MIC2777 Micrel, Inc. MIC2777 Dual Micro-Power Low Voltage Supervisor General Description Features The MIC2777 is a dual power supply supervisor that provides under-voltage monitoring, manual reset capability, and poweron reset generation in a compact 5-pin SOT package. Features include two under-voltage detectors, one fixed and one adjustable, and both active-high and active-low reset outputs. One under-voltage detector compares VDD against a fixed threshold. Ten factory-programmed thresholds are available. The other under-voltage detector is user-adjustable. The reset outputs are asserted for no less than 140ms at power-on and any time VDD or the input voltage drops below the corresponding reference voltage. They remain asserted for the timeout period after the input voltage and VDD subsequently rise back above the threshold boundaries. A reset can be generated at any time by pulling down on the adjustable input. Hysteresis is included to prevent chattering due to noise. Typical supply current is a low 3.5µA. • Monitors two independent power supplies for under-voltage conditions • One fixed and one user adjustable input • 1.5% theshold accuracy • Choice of factory-programmed thresholds • User-adjustable input can monitor supplies as low as 0.3V • Generates 140ms (minimum) power-on RESET pulse • Manual reset capability • Both active-high and active-low RESET outputs • Input may be pulled above VDD (abs. max.) • /RST output valid down to 1.2V • Ultra-low supply current, 3.5µA typical • Rejects brief input transients • IttyBitty™ 5-pin SOT-23 package Applications • Monitoring processor, ASIC, or FPGA core and I/O voltages • Computer systems • PDAs, hand-held PCs • Embedded controllers • Telecommunications systems • Power supplies • Wireless / Cellular systems • Networking hardware Ordering Information Part Number Standard Marking Pb-Free MIC2777-XXBM5 UNXX Marking MIC2777-XXYM5 Typical Application Junction Tem. Range Package –40ºC to +85ºC SOT-23-5 UNXX MICROPROCESSOR VCORE VCORE 1.0V VI/O VI/O 1.8V R1 MIC2777 VDD /RST IN R2 /RESET GND RST GND Manual Reset OTHER LOGIC Typical Application IttyBitty™ is a trademark of Micrel, Inc. Micrel, Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com November 2005 1 MIC2777 MIC2777 Micrel, Inc. Standard Voltage Options* Voltage Code Typical Application (VDD) Nominal Threshold Voltage (VTH) 44 5.0V ±10% 4.43 31 3.3V ±5% 3.09 29 3.3V ±10% 2.93 28 3.0V ±5% 2.81 26 2.85V ±5% 2.67 25 2.70V ±5% 2.53 23 2.5V ±5% 2.34 22 2.4V ±5% 2.25 17 1.8V ±5% 1.69 46 5.0V ±5% 4.68 *There are ten standard versions available with an order increment of 3000 pieces. Samples of standard versions are normally available from stock. Contact factory for information on non-standard versions. Available in tape-and-reel only. Pin Configuration RST GND /RST 3 2 1 4 5 IN VDD SOT-23-5 (M5) Pin Description Pin Number Pin Name 1 /RST Digital (Output): Asserted low whenever VIN or VDD falls below the reference voltage. It will remain asserted for no less than 140ms after VIN and VDD return above the threshold limits. 2 GND Ground 3 RST 4 IN Digital (Output): Asserted high whenever VIN or VDD falls below the reference voltage. It will remain asserted for no less than 140ms after VIN and VDD return within above the threshold limit. 5 VDD MIC2777 Pin Function Analog (Input): The voltage on this pin is compared to the internal 300mV reference. An undervoltage condition will trigger a reset sequence. Manual reset capability can be achieved by adding a switch between this pin and ground. Analog (Input): Power supply input for internal circuitry and input to the fixed voltage monitor: The voltage on this pin is compared against the internal voltage reference. An undervoltage condition will trigger a reset sequence. 2 November 2005 MIC2777 Micrel, Inc. Absolute Maximum Ratings (Note 1) Operating Ratings (Note 2) Supply Voltage (VDD).......................................–0.3V to +7V Input Voltage (VIN) ...........................................–0.3V to +7V RST, (/RST) Current ................................................... 20mA Storage Temperature (TS) ........................ –65°C to +150°C ESD Rating, Note 3 .................................................... 1.5kV Supply Voltage (VDD) .................................. +1.5V to +5.5V Input Voltage (VIN) .............................................................–0.3V to +6.0V Ambient Temperature Range (TA) .............. –40°C to +85°C Package Thermal Resistance (θJA) ....................... 256°C/W Electrical Characteristics Note 5; TA = +25°C, bold values indicate –40°C ≤ TA ≤ +85°C; unless noted Symbol Parameter Condition IDD Supply Current VDD = VIN Note 5; /MR, RST, /RST open Min Typ Max Units 3.5 µA VDD VOLTAGE THRESHOLD VHYST Under-Voltage Threshold On VDD Hysteresis Voltage VTH-1.5% VTH VTH+1.5% 295 300 305 1 V % IN, UNDER-VOLTAGE DECTECTOR INPUT VREF Under-Voltage Threshold IIN Input Current VHYST Hysteresis Voltage RST, /RST OUTPUTS tPROP Propagation Delay tRST Reset Pulse Width VOL VOH RST or /RST Output Voltage Low RST or /RST Output Voltage High 3 VIN = (VREF(MAX) + 100mV) to VIN = (VREF(MIN) – 100mV) or VDD = (VTH + 2.5%) to (VTH – 2.5%), VIN =VDD ISINK = 100µA; VDD ≥ 1.2V, Note 4 ISOURCE = 500µA; VDD ≥ 1.5V pA 10 20 140 ISINK = 1.6mA; VDD ≥ 1.6V RST asserted, ISOURCE = 10µA; VDD ≥ 1.2V, Note 4 nA µs 280 ms 0.3 V 0.3 V 0.8VDD V 0.8VDD V Note 1. Exceeding the absolute maximum rating may damage the device. Note 2. The device is not guaranteed to function outside its operating rating. Note 3. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF. Note 4. VDD operating range is 1.5V to 5.5V. Output is guaranteed to be asserted down to VDD = 1.2V. Note 5. mV 5 TMIN ≤ TA ≤ TMAX mV VDD equals the nominal “Typical Application (VDD)” as shown in “Standard Voltage Options Table.” November 2005 3 MIC2777 MIC2777 Micrel, Inc. Timing Diagram VDD 0V VIN A VHYST VTH A A VHYST VREF 0V V/RST (ACTIVE LOW) VRST (ACTIVE HIGH) VOH VOL VOH tRST tRST VOL Propagation delays not shown for clarity. Note A. MIC2777 The MIC2777 ignores very brief transients. See “Applications Information” for details. 4 November 2005 MIC2777 Micrel, Inc. Functional Diagram VDD R VREF Q /RST S /Q RST One Shot IN Delay Line VREF MIC2777 GND Functional Description RST, /RST Reset Output Typically, the MIC2777 is used to monitor the power supplies of intelligent circuits such as microcontrollers and microprocessors. By connecting the appropriate reset output of a MIC2777 to the reset input of a µC or µP, the processor will be properly reset at power-on and during power-down and brown-out conditions. The reset outputs are asserted any time VDD or VIN drops below the corresponding threshold voltage. The reset outputs remain asserted for tRST(min) after VIN and/or VDD subsequent return above the threshold boundaries and/or /MR is released. A reset pulse is also generated at power-on. Manual Reset The ability to initiate a reset via external logic or a manual switch is provided in addition to the MIC2777’s automatic supervisory functions. Typically, a momentary push-button switch is connected such that IN is shorted to ground when the switch contacts close. Assuming VDD and VIN are within tolerance when the switch is released, the reset outputs will be de-asserted no less than 140ms later. IN can also be driven by an open-drain or open-collector logic signal. IN, Under-Voltage Detector Input The voltage present at the IN pin is compared to the internal 300mV reference voltage. A reset is triggered if and when VIN falls below VREF. Typically, a resistor divider is used to scale the input voltage to be monitored such that VIN will fall below VREF as the voltage being monitored falls below the desired trip-point. Hysteresis is employed to prevent chattering due to noise. The comparator on the IN input is relatively immune to very brief negative-going transients. VDD Input The VDD pin is both the power supply terminal and a monitored input voltage. The voltage at this pin is continually compared against the internal reference. The trip-point at which a reset occurs is factory programmed. A reset is triggered if and when VDD falls below the trip-point. Hysteresis is employed to prevent chattering due to noise. The comparator on the VDD input is relatively immune to very brief negative-going transients. November 2005 5 MIC2777 MIC2777 Micrel, Inc. Application Information To summarize, the various potential error sources are: • Variation in VREF: specified at ±1.5% • Resistor tolerance: chosen by designer (typically ≤ ±1%) • Input bias current, IIN: calculated once resistor values are known, typically very small Taking the various potential error sources into account, the threshold voltage will be set slightly below the minimum VCORE specification of 0.950V so that when the actual threshold voltage is at its maximum, it will not intrude into the normal operating range of VCORE. The target threshold voltage will be set as follows: Given that the total tolerance on VTH for the IN pin is [VREF tolerance] + [resistor tolerance] = ±1.5% + ±1% = ±2.5%, and VTH(max) = VCORE(min), then VCORE(min) = VTH + 2.5% VTH = 1.025 VTH, therefore, solving for VTH results in VCORE(min) 0.950 = 0.9268V VTH = = 1.025 1.025 Programming the Voltage Threshold Referring to the “Typical Application Circuit”, the voltage threshold on the IN pin is calculated as follows: VTH = VREF × (R1+ R2) R2 where VREF = 0.300V In order to provide the additional criteria needed to solve for the resistor values, the resistors can be selected such that the two resistors have a given total value, that is, R1 + R2 = RTOTAL. Imposing this condition on the resistor values provides two equations that can be solved for the two unknown resistor values. A value such as 1MΩ for RTOTAL is a reasonable choice since it keeps quiescent current to a generally acceptable level while not causing any measurable errors due to input bias currents. The larger the resistors, the larger the potential errors due to input bias current (IIN). The maximum recommended value of RTOTAL is 3MΩ. Applying this criteria and rearranging the VTH expression to solve for the resistor values gives: R2 = (RTOTAL )( VREF ) Solving for R1 and R2 using this value for VTH and the equations above yields: R1 = 676.3kΩ ≈ 673kΩ R2 = 323.7kΩ ≈ 324kΩ The resulting circuit is shown in Figure 1. Input Bias Current Effects Now that the resistor values are known, it is possible to calculate the maximum potential error due to input bias current, IIN. As shown in the “Electrical Characteristics” table, the maximum value of IIN is 10nA. (Note that the typical value is a much smaller 5pA) The magnitude of the offset caused by IIN is given by: VERROR = IIN(max) × (R1||R2) = VERROR = ±1 × 10-8A × 2.189 ×105Ω = VERROR = ±2.189 × 10-3V = VERROR = ±2.189mV VTH R1 = RTOTAL – R2 Application Example Figure 1 illustrates a hypothetical MIC2777-23 application in which the MIC2777-23 is used to monitor the core and I/O supplies of a high-performance CPU or DSP. The core supply, VCORE, in this example is 1.0V ±5%. The main power rail and I/O voltage, VI/O, is 2.5V ±5%. As shown in Figure 1, the MIC2777 is powered by VI/O. The minimum value of VI/O is 2.5V –5% = 2.375V; the maximum is 2.5V +5% = 2.625V. This is well within the MIC2777’s power supply range of 1.5V to 5.5V. Resistors R1 and R2 must be selected to correspond to the VCORE supply of 1.0V. The goal is to insure that the core supply voltage is adequate to insure proper operation, i.e., VCORE ≥ (1.0V –5%) = 0.950V. Because there is always a small degree of uncertainty due to the accuracy of the resistors, variations in the devices’ voltage reference, etc., the threshold will be set slightly below this value. The potential variation in the MIC2777’s input voltage reference (VREF) is specified as ±1.5%. The resistors chosen will have their own tolerance specification. This example will assume the use of 1% accurate resistors. The potential worst-case error contribution due to input bias current can be calculated once the resistor values are chosen. If the guidelines above regarding the maximum total value of R1+R2 are followed, this error contribution will be very small thanks to the MIC2777’s very low input bias current. MIC2777 The typical error is about three orders of magnitude lower than this– close to one microvolt! Generally, the error due to input bias can be discounted. If it is to be taken into account, simply adjust the target threshold voltage downward by this amount and recalculate R1 and R2. The resulting value will be very close to optimum. If accuracy is more important than the quiescent current in the resistors, simply reduce the value of RTOTAL to minimize offset errors. 6 November 2005 MIC2777 Micrel, Inc. VCORE 1.0V 5% VI/O 2.5V 5% R1 676k 1% R2 324k 1% Ensuring Proper Operation at Low Supply At levels of VDD below 1.2V, the MIC2777’s /RST output cannot turn on sufficiently to produce a valid logic-low on /RST. In this situation, other 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 MIC2777 will be similarly inoperative at VDD ≤ 1.2V.) If a given application requires that /RST be valid below VDD = 1.2V, this can be accomplished by adding a pull-down resistor to the /RST output. A value of 100kΩ is recommended as this is usually an acceptable compromise of quiescent current and pull-down current. The resistor’s value is not critical, however. See Figure 3. The statements above also apply to the MIC2777’s RST output. That is, to ensure valid RST signal levels at VDD<1.2V, a pull-up resistor (as opposed to a pull-down) should be added to the RST output. A value of 100kΩ is typical for this application as well. See Figure 4. MICROPROCESSOR VCORE VI/O MIC2777-23 VDD /RST IN RST /RESET GND GND Figure 1. MIC2777 Example Design Transient response The MIC2777 is inherently immune to very short negativegoing “glitches.” Very brief transients may exceed the voltage thresholds without tripping the output. As shown in Figure 2a and 2b, in general the narrower the transient, the deeper the threshold overdrive that will be ignored by the MIC2777. The graph represents the typical allowable transient duration for a given amount of threshold overdrive that will not generate a reset. Typical INPUT Transient Response 40 MICROPROCESSOR VCC VCC R1 R2 35 MIC2777-XX VDD /RST IN RST /RESET GND 100k Rpull-down GND 30 25 20 Figure 3. MIC2777 Valid /RST Below 1.2V 15 10 5 0 MICROPROCESSOR VCC VCC 0 100 200 300 RESET COMP. OVERDRIVE, VREF–VIN(mV) R1 Figure 2a. Typical INPUT Transient Response R2 Typical V 100k Rpull-up RESET GND GND DD 100 MIC2777-XX VDD RST IN /RST Transient Response 80 Figure 4. MIC2777 Valid RST Below 1.2V 60 40 20 0 0 500 1000 1500 2000 RESET COMP. OVERDRIVE, VREF–VDD(mV) Figure 2b. Typical VDD Transient Response November 2005 7 MIC2777 MIC2777 Micrel, Inc. Package Information SOT-23-5 (M5) MICREL INC. TEL 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA + 1 (408) 944-0800 FAX + 1 (408) 474-1000 WEB http://www.micrel.com This information furnished by Micrel in this data sheet is believed to be accurate and reliable. However no responsibility is assumed by Micrel for its use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer. Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser's use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser's own risk and Purchaser agrees to fully indemnify Micrel for any damages resulting from such use or sale. © 2000 Micrel, Inc. MIC2777 8 November 2005