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TC1272AMVNBTR Scheda tecnica(PDF) 4 Page - Microchip Technology |
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TC1272AMVNBTR Scheda tecnica(HTML) 4 Page - Microchip Technology |
4 / 12 page TC1272A DS21877A-page 4 2004 Microchip Technology Inc. 3.0 APPLICATIONS INFORMATION 3.1 VDD Transient Rejection The TC1272A provides accurate VDD monitoring and reset timing during power-up, power-down and brown- out/sag conditions. These devices also reject negative- going transients (glitches) on the power supply line. Figure 3-1 shows the maximum transient duration vs. maximum negative excursion (overdrive) for glitch rejection. Any combination of duration and overdrive that lies under the curve will not generate a reset signal. FIGURE 3-1: Maximum Transient Duration vs. Overdrive for Glitch Rejection at +25°C. Combinations above the curve are detected as a brown-out or power-down condition. Transient immu- nity can be improved by adding a capacitor in close proximity to the VDD pin of the TC1272A. 3.2 RESET Signal Integrity During Power-Down The TC1272A RESET output is valid to VDD = 1.0V. Below this voltage, the output becomes an "open cir- cuit" and does not sink current. This means CMOS logic inputs to the microcontroller will be floating at an undetermined voltage. Most digital systems are com- pletely shut down well above this voltage. However, in situations where RESET must be maintained valid to VDD = 0V, a pull-down resistor must be connected from RESET to ground to discharge stray capacitances and hold the output low (Figure 3-2). This resistor value, though not critical, should be chosen such that it does not appreciably load RESET under normal operation (100 k Ω will be suitable for most applications). FIGURE 3-2: The addition of R1 at the RESET output of the TC1272A ensures that the RESET output is valid to VDD = 0V. Reset Comparator Overdrive [V TH - VDD] (mv) 400 240 160 320 80 0 1 5 100 1000 T A = +25°C V TH Duration Overdrive V DD TC1272A/L/M/J TC1272A/Z/R/S/T [V TH - VDD] (mv) 30 40 50 60 70 80 90 100 110 120 130 1 10 100 1000 Reset Comparator Overdrive (mV) TC1272A/L/M/J TC1272A/Z/R/S/T TC1272A V DD V DD R1 100 k Ω RESET GND |
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