) 4 Page - Maxim Integrated Products |
4 / 8 page Power-Supply Monitor with Reset 4 _______________________________________________________________________________________ __________Applications Information Negative-Going VCC Transients In addition to issuing a reset to the microprocessor (µP) during power-up, power-down, and brownout con- ditions, the MAX709 is relatively immune to short dura- tion negative-going VCC transients (glitches). Figure 1 shows typical transient duration vs. reset com- parator overdrive, for which the MAX709 does not gen- erate a reset pulse. The graph was generated using a negative-going pulse applied to VCC, starting 1.5V above the actual reset threshold and ending below it by the magnitude indicated (reset comparator over- drive). The graph indicates the typical maximum pulse width that a negative-going VCC transient may have without causing a reset pulse to be issued. As the magnitude of the transient increases (goes farther below the reset threshold), the maximum allowable pulse width decreases. Typically, for the MAX709L/MAX709M, a VCC transient that goes 100mV below the reset threshold and lasts 40µs or less will not cause a reset pulse to be issued. A 0.1µF bypass capacitor mounted as close as possible to pin 2 (VCC) provides additional transient immunity. Ensuring a Valid RESET Output Down to VCC = 0V When VCC falls below 1V, the MAX709 RESET output no longer sinks current—it becomes an open circuit. Therefore, high-impedance CMOS logic inputs con- nected to the RESET output can drift to undermined voltages. This presents no problem in most applica- tions, since most µP and other circuitry is inoperative with VCC below 1V. However, in applications where the RESET output must be valid down to 0V, adding a pull- down resistor to the RESET pin will cause any stray leakage currents to flow to ground, holding RESET low (see Figure 2). The resistance value of R1 is not criti- cal. It should be about 100k Ω, which is large enough not to load RESET and small enough to pull RESET to ground. ____________________Pin Description PIN NAME FUNCTION N.C. No Connect. There is no internal connection to this pin. 2 VCC +5V, +3.3V, or +3V Supply Voltage 3 GND Ground 7 RESET 1, 4, 5, 6, 8 Reset Output remains low while VCC is below the reset threshold, and for 280ms after VCC rises above the reset threshold. 150 0 10 100 10k 50 100 RESET COMPARATOR OVERDRIVE, VTH - VCC (mV) 1k MAX709L/M MAX709R/S/T TA = +25°C 10ns Figure 1. Maximum Transient Duration without Causing a Reset Pulse vs. Reset Comparator Overdrive |
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