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LM809M3X-2.45 Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LM809M3X-2.45 Datasheet(HTML) 6 Page - National Semiconductor (TI) |
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6 / 8 page  Applications Information Benefits of Precision Reset Thresholds A microprocessor supply supervisor must provide a reset output within a predictable range of the supply voltage. A common threshold range is between 5% and 10% below the nominal supply voltage. The 4.63V and 3.08V options of the LM809/810 use highly accurate circuitry to ensure that the reset threshold occurs only within this range (for 5V and 3.3V supplies). The other voltage options have the same tight tolerance to ensure a reset signal for other narrow monitor ranges. See Table 1 for examples of how the standard reset thresholds apply to 3V, 3.3V, and 5V nominal supply volt- ages. TABLE 1. Reset Thresholds Related to Common Supply Voltages Reset Threshold 3.0V 3.3V 5.0V 4.63 ± 3% 90 - 95% 4.38 ± 3% 85 - 90% 4.00 ± 3% 78 - 82% 3.08 ± 3% 90 - 95% 2.93 ± 3% 86 - 90% 2.63 ± 3% 85 - 90% 77 - 81% 2.45 ± 3% 79 - 84% 72 - 76% Ensuring a Valid Reset Output Down to V CC =0V When V CC falls below 1V, the LM809 RESET output no longer sinks current. A high-impedance CMOS logic input connected to RESET can therefore drift to undetermined 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 LM810, if RESET is required to remain valid for V CC < 1V. Negative-Going V CC Transients The LM809/810 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, for the 4.63V and 4.38V version of the LM809/810, aV CC transient that goes 100mV below the reset threshold and lasts 20µs 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 LM809 RESET output. To ensure a correct output on the LM809 even when the microprocessor reset pin is in the opposite state, connect a 4.7k Ω resistor between the LM809 RESET output and the µP reset pin, as shown in Figure 3. Buffer the LM809 RESET output to other system components. LLP Mounting The LLP package requires special mounting techniques which are detailed in National Semiconductor Application Note AN-1187. Referring to the section PCB Design Recom- mendations, it should be noted that the pad style which should be used with the LLP package is the NSMD (non- solder mask defined) type. 10105710 FIGURE 1. RESET Valid to V CC = Ground Circuit 10105709 FIGURE 2. Maximum Transient Duration without Causing a Reset Pulse vs. Reset Comparator Overdrive 10105711 FIGURE 3. Interfacing to Microprocessors with Bidirectional Reset I/O www.national.com 6 |
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