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LM3723EM5X-4.63 Datenblatt(PDF) 9 Page - National Semiconductor (TI) |
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LM3723EM5X-4.63 Datenblatt(HTML) 9 Page - National Semiconductor (TI) |
9 / 12 page Applications Information (Continued) TABLE 1. Monitored Tolerance Table Reset Threshold Supply Voltage 2.5V 3.3V 5.0V 4.63 ± % 90.8-94.4% 3.08 ± % 91.8-95.2% 2.32 ± % 90.8-94.8% Ensuring a Valid Reset Output Down to V CC =0V When V CC falls below 1V, the LM3722 RESET output is unable to sink the rated current. A high-impedance CMOS logic input connected to RESET can therefore drift to unde- termined voltages. To prevent this situation, a 100k Ω resistor should be connected from the RESET output to ground, as shown in Figure 1. A 100k Ω pull-up resistor to V CC is also recommended for the LM3723, if RESET is required to remain valid for V CC < 1V. Open Drain Output (LM3724) An open drain output allows easy paralleling of multiple microprocessor reset circuits without requiring additional logic gates. Open drain outputs also allow interfacing de- vices of differing logic levels or families, since the output pull-up resistor may be connected to any supply voltage up to 5.5V, regardless of LM3724 V CC. The pull up resistor is calculated so that maximum current flow into RESET is less than 10 mA when activated. The resistor must be small enough so that the leakage current of all connected devices does not create an excessive voltage drop when the output is not activated. A resistor value of 100 k Ω will generally suffice. Negative-Going V CC Transients The LM3722/LM3723/LM3724 are relatively immune to short negative-going transients or glitches on V CC. Figure 2 shows the maximum pulse width a negative-going V CC transient can have without causing a reset pulse. In general, as the magnitude of the transient increases, going further below the threshold, the maximum allowable pulse width decreases. Typically, a V CC transient that goes 125 mV below the reset threshold and lasts 40 µs or less will not cause a reset pulse. A 0.1 µF bypass capacitor mounted as close as possible to the V CC pin will provide additional transient rejection. Interfacing to µPs with Bidirectional Reset Pins Microprocessors with bidirectional reset pins, such as the Motorola 68HC11 series, can be connected to the LM3722 RESET output. To ensure a correct output on the LM3722 even when the microprocessor reset pin is in the opposite state, connect a 4.7k Ω resistor between the LM3722 RESET output and the µP reset pin, as shown in Figure 3. Buffer the LM3722 RESET output to other system components. 20014410 FIGURE 1. Circuit for RESET Valid from V CC =0V 20014414 FIGURE 2. Maximum Transient Duration without Causing a Reset Pulse vs. Reset Comparator Overdrive www.national.com 9 |
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