AN2504 Application note Using STMicroelectronics Voltage Detectors 1 Introduction Voltage detectors are designed to monitor the system supply voltage and assert the output signal, OUT, every time it goes below a defined voltage threshold. The major advantages of these circuits are low current consumption and a precise temperature-compensated voltage reference, VREF (see Figure 1). STMicroelectronics voltage detectors are also laser programmed to the desired voltage threshold over the range of 1.6V to 6.0V in 100mV steps and they have a good transient immunity (see Figure 5). Figure 1. N-Channel Open Drain Output Block Diagram VCC VOUT R1 + – R2 VREF VSS R3 AI10482b Note that voltage detector output state simply indicates if the supply voltage is above or below a specific threshold, which can be used for early power fail warning. However microprocessors' inputs require some minimum input pulse width to register the change of signal on the logical input. In this case we recommend using reset circuits, which guarantee the minimum pulse width known as reset time-out period. Reset circuits have some other advantages compared to voltage detectors (e.g. specification across temperature, better voltage threshold accuracy, and lower temperature coefficient). On the other hand voltage detectors are usually able to operate at higher voltages, have lower current consumption, greater hysteresis and usually a lower price. The qualities mentioned above cause voltage detectors to be preferentially used in the following applications: • Battery Monitoring • Power-Supply Monitoring • Portable Medical Devices • Power Fail Detection • PDAs • Notebook Computers • Back-up Supply Switching • Portable/Battery- • Cell Phones. Powered Electronics February 2007 Rev 1 1/14 www.st.com Contents AN2504 - Application note Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3 Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4 Transient Immunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5 Open Drain Output Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 6 Modifying the Voltage Threshold, VTH– . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7 Simulation of the Reset Time-out Period . . . . . . . . . . . . . . . . . . . . . . . . 9 8 Output MOSFET protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 9 2/14 8.1 External N-channel transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 8.2 External P-channel transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 AN2504 - Application note List of figures List of figures Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. N-Channel Open Drain Output Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 STM1061N Active-Low, Open Drain typical Hardware Hookup . . . . . . . . . . . . . . . . . . . . . . 4 Voltage timing waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Maximum Transient Duration vs. Reset Threshold Overdrive . . . . . . . . . . . . . . . . . . . . . . . 6 Additional Transient Immunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Voltage Detectors with Wire OR Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Voltage Detector Monitors Different Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Modifying the Voltage Threshold, VTH– . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Hardware Hookup with RC element on the Voltage Detector Output . . . . . . . . . . . . . . . . . . 9 Voltage Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Circuit with external N-channel transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Circuit with external P-channel transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3/14 Description 2 AN2504 - Application note Description STMicroelectronics offers e.g. the STM1061 voltage detector. One of its typical uses is shown in the Figure 2. The STM1061N open drain output voltage detector is monitoring the 3.6V battery. This allows the voltage detector to detect a voltage drop early, assert the output signal (OUT) so the MCU can start safeguard routines even before the regulated supply voltage for the MCU starts to fall. The voltage detector open drain output sinks current when the output is asserted. It is necessary to connect a pull-up resistor from OUT to any supply voltage (see Figure 2). The resistor value must be large enough to register a logic low and small enough to register a logic high while all of the input current and leakage paths connected to the reset output line are being supplied. A10kΩ pull-up is sufficient in most applications. Figure 2. STM1061N Active-Low, Open Drain typical Hardware Hookup DC/DC Converter VIN VOUT Regulated 3V VSS R VCC 3.6V + STM1061N (Open Drain) OUT VCC VSS MCU VOUT Suspend VSS AI10484a 4/14 AN2504 - Application note 3 Operation Operation The voltage detector monitors VCC voltage input continuously (see Figure 1) and compares it with the precision voltage reference, V REF. When VCC falls below a specified trip point threshold, the output (OUT) is forced low and remains asserted as long as the VCC input remains below VTH+, where VTH+ = VTH– + VHYST (hysteresis) see Figure 3. Remember that a pull-up resistor on the voltage detector open-drain output is required for proper functionality (see Figure 2). Figure 3. Voltage timing waveform VCC VTH+ VTH– Release Voltage (VTH+ = VTH– + VHYST) Detect Voltage VCC (min) VSS VOUT tPD Detect Delay VSS tPR Release Delay AI10483 5/14 Transient Immunity 4 AN2504 - Application note Transient Immunity The STM1061 device is relatively immune to negative-going V CC transients (glitches). The graph (see Figure 4) indicates the maximum pulse width a negative VCC transient can have without asserting output signal, OUT. As the magnitude of the transient increases (further below the threshold), the maximum allowable pulse width decreases. Any combination of duration and overdrive which lies under the curve will NOT assert the output signal, OUT. A 0.1µF bypass capacitor, Cb, mounted as close as possible to the VCC pin provides additional transient immunity (see Figure 5). Figure 4. Maximum Transient Duration vs. Reset Threshold Overdrive 160 Transient Duration (µs) 140 VTH– = 3.4V 120 VTH– = 1.6V 100 80 60 40 20 0 1 10 100 1000 10000 Threshold Overdrive (mV) Figure 5. AI11123b Additional Transient Immunity VCC Cb R VCC STM1061N OUT Power Good VSS AI12905a 6/14 AN2504 - Application note 5 Open Drain Output Advantages Open Drain Output Advantages The advantages of open drain output are the ability to connect more open drain outputs in parallel (wired OR connections, see Figure 6) as well as connect the output to a power supply voltage different from VCC (see Figure 7). The hook up on Figure 6 monitors 2 independent supply voltages (3V and 5V). Every time either the first OR the second supply voltage goes below defined threshold, the input signal of the MCU, Suspend, goes low. Figure 6. Voltage Detectors with Wire OR Connection 3V VCC R VCC MCU STM1061N OUT Suspend VSS VSS 5V VCC STM1061N OUT VSS AI12906 A voltage detector with an open drain output can also monitor supply voltages different from the MCU supply voltage (see Figure 7). The logic high on the MCU Suspend input is adequate to supply voltage of the MCU. Figure 7. Voltage Detector Monitors Different Supply Voltage 5V 3V R VCC VCC MCU STM1061N OUT VSS Suspend VSS AI12907 7/14 Modifying the Voltage Threshold, VTH– 6 AN2504 - Application note Modifying the Voltage Threshold, VTH– Although the STM1061 voltage thresholds, V TH–, are adjusted in fine steps (100mV), it is sometimes necessary to make adjustments during prototyping. This can be achieved by connecting of external divider (see Figure 8). This hook up can be used if the required threshold of voltage detector is lower than the desired monitored voltage. To maintain detector accuracy, the current flow through the divider should be significantly higher than the 0.9µA operating current required by the STM1061. A 90µA bleeder current is sufficient in most applications. Figure 8. Modifying the Voltage Threshold, VTH– VSUPPLY R2 STM1061N VCC R1 OUT VSS AI12910 Example: Threshold of STM1061 detector: VTH– = 1.6V Desired threshold: VSUPPLY = 1.8V Recommended bleeder current: I = 90µA R1+R2 is then: (R1+R2) = VSUPPLY/I = 1.8V/90µA = 20kΩ The voltage on the divider is: R1 V CC = --------------------V = V TH – R 1 + R 2 SUPPLY That is why: V TH – 1.6V R 1 = ------------------------ ( R + R 2 ) = ------------20kΩ = 17.7kΩ 1.8V V SUPPLY 1 The value of R2 resistor is then: R 2 = 20kΩ – 17.7k Ω = 2.2kΩ We will choose the nearest values of the resistors (e.g. from E24): R1 = 18kΩ, R2 = 2.2kΩ. 8/14 AN2504 - Application note 7 Simulation of the Reset Time-out Period Simulation of the Reset Time-out Period It is possible to simulate reset time-out period with an RC element on the output of voltage detector (see Figure 9). Figure 9. Hardware Hookup with RC element on the Voltage Detector Output VCC VCC R VCC MCU STM1061N OUT Suspend C VSS VCAP VSS AI12911a If the supply voltage, V CC, drops below the defined voltage threshold, VRST, the capacitor, C, is discharged through the output transistor of the voltage detector (see Figure 10). When the supply voltage, VCC, rises above the voltage threshold plus hysteresis, VRST + VHYST, the output transistor disconnects the output from the ground and the capacitor, C, is charged through the resistor, R. Figure 10. Voltage Waveforms V VCC VRST – VHYST VRST VCAP VIH Capacitor is charging Capacitor is discharging t1 t2 t3 t AI12912 9/14 Simulation of the Reset Time-out Period AN2504 - Application note The voltage on the STM1061N output resp. capacitor, VCAP, rises as capacitor is charged. When it reaches input high voltage trip point of the MCU (time t3), the high state is detected. If there is no RC element on the output, the OUT pin goes high at the time t2. That is why simulated reset time-out period is t3-t2. Use following formula for determining the correct capacitor value: t C = -------------------------------------------V CC R ⋅ I n -------------------------V CC – V IH where, C = Capacitor of RC element in Farads, R = resistor of RC element in Ohms, t = desired time-out period (t3-t 2) in seconds, VCC = supply voltage, VIH = input voltage trip point for the MCU ln = natural logarithm. Example: The STM1061N27WX6F is used to monitor a 3V supply voltage. The MCU detects input high state at VIH =0.7·VCC = 0.7·3V = 2.1V. Desired time-out period is 140ms. An R = 100Ω resistor is chosen. The value of the capacitor is following: –3 –6 140 ⋅ 10 t C = ------------------------------------------ = --------------------------------------------------------- = 1.16 ⋅ 10 = 1.16µF 3 V CC 3 100 ⋅ 10 ⋅ I n ----------------R ⋅ I n -------------------------3 – 2.1 V CC – V IH The closest higher capacitance value from the most common E12 series (i.e. C = 1.2µF) is chosen. The time-out delay will be slightly greater by choosing a higher value of capacitor. Remember that this circuitry should be used only for temporary purposes. In the case of real need of time-out delay the usage of reset device is highly recommended as mentioned in the Introduction. 10/14 AN2504 - Application note 8 Output MOSFET protection Output MOSFET protection For applications requiring higher current drive capabilities an external MOSFET might be used, which protect the output of a voltage detector from overloading and destroying. It is possible to use either N channel or P channel transistor (see below). 8.1 External N-channel transistor If the load should be connected to the supply voltage during the regular operation (output, OUT, is not asserted), an external N-channel transistor will be needed (see Figure 11). When the supply voltage VCC drops below the threshold of the voltage detector, the output signal, OUT, is asserted, the external N-channel MOSFET is switched off and disconnects the load from the ground. Figure 11. Circuit with external N-channel transistor R VCC VCC RLOAD VCC STM1061N OUT VSS AI12913 11/14 Output MOSFET protection 8.2 AN2504 - Application note External P-channel transistor If the load should be connected to the supply voltage during the output, OUT, assertion, an external P-channel transistor will be needed (see Figure 12). When the supply voltage V CC drops below the threshold of the voltage detector, the output signal, OUT, is asserted, the external P-channel MOSFET is switched on and connects the load to the supply voltage, VCC. Figure 12. Circuit with external P-channel transistor R VCC VCC VCC STM1061N OUT RLOAD VSS AI12913a 12/14 AN2504 - Application note 9 Revision history Revision history Table 1. Document revision history Date Revision 06-Feb-2007 1 Changes Initial release. 13/14 AN2504 - Application note W Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST’s terms and conditions of sale. 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