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MIC2753-LBM5 Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MIC2753-LBM5 Datasheet(HTML) 9 Page - Micrel Semiconductor |
9 / 12 page June 1999 9 MIC2753/2754 MIC2753/2754 Micrel Application Information Supply Transients The MIC2753/4 are inherently immune to very short “glitches” on V IN. In the case of very brief transients, VIN may drop below the reset threshold or exceed the overvoltage thresh- old without activating the RST, /RST or /HV output. As shown in the graphs of Figure 1, the narrower the transient, the deeper the threshold overdrive that will be ignored by the MIC2753/4. The lines on the graph represent the typical allowable transient duration for a given amount of threshold overdrive that will not generate a reset or overvoltage indica- tion. The data from which Figure 1 is derived was taken by adding negative-going square-wave pulses to a dc V IN set at 0.5V above or below the actual measured threshold for the part being characterized. In the case of the MIC2754L/M for example (refer to Figure 1), a transient on V IN which goes below the reset threshold by 100mV and lasts no more than 8.4 µs will typically not cause a reset to occur. Bypass capacitance placed as close as physically possible to the device’s VIN and GND pins will increase the transient immunity (0.1 µF, for example). Ensuring Proper Operation at Low Supply At levels of V IN below 1.5V, the MIC2754’s /RST output driver cannot turn on sufficiently to sink current and produce a valid logic-low on the /RST output. In this situation, other CMOS circuits driven by /RST could be allowed to float, causing undesired operation. (In most cases, however, it is expected that the circuits driven by the MIC2754 will be similarly inoperative at V CC ≤ 1.5V.) If a given application requires that /RST be valid below V IN = 1.5V, this can be accomplished by adding a pull-down resis- tor to the /RST output. A value of 100k Ω is recommended as this is usually an acceptable compromise of leakage current and pull-down current. The resistor’s value is not critical, however. The statements above also apply to the MIC2753’s RST output. That is, to ensure valid RST signal levels at V IN < 1.5V, a pull-up resistor (as opposed to a pull-down) should be added to the RST output. A value of 100k Ω is typical for this application as well. See Figure 2. Interfacing to Processors with Bidirectional Reset Pins Some microcontrollers and microprocessors utilize reset signal pins that are bidirectional in nature, rather than simply being input only. The Motorola 68HC11 family is one ex- ample. To use the MIC2753/4 with these processors, it is necessary to insert a resistor into the signal path between the MIC2753/4’s reset output and the microprocessor’s reset input. This prevents excessive current from flowing due to contention between the two drivers. The signal present at the processor’s reset pin will now be of limited drive capability, so it is necessary to buffer it in order to drive other circuits. This technique is shown in Figure 3. 0 10 20 30 40 1 10 100 1000 OVERDRIVE (mV) Reset Comparator Overdrive vs. Duration T A = 25°C A. Reset 0 1 2 3 4 5 6 7 8 1 10 100 1000 OVERDRIVE (mV) Overvoltage Comparator Overdrive vs. Duration T A = 25°C B. Overvoltage Figure 1. MIC2753/2754 Transient Immunity Behavior |
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