SM72240 SM72240 SolarMagic 5-Pin Microprocessor Reset Circuit Literature Number: SNIS154B SM72240 SolarMagic 5-Pin Microprocessor Reset Circuit General Description Features The SM72240 microprocessor supervisory circuit monitors the power supplies in microprocessor and digital systems. It provides a reset to the microprocessor during power-up, power-down, brown-out conditions, and manual reset. The SM72240 asserts a reset signal whenever the supply decreases below the factory-programmed reset threshold. Reset will be asserted for at least 100ms even after VCC rises above the reset threshold. The SM72240 has an active-low open-drain RESET output. The SM72240 is suitable for monitoring 5V. With a low supply current of only 6µA, the SM72240 is ideal for use in portable equipment. The SM72240 is available in the 5-pin SOT23 package. ■ Renewable Energy Grade ■ Precise monitoring of 5V supply voltages ■ Fully specified over temperature −40°C to +125°C ■ 100 ms minimum Power-On Reset pulse width, 190 ms ■ ■ ■ ■ typical: Active-Low RESET Open Drain Output Guaranteed RESET Output valid for VCC ≥ 1V Low Supply Current, 6µA typical Power supply transient immunity Compatible with MAX811/812 applications Applications ■ ■ ■ ■ ■ Microprocessor Systems Computers Controllers Intelligent Instruments Portable/Battery-Powered Equipment Typical Application Circuit 30141437 © 2011 National Semiconductor Corporation 301414 www.national.com SM72240 SolarMagic 5-Pin Microprocessor Reset Circuits June 1, 2011 SM72240 Connection Diagram SOT23-5 30141402 Pin Descriptions Pin Name 1 GND Ground reference Function 2 GND Ground reference, device substrate, connect to ground. 3 RESET 4 MR Active-low input. Reset is asserted whenever this pin is pulled low and remains asserted for 190 ms after the MR pin goes high. May be left open. 5 VCC Supply Voltage (+5V, nominal) Active-low output. RESET remains low while VCC is below the reset threshold voltage, and for 190 ms after VCC rises above the reset threshold voltage. Ordering Information Reset Threshold (V) Supplied as 250 units, tape & reel Supplied as 1000 units, tape & reel Supplied as 3000 units, tape & reel 4.63 SM72240MFE–4.63 SM72240MF–4.63 SM72240MFX–4.63 3.08 SM72240MFE–3.08 SM72240MF–3.08 SM72240MFX–3.08 www.national.com 2 Package Type Package Top Mark NSC Package SOT23-5 2240 MF05A SOT23–5 R133 MF05A SM72240 Block Diagram SM72240 Block Diagram 30141435 3 www.national.com SM72240 ESD Rating (Note 2) 2kV Continuous Power Dissipation (TA = +70°C) SOT-23 (Note 3) 320mW Operating Temperature Range −40°C to +125°C Maximum Junction Temperature 125°C Storage Temperature Range −65°C to +160°C Lead Temperature (soldering, 10sec) +300°C Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VCC, MR RESET, RESET Input Current, VCC Pin Output Current, RESET, RESET Pin −0.3V to 6.0V −0.3V to (VCC+ 0.3V) 20mA 20mA Electrical Characteristics Typical values are at TA = +25°C. Limits with standard typeface are for TA = +25°C, and limits in boldface type apply for the operating temperature range −40°C to +125°C , unless otherwise noted. (Note 4) Symbol VCC ICC VTH Parameter Conditions Min VCC Range Supply Current (ILOAD = 0A) 1.0 SM72240-3.08 VCC = 3.6V 7 10 SM72240-4.63 4.54 4.50 4.63 4.72 4.75 SM72240-3.08 3.03 3.00 3.08 3.14 3.15 30 VCC to Reset Delay (Note 5) Reset Active Timeout Period 100 tMR MR Minimum Pulse Width 10 tMD MR to Reset Propagation Delay VCC = VTH to (VTH − 100mV) VCC > VTH(MAX) RESET Output Leakage Current (SM72240) ms µs 2 µs 100 ns 0.8 22 V kΩ VCC = VTH min, ISINK = 3.2 mA 0.4 VCC > 1V, ISINK = 50µA 0.3 VCC > VTH, RESET = 5.5V V µs 560 2.3 MR Pull-Up Resistance IIN 190 µA ppm/°C 20 MR Glitch Immunity (Note 6) VOL V 15 tRP RESETOutput Voltage Low 5.5 8 Reset Threshold MR Input Threshold Units VCC = 5.5V tRD VIL Max SM72240-4.63 VTH Reset Threshold Temperature Tempco Coefficient VIH Typ 0.5 V µA Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which the device operates correctly. Operating ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the Electrical Characteristics. Note 2: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. Note 3: At elevated temperatures, devices must be derated based on package thermal resistance. The device must be derated at 4.5mW/°C at ambient temperatures above 70°C. The device has internal thermal protection. Note 4: Production testing done at TA = +25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical Quality Control (SQC) methods. Note 5: RESET Output. Note 6: Glitches of 100 ns or less typically will not generate a reset pulse. www.national.com 4 SM72240 Typical Performance Characteristics Supply Current vs VCC Supply Current vs Temperature 30141407 30141403 Reset Timeout vs VCC Reset Timeout vs Temperature 30141408 30141404 Normalized VTH vs Temperature VOL vs Current 30141416 30141405 5 www.national.com SM72240 Low VCC Characteristics 30141406 Timing Diagram 30141412 www.national.com 6 RESET OUTPUT The reset input of a µP initializes the device into a known state. The SM72240 microprocessor voltage monitoring circuit asserts a forced reset output to prevent code execution errors during power-up, power-down, and brownout conditions. RESET is guaranteed valid for VCC ≥ 1V. Once VCC exceeds the reset threshold, an internal timer maintains the output for the reset timeout period. After this interval, reset goes high and the microprocessor initializes itself into a known state. RESET is active low. As VCC drops below the reset threshold (such as during a brownout), the reset activates (see the Negative-Going VCC Transients section). When VCC again rises above the reset threshold, the internal timer starts. Reset holds until VCC exceeds the reset threshold for longer than the reset timeout period. After this time, reset releases. Additionally, the Manual Reset input (MR) will initiate a forced reset. See the Manual Reset Input section. The SM72240 reset output ignores short duration glitches on VCC and MR. See the Applications Information section for details. TABLE 1. Monitored Tolerance Table Reset Threshold Supply Voltage 3.3V 4.63 ± % 3.08 ± % 5.0V 90.8-94.4% 91.8–95.2% ENSURING A VALID RESET OUTPUT DOWN TO VCC = 0V When VCC falls below 1V, the SM72240 RESET output is unable to sink the rated current. A high-impedance CMOS logic input connected to RESET can therefore drift to undetermined voltages. To prevent this situation, a 100kΩ resistor should be connected from the RESET output to ground, as shown in Figure 1. RESET THRESHOLD The SM72240 is available with a reset voltage of 4.63V or 3.08V which are suitable for monitoring 5.0V or 3.3V supplies respectively. 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 22 kΩ pull-up. When the MR input is pulled below VIL (0.25VCC) for more than 100 ns, reset is asserted after a typical delay of 2 µs. Reset remains active as long as MR is held low, and releases after MR rises above VIH and the reset timeout period expires. Use MR with digital logic to assert reset or to daisy chain supervisory circuits. 30141410 FIGURE 1. Circuit for RESET Valid from VCC = 0V OPEN DRAIN OUTPUT An open drain output allows easy paralleling of multiple microprocessor reset circuits without requiring additional logic gates. Open drain outputs also allow interfacing devices of differing logic levels or families, since the output pull-up resistor may be connected to any supply voltage up to 5.5V, regardless of VCC. The pull up resistor is calculated so that maximum current flow into RESET is less than 10 mA when activated. The resistor must be small enough so that the leakage current of all connected devices does not create an excessive voltage drop when the output is not activated. A resistor value of 100 kΩ will generally suffice. 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 SM72240 uses highly accurate 7 www.national.com SM72240 circuitry to ensure that the reset threshold occurs only within this range (for 5.0V and 3.3V supplies). Table 1 shows how the standard reset threshold applies to 5.0V and 3.3V nominal supply voltages. Circuit Information SM72240 NEGATIVE-GOING VCC TRANSIENTS The SM72240 is relatively immune to short negative-going transients or glitches on VCC. Figure 2 shows the maximum pulse width a negative-going VCC 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, a VCC transient that goes 125 mV below the reset threshold and lasts 40 µs or less will not cause a reset pulse. A 0.1 µF bypass capacitor mounted as close as possible to the VCC pin will provide additional transient rejection. 30141414 FIGURE 2. Maximum Transient Duration without Causing a Reset Pulse vs. Reset Comparator Overdrive www.national.com 8 SM72240 Physical Dimensions inches (millimeters) unless otherwise noted 5-Lead SOT23-5 For Ordering, refer to Ordering Information table NS Package Number MF05A 9 www.national.com SM72240 SolarMagic 5-Pin Microprocessor Reset Circuits Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Design Support Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage References www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Applications & Markets www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise® Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic PLL/VCO www.national.com/wireless www.national.com/training PowerWise® Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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