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MIC2753-TBM5 Datasheet(PDF) 8 Page - Micrel Semiconductor |
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MIC2753-TBM5 Datasheet(HTML) 8 Page - Micrel Semiconductor |
8 / 12 page MIC2753/2754 Micrel MIC2753/2754 8 June 1999 Functional Description The MIC275x family of devices are typically used to monitor power supplies of intelligent circuits such as microcontrollers and microprocessors. By connecting the appropriate reset output of an MIC275x to the reset input of a microcontroller or microprocessor, the processor will be properly reset at power- on, and during power-down and brownout conditions. In addition, asserting /MR (manual reset input) activates the reset function. Reset Outputs The /RST and/or RST output is asserted any time /MR is asserted or if V IN deviates beyond the preset threshold voltages, V THU and VTHO. Reset output(s) remain asserted for 140ms minimum after V IN returns within the threshold boundaries and/or /MR is released. A minimum 140ms reset pulse is also generated at power-on. Hysteresis is included in the overvoltage and undervoltage comparators to prevent chattering of the outputs due to noise. The MIC275x family offers a choice of two reset output options: the MIC2753 has an active-high RST output, and the MIC2754 has an active-low /RST output. Manual Reset Input The ability to initiate a reset from external logic or a manual switch is provided in addition to the MIC275x’s automatic supervisory functions. Driving the /MR input low causes an immediate and unconditional reset to occur. Assuming V IN is within the tolerance thresholds when /MR is released (inter- nally pulled to logic high), the reset output will be deasserted no less than 140ms later. /MR may be driven by a CMOS or TTL logic signal or a mechanical switch. Typically, a momen- tary push-button switch is connected such that /MR is shorted to ground when the contacts close. Switch debouncing is performed internally; the switch may be connected directly between /MR and GND. /MR has an internal pull-up to V IN and may be left open if unused. Overvoltage Output The /HV and reset outputs are immediately and uncondition- ally asserted any time V IN exceeds VTHO (overvoltage thresh- old). /HV is an active-low, open-drain logic output. Overvolt- age on a system power supply rail is generally considered to be a catastrophic condition which may cause permanent system damage. If and when an overvoltage condition occurs using a MIC275x, the system is immediately driven into reset mode and /HV (overvoltage flag), is driven active. Typically, an overvoltage signal, such as /HV, is connected to a “crowbar circuit” using a latching power device such as an SCR. See “Typical Application”. The crowbar circuit will clamp the power rail to some low voltage by shunting power supply current to ground. This should cause some upstream power supply protection device to be activated, such as fold- back current limiting, a fuse, or circuit breaker. In this way, the system is most likely to be protected against damage and spurious operation until the condition can be remedied. /HV is an open-drain output and may be wire-ORed with other open-drain logic signals. Some systems may require a pull- up resistor. System Block Diagram Reset Switch Voltage Monitor and Reset Generator Microprocessor System Circuitry Reset(s) Crowbar Circuit HV |
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