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SS8039L438TCTR Datenblatt(PDF) 6 Page - Silicon Standard Corp. |
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SS8039L438TCTR Datenblatt(HTML) 6 Page - Silicon Standard Corp. |
6 / 7 page www.SiliconStandard.com 6 of 7 Rev.2.02 1/06/2004 SS8039 Applications Information Negative-Going VCC Transients In addition to issuing a reset to the µP during power-up, power-down, and brownout conditions, the SS8039H and SS8039L are relatively immune to short-duration negative-going VCC transients (glitches). Figure1 shows typical transient durations, vs. the reset comparator overdrive, for which both the SS8039H and SS8039L do not generate a reset pulse. The graph was generated using a negative-going pulse applied to VCC, starting 0.5V above the actual reset threshold and ending below it by the magnitude indicated (reset comparator overdrive). The graph indicates the maximum pulse width a negative-going VCC transient can have without causing a reset pulse. As the magnitude of the transient increases (goes farther below the reset threshold), the maximum allowable pulse width decreases. For the SS8039x463 and SS8039x438, a VCC transient that goes 100mV below the reset threshold and lasts 15µs or less will not typically cause a reset pulse. A 0.1µF bypass capacitor mounted as close as possible to the VCC pin provides additional transient immunity. Ensuring a Valid Reset Output Down to VCC = 0 When VCC falls below 1V, the SS8039 RESET output no longer sinks current—it becomes an open circuit. Therefore, high-impedance CMOS logic inputs con- nected to RESET can drift to undetermined voltages. This presents no problem in most applications since most µP and other circuitry is inoperative with VCC below 1V. However, in applications where RESET must be valid down to 0V, adding a pull-down resistor to RESET causes any stray leakage currents to flow to ground, holding RESET low (Figure 2). R1’s value is not critical; 100k Ω is large enough not to load RESET and small enough to pull RESET to ground. Benefits of Highly Accurate Reset Threshold Most µP supervisor ICs have reset threshold voltages between 5% and 10% below the value of nominal supply voltages. This ensures a reset will not occur within 5% of the nominal supply, but will occur when the supply is 10% below nominal. When using ICs rated at only the nominal supply ±5%, this leaves a zone of uncertainty where the supply is between 5% and 10% low, and where the reset may or may not be asserted. The SS8039x uses highly accurate circuitry to ensure that reset is asserted close to the 5% limit, and long before the supply has declined to 10% below nominal. |
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