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LM3705YBBPX-308 数据表(PDF) 9 Page - National Semiconductor (TI) |
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LM3705YBBPX-308 数据表(HTML) 9 Page - National Semiconductor (TI) |
9 / 17 page Circuit Information Reset Output The Reset input of a µP initializes the device into a known state. The LM3704/LM3705 microprocessor supervisory cir- cuits assert a forced reset output to prevent code execution errors during power-up, power-down, and brownout condi- tions. RESET is guaranteed valid for V CC > 1V. Once VCC ex- ceeds the reset threshold, an internal timer maintains the output for the reset timeout period. After this interval, reset goes high. The LM3704 offers an active-low RESET; The LM3705 offers an active-high RESET. Any time V CC drops below the reset threshold (such as during a brownout), the reset activates. When V CC again rises above the reset threshold, the internal timer starts. Reset holds until V CC exceeds the reset threshold for longer than the reset timeout period. After this time, reset releases. The Manual Reset input (MR) will initiate a forced reset also. See the Manual Reset Input section. Reset Threshold The LM3704/LM3705 family is available with a reset voltage of 3.08V. Other reset thresholds in the 2.20V to 5.0V range, in steps of 10 mV, are available; contact National Semicon- ductor for details. Manual Reset Input (MR) Many µP-based products require a manual reset capability, allowing the operator to initiate a reset. The MR input is fully debounced and provides an internal 56 k Ω pull-up. When the MR input is pulled below V MRT (1.225V) for more than 25 µs, reset is asserted after a typical delay of 12 µs. Reset remains active as long as MR is held low, and releases after the reset timeout period expires after MR rises above V MRT. Use MR with digital logic to assert or to daisy chain supervisory circuits. It may be used as another low-line comparator by adding a buffer. Power-Fail Comparator (PFI/PFO) The PFI is compared to a 1.225V internal reference, V PFT.If PFI is less than V PFT, the Power Fail Output PFO drops low. The power-fail comparator signals a falling power supply, and is driven typically by an external voltage divider that senses either the unregulated supply or another system supply voltage. The voltage divider generally is chosen so the voltage at PFI drops below V PFT several milliseconds before the main supply voltage drops below the reset thresh- old, providing advanced warning of a brownout. The voltage threshold is set by R 1 and R2 and is calculated as follows: Note this comparator is completely separate from the rest of the circuitry, and may be employed for other functions as needed. Low-Line Output (LLO) The low-line output comparator is typically used to provide a non-maskable interrupt to a µP when V CC begins falling. LLO monitors V CC and goes low when VCC falls below VLLOT (typically 1.02 • V RST) with hysteresis of 0.0032 • VRST. Special Precautions for the micro SMD Package As with most integrated circuits, the LM3704 and LM3705 are sensitive to exposure from visible and infrared (IR) light radiation. Unlike a plastic encapsulated IC, the micro SMD package has very limited shielding from light, and some sensitivity to light reflected from the surface of the PC board or long wavelength IR entering the die from the side may be experienced. This light could have an unpredictable affect on the electrical performance of the IC. Care should be taken to shield the device from direct exposure to bright visible or IR light during operation. Micro SMD Mounting The micro SMD package requires specific mounting tech- niques which are detailed in National Semiconductor Appli- cation Note AN-1112. Referring to the section Surface Mount Technology (SMT) Assembly Considerations,it should be noted that the pad style which must be used with the 9-pin package is the NSMD (non-solder mask defined) type. For best results during assembly, alignment ordinals on the PC board may be used to facilitate placement of the micro SMD device. www.national.com 9 |
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类似说明 - LM3705YBBPX-308 |
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