SECTION 7 OVERVOLTAGE EFFECTS ON ANALOG INTEGRATED CIRCUITS Amplifier Input Stage Overvoltage Amplifier Output Voltage Phase Reversa Understanding and Protecting Integrated om Electrostatic Discharge(ESD)
1 SECTION 7 OVERVOLTAGE EFFECTS ON ANALOG INTEGRATED CIRCUITS Amplifier Input Stage Overvoltage Amplifier Output Voltage Phase Reversal Understanding and Protecting Integrated Circuits from Electrostatic Discharge (ESD)
sectiOn 7 OⅴERⅴ OLTAGE EFFECTS ON ANALOG INTEGRATED CIRCUITS Adolfo garcia, Wes Freeman One of the most commonly asked applications questions is: What happens if external voltages are applied to an analog integrated circuit with the supplies turned off? This question describes situations that can take on many different forms: from lightning strikes on cables which propagate very high transient voltages into signal conditioning circuits, to walking across a carpet and then touching a printed circuit board full of sensitive precision circuits. Regardless of the situation the general issue is the effect of overvoltage stress(and, in some cases, abuse)on analog integrated circuits. The discussion which follows will be limited in general to operational amplifiers, because it is these devices that most often interface to the integrated circuits which are required to condition or digitize analog waveform \o% outside world. The principles developed here can and should be applied to all ana comparators, sample-and-hold amplifiers, analog switches and multiplexers, and analog-to-digital converters AMPLIFIER INPUT STAGE OVERVOLTAGE In real world signal conditioning, sensors are often used in hostile environments where faults can and do occur. When these faults take place, signal conditioning circuitry can be exposed to large voltages which exceed the power supplies. The likelihood for damage is quite high, even though the components' power supplies may be turned on. Published specifications for operational amplifier absolute maximum ratings state that applied input signal levels should never exceed the power supplies by more than 0.3V or, in some devices, 0.7V. Exceeding these levels exposes amplifier input stages to potentially destructive fault currents which flow through internal metal traces and parasitic p-n junctions to the supplies. without some type of current limiting, unprotected input differential pairs(BJTs or FETs) can be destroyed in a matter of microseconds There are, however, some devices with built-in circuitry that can provide protection beyond the supply voltages, but in general, absolute maximum ratings must still be observed
2 SECTION 7 OVERVOLTAGE EFFECTS ON ANALOG INTEGRATED CIRCUITS Adolfo Garcia, Wes Freeman One of the most commonly asked applications questions is: “What happens if external voltages are applied to an analog integrated circuit with the supplies turned off?” This question describes situations that can take on many different forms: from lightning strikes on cables which propagate very high transient voltages into signal conditioning circuits, to walking across a carpet and then touching a printed circuit board full of sensitive precision circuits. Regardless of the situation, the general issue is the effect of overvoltage stress (and, in some cases, abuse) on analog integrated circuits. The discussion which follows will be limited in general to operational amplifiers, because it is these devices that most often interface to the outside world. The principles developed here can and should be applied to all analog integrated circuits which are required to condition or digitize analog waveforms. These devices include (but are not limited to) instrumentation amplifiers, analog comparators, sample-and-hold amplifiers, analog switches and multiplexers, and analog-to-digital converters. AMPLIFIER INPUT STAGE OVERVOLTAGE In real world signal conditioning, sensors are often used in hostile environments where faults can and do occur. When these faults take place, signal conditioning circuitry can be exposed to large voltages which exceed the power supplies. The likelihood for damage is quite high, even though the components’ power supplies may be turned on. Published specifications for operational amplifier absolute maximum ratings state that applied input signal levels should never exceed the power supplies by more than 0.3V or, in some devices, 0.7V. Exceeding these levels exposes amplifier input stages to potentially destructive fault currents which flow through internal metal traces and parasitic p-n junctions to the supplies. Without some type of current limiting, unprotected input differential pairs (BJTs or FETs) can be destroyed in a matter of microseconds. There are, however, some devices with built-in circuitry that can provide protection beyond the supply voltages, but in general, absolute maximum ratings must still be observed
INPUT STAGE OVERVOLTAGE INPUT SHOULD NOT EXCEED ABSOLUTE MAXIMUM RATINGS (Usually Specified With Respect to Supply Voltages) A Common Specification Requires the Input Signal <Vs+0.3v Input voltage should be held Near Zero in the absence of Input Stage Conduction Current Needs to be Limited( Rule of Thumb: s 5mA) Avoid Reverse Bias Junction Breakdown in Input Stage Base Emitter junctions Differential and Common-Mode Ratings may Differ No Two Amplifiers are exactly the Same Some Op Amps Contain Input Protection(Voltage Clamps Current Limits, or Both), but Absolute Maximum Ratings Must Still be observed Figure 7.1 Although more recent vintage operational amplifiers designed for single-supply rail-to-rail operation are now including information with regard to input stage overvoltage effects, there are very many amplifiers available today without such the device in question before steps can be taken to protect it. All amplifiers il or a information provided by the manufacturer. In those cases, the circuit designer usir these components must ascertain the input stage current-voltage characteristic conduct current to the positive/negative supply, provided the applied input voltage exceeds some internal threshold. This threshold is device dependent, and can rang from 0. 7V to 30V, depending on the internal construction of the input stage Regardless of the threshold level, externally generated fault currents should be limited to no more than + 5mA Many factors contribute to the current-voltage characteristic of an amplifiers input stage: internal differential clamping diodes, current-limiting series resistances substrate potential connections, and differential input stage topologiesBjTs or s) Input protection diodes used as differential input clamps are typically constructed from the base-emitter junctions of NPn transistors. These diodes usually form a parasitic p-n-junction to the negative supply when the applied input voltage exceeds the negative supply Current-limiting series resistances used in the input stages of operational amplifiers can be fabricated from three types of material thin-film resistors are fabricated over thin layers of oxide which provide ap on and n-or p-type diffusions, polysilicon, or thin-films(SiCr, for example). polysilic junctions to either supply. Diffused resistors, on the other hand exhibit p-l o-n insulating barrier to the substrate; as such, they do not exhibit any parasitic
3 INPUT STAGE OVERVOLTAGE INPUT SHOULD NOT EXCEED ABSOLUTE MAXIMUM RATINGS (Usually Specified With Respect to Supply Voltages) A Common Specification Requires the Input Signal <|Vs | 0.3V Input Voltage Should be Held Near Zero in the Absence of Supplies Input Stage Conduction Current Needs to be Limited (Rule of Thumb: 5mA) Avoid Reverse Bias Junction Breakdown in Input Stage BaseEmitter Junctions Differential and Common-Mode Ratings may Differ No Two Amplifiers are exactly the Same Some Op Amps Contain Input Protection (Voltage Clamps, Current Limits, or Both), but Absolute Maximum Ratings Must Still be Observed Figure 7.1 Although more recent vintage operational amplifiers designed for single-supply or rail-to-rail operation are now including information with regard to input stage overvoltage effects, there are very many amplifiers available today without such information provided by the manufacturer. In those cases, the circuit designer using these components must ascertain the input stage current-voltage characteristic of the device in question before steps can be taken to protect it. All amplifiers will conduct current to the positive/negative supply, provided the applied input voltage exceeds some internal threshold. This threshold is device dependent, and can range from 0.7V to 30V, depending on the internal construction of the input stage. Regardless of the threshold level, externally generated fault currents should be limited to no more than ±5mA. Many factors contribute to the current-voltage characteristic of an amplifier’s input stage: internal differential clamping diodes, current-limiting series resistances, substrate potential connections, and differential input stage topologies (BJTs or FETs). Input protection diodes used as differential input clamps are typically constructed from the base-emitter junctions of NPN transistors. These diodes usually form a parasitic p-n-junction to the negative supply when the applied input voltage exceeds the negative supply. Current-limiting series resistances used in the input stages of operational amplifiers can be fabricated from three types of material: n- or p-type diffusions, polysilicon, or thin-films (SiCr, for example). Polysilicon and thin-film resistors are fabricated over thin layers of oxide which provide an insulating barrier to the substrate; as such, they do not exhibit any parasitic p-n junctions to either supply. Diffused resistors, on the other hand, exhibit p-n
junctions to the supplies because they are constructed from either p-or n-type diffusion regions. The substrate potential of the amplifier is the most critica component, for it will determine the sensitivity of an amplifiers input current voltage characteristic to supply voltage The configuration of the amplifiers input stage also plays a large role in the curren voltage characteristic of the amplifier Input differential pairs of operationa amplifiers are constructed from either bipolar transistors(NPN or PNP)or field- effect transistors junction or MOs, N-or P-channel). While the bipolar input differential pairs do not have any direct path to either supply, fet differential pairs do. For example, an n-channel JFET forms a parasitic p-n junction between its backgate and the psubstrate that energizes when Vin+0.7V< VNEG As mentioned previously, many manufacturers of analog integrated circuits do not provide any details with regard to the behavior of the device's input structure Either simplified schematics are not provided or, if they are shown, the behavior of the input stage under an overvoltage condition is omitted Therefore, other measures must be taken in order to identify the conduction paths a standard transistor curve tracer can be configured to determine the current voltage characteristic of any amplifier regardless of input circuit topology As showr in Figure 7.2, both amplifier supply pins are connected to ground, and the collector output drive is connected to one of the amplifiers inputs. The curve tracer applies a DC ramp voltage and measures the current flowing through the input stage. In the event that a transistor curve tracer is not available, a dc voltage source and a multimeter can be substituted for the curve tracer. A 10kohm resistor should be used between the dC voltage source and the amplifier input for additional protection. Ammeter readings from the multimeter at each applied dC voltage will yield the same result as that produced by the curve tracer. Although either input can be tested(both inputs should), it is recommended that the unused input is left open; otherwise, additional junctions could come into play and would complicate matters further Evaluations of current feedback amplifier input stages are more difficult because of the lack of symmetry between the inputs. As a result, both inputs should be characterized for their individual current-voltage characteristics
4 junctions to the supplies because they are constructed from either p- or n-type diffusion regions. The substrate potential of the amplifier is the most critical component, for it will determine the sensitivity of an amplifier’s input currentvoltage characteristic to supply voltage. The configuration of the amplifier’s input stage also plays a large role in the currentvoltage characteristic of the amplifier. Input differential pairs of operational amplifiers are constructed from either bipolar transistors (NPN or PNP) or fieldeffect transistors (junction or MOS, N- or P-channel). While the bipolar input differential pairs do not have any direct path to either supply, FET differential pairs do. For example, an n-channel JFET forms a parasitic p-n junction between its backgate and the p-substrate that energizes when VIN + 0.7V < VNEG. As mentioned previously, many manufacturers of analog integrated circuits do not provide any details with regard to the behavior of the device’s input structure. Either simplified schematics are not provided or, if they are shown, the behavior of the input stage under an overvoltage condition is omitted. Therefore, other measures must be taken in order to identify the conduction paths. A standard transistor curve tracer can be configured to determine the currentvoltage characteristic of any amplifier regardless of input circuit topology. As shown in Figure 7.2, both amplifier supply pins are connected to ground, and the collector output drive is connected to one of the amplifier’s inputs. The curve tracer applies a DC ramp voltage and measures the current flowing through the input stage. In the event that a transistor curve tracer is not available, a DC voltage source and a multimeter can be substituted for the curve tracer. A 10kohm resistor should be used between the DC voltage source and the amplifier input for additional protection. Ammeter readings from the multimeter at each applied DC voltage will yield the same result as that produced by the curve tracer. Although either input can be tested (both inputs should), it is recommended that the unused input is left open; otherwise, additional junctions could come into play and would complicate matters further. Evaluations of current feedback amplifier input stages are more difficult because of the lack of symmetry between the inputs. As a result, both inputs should be characterized for their individual current-voltage characteristics
OVER-VOLTAGE CURVE TRACER TEST SETU CURVE DUT TRACER Test Both Inputs of Op Amp-Results are ldentical for Voltage Feedback Types, but Not Current Feedback Force a Voltage Using the Collector Output Display Collector Current Versus Voltage Results May Differ Depending on Whether the Other Input is Open or Grounded Once the input current-voltage characteristic has been determined for the device in question, the next step is to determine the minimum level of resistance required to limit fault currents to +5mA. Equation 7.1 illustrates the computation for Rs when the input overvoltage level is known Rs VIN(MAX)-VSUPPLY Eq.7.1 5 mA The worst case condition for overvoltage would be when the power supplies are litially turned off or disconnected. In this case, VSUPPLY is equal to zero. For xample, if the input overvoltage could reach 100V under some type of fault condition then the external resistor should be no smaller than 20kohms Most operational amplifier applications only require protection at one input however there are a few configurations(difference amplifiers, for example)where both inputs can be subjected to overvoltage and both must be protected The need for protection on both inputs is much more common with instrumentation amplifiers
5 OVER-VOLTAGE CURVE TRACER TEST SETUP Figure 7.2 Once the input current-voltage characteristic has been determined for the device in question, the next step is to determine the minimum level of resistance required to limit fault currents to ±5mA. Equation 7.1 illustrates the computation for Rs when the input overvoltage level is known: Rs VIN(MAX) VSUPPLY 5 mA = - Eq. 7.1 The worst case condition for overvoltage would be when the power supplies are initially turned off or disconnected. In this case, VSUPPLY is equal to zero. For example, if the input overvoltage could reach 100V under some type of fault condition, then the external resistor should be no smaller than 20kohms. Most operational amplifier applications only require protection at one input; however, there are a few configurations (difference amplifiers, for example) where both inputs can be subjected to overvoltage and both must be protected. The need for protection on both inputs is much more common with instrumentation amplifiers
OVERVOLTAGE EFFECTS Junctions may be Forward Biased if the current is Limited In general a safe Current limit is 5mA Reverse Bias Junction Breakdown is Damaging Regardless of the current level When in Doubt, Protect with External Diodes and series Resistances Curve Tracers Can be Used to Check the Overvoltage Characteristics of a device Simplified Equivalent Circuits in Data Sheets do not tell the Entire Story!!! Figure 7.3 AMPLIFIER OUTPUT VOLTAGE PHASE REVERSAL Some operational amplifiers exhibit output voltage phase reversal when one or both of their inputs exceeds their input common-mode voltage range. Phase reversal is usually associated with JFET (n-or p-channel)input amplifiers, but some bipolar devices(especially single-supply amplifiers operating as unity-gain followers)may also be susceptible In the vast majority of applications, output voltage phase reversal does not harm the amplifier nor the circuit in which the amplifier is used. Although a number of operational amplifiers suffer from phase reversal, it is rarely a problem in system design. However, in servo loop applications, this effect can be quite hazardous. Fortunately, this is only a temporary condition. Once the amplifiers inputs return to within its normal operating common-mode range, output voltage phase reversal ceases. It may still be necessary to consult the amplifier manufacturer, since phase reversal information rarely appears on device data sheets. Summarized as follows is a list of recent vintage Analog devices amplifier products that are now including output voltage phase reve characterization/commentary: Single-Supply/ Dual Supply Rail-to-Rail OP295/OP495 OP282OP482 OP113/OP213/OP413 OP285 OP183/OP283 OP467 OP292OP492 OP176 OP191/OP291OP491 BUF04 AD820/AD822/AD824 OP193/OP293/OP493
6 OVERVOLTAGE EFFECTS Junctions may be Forward Biased if the Current is Limited In General a Safe Current Limit is 5mA Reverse Bias Junction Breakdown is Damaging Regardless of the Current Level When in Doubt, Protect with External Diodes and Series Resistances Curve Tracers Can be Used to Check the Overvoltage Characteristics of a Device Simplified Equivalent Circuits in Data Sheets do not tell the Entire Story!!! Figure 7.3 AMPLIFIER OUTPUT VOLTAGE PHASE REVERSAL Some operational amplifiers exhibit output voltage phase reversal when one or both of their inputs exceeds their input common-mode voltage range. Phase reversal is usually associated with JFET (n- or p-channel) input amplifiers, but some bipolar devices (especially single-supply amplifiers operating as unity-gain followers) may also be susceptible. In the vast majority of applications, output voltage phase reversal does not harm the amplifier nor the circuit in which the amplifier is used. Although a number of operational amplifiers suffer from phase reversal, it is rarely a problem in system design. However, in servo loop applications, this effect can be quite hazardous. Fortunately, this is only a temporary condition. Once the amplifier’s inputs return to within its normal operating common-mode range, output voltage phase reversal ceases. It may still be necessary to consult the amplifier manufacturer, since phase reversal information rarely appears on device data sheets. Summarized as follows is a list of recent vintage Analog Devices amplifier products that are now including output voltage phase reversal characterization/commentary: Single-Supply/ Rail-to-Rail Dual Supply OP295/OP495 OP282/OP482 OP113/OP213/OP413 OP285 OP183/OP283 OP467 OP292/OP492 OP176 OP191/OP291/OP491 BUF04 OP279 AD820/AD822/AD824 OP193/OP293/OP493
In BiFET operational amplifiers, phase reversal may be prevented by adding an appropriate resistance in series with the amplifiers input to limit the current Bipolar input devices can be protected by using a Schottky diode to clamp the input to within a few hundred millivolts of the negative rail For a complete description of the output voltage phase reversal effect, please consult Reference 1 BEWARE OF AMPLIFIER OUTPUT PHASE REVERSAL a Sometimes Occurs in FET and Bipolar Input(Especially Single Supply )Op Amps when Input Exceeds Common Mode Range Does Not Harm Amplifier, but may be Disastrous in Servo Not Usually Specified on Data Sheet, so Amplifier Must be Checked Easily Prevented BiFETs: Add Appropriate Input Series Resistance Determined Empirically, Unless Provided in Data Sheet) Bipolars Use Schottky Diode Clamps to the Supply Rails Figure 7. 4 Rail-to-rail operational amplifiers present a special class of problems to the integrated circuit designer, because these types of devices should not exhibit any abnormal behavior throughout the entire input common-mode range. In fact, it is desirable that devices used in these applications also not exhibit any abnormal behavior if the applied input voltages exceed the power supply range. One of the more recent vintage rail-to-rail input/output operational amplifiers, the OPX91 family(the OP191, the OP291, and the OP491), includes additional components that prevent overvoltage and damage to the device. As shown in Figure 7.5, the input tage of the opX9l devices use six diodes and two resistors to clamp the input terminals to each other and to the supplies. Di and D2 are base-emitter nPn diodes which are used to protect the bases of Q1-Q2 and Q3-Q4 against avalanche breakdown when the applied differential input voltage to the device exceeds 0.7V Diodes D3-D6 are diodes formed from substrate PNp transistors that clamp the applied input voltages on the opX9 1 to the supply rails
7 In BiFET operational amplifiers, phase reversal may be prevented by adding an appropriate resistance in series with the amplifier’s input to limit the current. Bipolar input devices can be protected by using a Schottky diode to clamp the input to within a few hundred millivolts of the negative rail. For a complete description of the output voltage phase reversal effect, please consult Reference 1. BEWARE OF AMPLIFIER OUTPUT PHASE REVERSAL Sometimes Occurs in FET and Bipolar Input (Especially SingleSupply) Op Amps when Input Exceeds Common Mode Range Does Not Harm Amplifier, but may be Disastrous in Servo Systems! Not Usually Specified on Data Sheet, so Amplifier Must be Checked Easily Prevented: BiFETs: Add Appropriate Input Series Resistance (Determined Empirically, Unless Provided in Data Sheet) Bipolars: Use Schlottky Diode Clamps to the Supply Rails. Figure 7.4 Rail-to-rail operational amplifiers present a special class of problems to the integrated circuit designer, because these types of devices should not exhibit any abnormal behavior throughout the entire input common-mode range. In fact, it is desirable that devices used in these applications also not exhibit any abnormal behavior if the applied input voltages exceed the power supply range. One of the more recent vintage rail-to-rail input/output operational amplifiers, the OPX91 family (the OP191, the OP291, and the OP491), includes additional components that prevent overvoltage and damage to the device. As shown in Figure 7.5, the input stage of the OPX91 devices use six diodes and two resistors to clamp the input terminals to each other and to the supplies. D1 and D2 are base-emitter NPN diodes which are used to protect the bases of Q1-Q2 and Q3-Q4 against avalanche breakdown when the applied differential input voltage to the device exceeds 0.7V. Diodes D3-D6 are diodes formed from substrate PNP transistors that clamp the applied input voltages on the OPX91 to the supply rails
A CLOSER LOOK AT THE OP-X91 INPUT STAGE REVEALS ADDITIONAL DEVICES AAAI D3-D6: SUBSTRATE PNPs(COLLECTORS TO VNEG) Figure 7.5 An interesting benefit from using substrate PNPs as clamp diodes is that their collectors are connected to the negative supply; thus, when the applied input voltage exceeds either supply rail, the diodes energize, and the fault currents are diverted directly to the supply and not through or into the device's input stage There are al 5kohm resistors in series with each of the inputs to the opX9 1 to limit the fault that these kohm resistors are p-type diffusions placed inside an n-well, which lo current through D1 and D2 when the differential input voltage exceeds 0. 7V. Note then connected to the positive supply. when the applied input voltage exceeds the positive supply, some of the fault current generated is also diverted to Vpos and away from the input stage. As a result of these measures, the input overvoltage characteristic of the oPX91 is well behaved as shown in Figure 7.6. Note that the combination of the kohm resistors and clamp diodes safely limits the input current to less than 2mA, even when the inputs of the device exceed the supply rails by 10v
8 A CLOSER LOOK AT THE OP-X91 INPUT STAGE REVEALS ADDITIONAL DEVICES Figure 7.5 An interesting benefit from using substrate PNPs as clamp diodes is that their collectors are connected to the negative supply; thus, when the applied input voltage exceeds either supply rail, the diodes energize, and the fault currents are diverted directly to the supply and not through or into the device’s input stage. There are also 5kohm resistors in series with each of the inputs to the OPX91 to limit the fault current through D1 and D2 when the differential input voltage exceeds 0.7V. Note that these 5kohm resistors are p-type diffusions placed inside an n-well, which is then connected to the positive supply. When the applied input voltage exceeds the positive supply, some of the fault current generated is also diverted to VPOS and away from the input stage. As a result of these measures, the input overvoltage characteristic of the OPX91 is well behaved as shown in Figure 7.6. Note that the combination of the 5kohm resistors and clamp diodes safely limits the input current to less than 2mA, even when the inputs of the device exceed the supply rails by 10V
INTERNAL 5kQ RESISTORS PLUS IINPUT CLAMP DIODES COMBINE TO PROTECT OP-X91 DEVICES AGAINST OVERVOLTAGE 1mA 10V 10v 1mA Figure 7.6 As an added safety feature, an additional pair of diodes is used in the input stage across Q3 and Q4 to prevent subsequent stages internal to the oPX91 from collapsing(that is, forced into cutoff). If these stages were forced into cutoff, then th amplifier would undergo output voltage phase reversal when the inputs exceeded the positive input common mode voltage. An illustration of the diodes' effectiveness is shown in Figure 7.7. Here, the oPX91 family can safely handle a 20Vp-p input signal on 5V supplies without exhibiting any sign of output voltage phase reversal or other anomalous behavior. With these amplifiers, no external clamping diodes are
9 INTERNAL 5k RESISTORS PLUS IINPUT CLAMP DIODES COMBINE TO PROTECT OP-X91 DEVICES AGAINST OVERVOLTAGE Figure 7.6 As an added safety feature, an additional pair of diodes is used in the input stage across Q3 and Q4 to prevent subsequent stages internal to the OPX91 from collapsing (that is, forced into cutoff). If these stages were forced into cutoff, then the amplifier would undergo output voltage phase reversal when the inputs exceeded the positive input common mode voltage. An illustration of the diodes’ effectiveness is shown in Figure 7.7. Here, the OPX91 family can safely handle a 20Vp-p input signal on ±5V supplies without exhibiting any sign of output voltage phase reversal or other anomalous behavior. With these amplifiers, no external clamping diodes are required
ADDITION OF TWO CLAMP DIODES PROTECTS OP-X9 DEVICES AGAINST OUTPUT PHASE REVERSAL 20Vp-p oP291 TIME-200uS/DIV Figure 7.7 For those amplifiers where external protection is clearly required against both overvoltage abuse and output phase reversal, a common technique is to use a series resistance, Rs, to limit fault current, and Schottky diodes to clamp the input signa to the supplies, as shown in Figure 7.8
1 0 ADDITION OF TWO CLAMP DIODES PROTECTS OP-X91 DEVICES AGAINST OUTPUT PHASE REVERSAL Figure 7.7 For those amplifiers where external protection is clearly required against both overvoltage abuse and output phase reversal, a common technique is to use a series resistance, Rs , to limit fault current, and Schottky diodes to clamp the input signal to the supplies, as shown in Figure 7.8
