Robust Amplifiers Provide Integrated Overvoltage Protection

Robust Amplifiers Provide Integrated Overvoltage Protection
Robust Amplifiers Provide
Integrated Overvoltage
Protection
By Eric Modica and Michael Arkin
Faulty performance, or even damage, can occur when an op
amp’s input voltage exceeds the specified input-voltage range,
or—in extreme cases—the amplifier’s supply voltage. This
article discusses some common causes and effects of overvoltage
conditions, how cumbersome overvoltage protection can be added
to an unprotected amplifier, and how the integrated overvoltage
protection of newer amplifiers provides designers with a compact,
robust, transparent, cost-effective solution.
All electronic components have upper limits to the applied
voltages they can tolerate. When any of these upper limits are
exceeded, the effects can range from momentary interruption of
operation to system latch-up to permanent damage. The amount
of overvoltage a given component can tolerate depends on several
factors, including whether the part is installed or incidentally
contacted, the amplitude and duration of the overvoltage event,
and the robustness of the device.
Precision amplifiers, often the first component in sensor
measurement signal chains, are the most exposed to overvoltage
faults. When selecting a precision amplifier, system designers
must be aware of the common-mode input range of the amplifier. On
the data sheet, the common-mode input range may be specified
by the input voltage range (IVR), or in the test conditions for the
common-mode rejection ratio (CMRR), or both.
Real World Causes of Overvoltage Conditions
Amplifiers require: overvoltage protection to protect against faults
caused by power-supply sequencing, sleep-mode switching, and
voltage spikes; and ESD protection to protect against faults caused
by electrostatic discharge (ESD), even during handling. When
installed, the device can be subjected to system power sequence
conditions, which cause repetitive overvoltage stress. System
designers seek methods of routing the fault currents away from
sensitive components, or limiting those fault currents enough to
avoid damage.
In complex distributed power architecture (DPA) systems with multiple
supply voltages, power-supply sequencing allows the supplies
powering various portions of the system circuitry to turn on and off
at different times. Improper sequencing can cause overvoltage and
latch-up conditions to occur on any pin on any device.
With an increasing focus on energy efficiency, many systems
implement complex sleep and standby modes. This means that
some sections of a system may be powered down while others
may remain powered and active. As with supply sequencing,
these situations can cause unpredictable overvoltage events, but
primarily on input pins.
Many types of sensors can generate unexpected output spikes
that are unrelated to the physical phenomena they are meant to
measure. This type of overvoltage condition generally affects only
input pins.
Electrostatic discharge is a well-known overvoltage event that
often occurs before the component is installed. The damage it
can cause is so prevalent that industry-driven specifications,
such as JESD22-A114D, determine how to test and specify the
Analog Dialogue 46-02, February (2012)
semiconductor’s ability to withstand various types of ESD events.
Almost all semiconductor products incorporate some form of
integrated protection devices. The AN-397 Application Note,
“Electrically Induced Damage to Standard Linear Integrated
Circuits: The Most Common Causes and the Associated Fixes
to Prevent Reccurrence,” is a good reference that covers this topic
in detail. ESD cells are designed to go into a low-impedance state
after a high-energy pulse. This does not limit the input current,
but it does provide a low-impedance path to the supply rails.
A Simple Case Study: Power-Supply Sequencing
As mixed-signal circuits become ubiquitous, so too does the
need for multiple supplies on a single PCB. See the AN-932
Application Note, “Power Supply Sequencing,” for a look at some
subtle issues to consider in new designs, especially when several
unrelated power supplies are required.
Precision amplifiers can fall victim to this condition. Figure 1 shows
an op amp configured as a differential amplifier. The amplifier
senses the current through R SENSE and provides an output
proportional to the resultant voltage drop. Care must be taken that
the divider formed by R 3 and R4 biases the inputs somewhere within
the specified IVR. If the amplifier’s supply voltage is not derived
from VSY, and VCC comes up after VSY, the voltage at the inverting
input of A1 will be:
V– = VSY – (I– × R1)
(1)
where I– depends on the input impedance of A1 with no supply.
If the amplifier is not designed to handle overvoltage conditions,
the most likely current path will be through an ESD diode, clamp
diode, or parasitic diode to the power supply or ground. Damage
can occur if this voltage falls outside of the IVR or if the current
exceeds the data sheet maximum rating.
The ESD structures used on overvoltage-protected amplifiers,
such as the ADA4091 and ADA4096, are not diodes, but DIAC
(bidirectional “diode for alternating current”) devices, making these
amplifiers tolerant of overvoltage conditions, even without power.
VCC
VSY
R1
V–
RSENSE R3
I–
LOAD
R2
A1
R2
R4
R1
VOUT
=
VOUT =
R4
R3
R2
(I
× RSENSE)
R1 LOAD
Figure 1. Differential amplifier high-side current sensor. If
VSY powers up before VCC, the amplifier’s input voltage or
current can exceed the data sheet maximum.
Fault Conditions in Operational Amplifiers
Figure 2 shows an N-channel JFET input stage (J1, J2 , R1, and
R 2), followed by a secondary gain stage and output buffer (A1).
When the open-loop amplifier is within its specified IVR, the
differential input signal (V IN+ – V IN–) is 180 degrees out of phase
with V DIFF. When connected as a unity-gain buffer, as shown, if
the common-mode voltage at V IN+ exceeds the amplifier’s IVR,
J1’s gate-drain will un-pinch and conduct the entire 200-µA stage
current. As long as J1’s gate-drain voltage remains reverse-biased,
a further increase at V IN+ causes no change in V DIFF (VOUT stays
at the positive rail). Once J1’s gate-drain becomes forward-biased,
however, a further increase in V IN+ raises the voltage at A1’s
inverting input, causing an undesirable phase-reversal between
the input signal and V DIFF.
www.analog.com/analogdialogue
1
VCC
R1
10k𝛀
VCC
D1ESD
R2
10k𝛀
–
VOUT
A1
D4ESD
VEE
J2
Figure 4. A bipolar input stage showing ESD, and
differential protection diodes.
VEE
Figure 2. A conceptual N-channel, JFET-input op amp.
Figure 3 shows an example of phase reversal at the output of
A1. Unlike bipolar input amplifiers, JFET amplifiers are prone
to phase reversal because their inputs are not clamped. CMOS
amplifiers are typically immune to phase reversal because the
gates are electrically isolated from the drains. If phase inversion
doesn’t occur, op amp manufacturers will often state this on the
data sheet. Phase inversion is possible if: the amplifier inputs are
not CMOS, the maximum differential input is VSY, and the data
sheet does not claim immunity to phase inversion. Although phase
inversion by itself is nondestructive, it can cause positive feedback,
which leads to instability in servo loops.
15
Figure 5 shows the input current-voltage relationship of an
unprotected bipolar op amp with differential input and overvoltage
applied simultaneously. Once the applied voltage exceeds a diode
drop, the current can become destructive, degrading or even
destroying the op amp.
5
4
3
INPUT CURRENT (mA)
VIN–
I1 = 200𝛍A
2
1
0
–1
–2
–3
VIN
10
–4
–5
5
VOLTAGE (V)
VIN–
D3ESD
+
J1
D2ESD
VIN+
VDIFF
VIN+
CLAMP
0
–5
–4
–3
–2
–1
0
1
2
INPUT VOLTAGE (V)
3
4
5
OVERVOLTAGE CURVE TRACER TEST SETUP
COLLECTOR
CURVE
TRACER BASE
–5
3
2
7
DUT
4
–10
–20
0.7
Figure 5. Op amp input current as the differential input
voltage exceeds a diode drop.
VOUT
–15
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
TIME (ms)
Figure 3. When VIN exceeds the specified IVR, input
phase inversion causes the output of the amplifier to
become negative.
System designers also have to be concerned about what happens
when the amplifier inputs are pulled outside the power supplies.
Most often this fault condition occurs when power-supply
sequencing causes a source signal to be active before the amplifier
supplies turn on, or when a power supply spikes during turn-on,
turn-off, or in operation. This condition is destructive for most
amplifiers, especially if the overvoltage is greater than a diode drop.
Figure 4 shows a typical bipolar input stage with ESD protection
diodes and clamp diodes. In a buffer configuration, when V IN+
exceeds either rail, ESD and clamp diodes will be forward biased.
With very low source impedance, these diodes will conduct as
much current as the source will allow. Precision amplifiers, such
as the AD8622, provide a modicum of differential protection by
including 500-Ω resistors in series with the inputs to limit the input
current when a differential voltage is applied, but they protect only
as long as the maximum input current specification isn’t exceeded.
If the maximum input current is 5 mA, then the maximum allowed
differential voltage is 5 V. Note that these resistors are not in series
with the ESD diodes, so they cannot limit current to the rails (for
example, during an overvoltage condition).
2
External Input Overvoltage Protection
From the earliest days of semiconductor op amps, IC designers
have had to deal with trade-offs between chip architecture and
the external circuitry needed to deal with its weaknesses. Fault
protection has been among the most difficult of problem areas
(for examples, see MT-036, “Op Amp Output Phase-Reversal
and Input Over-Voltage Protection” and MT-069, “In-Amp Input
Overvoltage Protection”).
Two properties system designers need precision op amps for are
their low offset voltage (VOS) and high common-mode rejection
ratio (CMRR), both of which simplify calibration and minimize
dynamic error. To maintain these specifications in the presence
of electrical overstress (EOS), bipolar op amps often include
internal clamp diodes and small limiting resistors in series with
their inputs, but these cannot address fault conditions caused
when the inputs exceed the rails. To add protection, the system
designer can implement circuitry such as that shown in Figure 6.
VCC = +15V
RFB
VIN
ROVP
D2
D1
VEE = –15V
Figure 6. Precision op amp with external protection using a
current-limiting resistor and two Schottky diodes. RFB is set
equal to ROVP to balance offsets due to input bias currents.
Analog Dialogue 46-02, February (2012)
ROVP will limit the current into the op amp if the signal source at VIN
powers up first. Schottky diodes have a forward voltage 200 mV less
than typical small-signal diodes, so all of the overvoltage current
will be shunted through external diodes D1 and D2. However,
these diodes can degrade the op amp specifications. For example,
the reverse leakage plots from the 1N5711 (see Figure 7) can be
used to determine the CMRR penalty for a given OVP resistor.
The reverse leakage of the 1N5711 is 0 nA at 0 V and 60 nA at
30 V. With a common mode of 0 V, the additional IOS caused by
D1 and D2 depends on how well their leakages match. When V IN
is taken to +15 V, D1 will be reverse biased by 30 V, and D2 will
have 0 V bias. Thus, an additional 60 nA flows into ROVP. When
the input is taken to –15 V, D1 and D2 swap positions electrically,
and 60 nA flows out of ROVP. The additional IOS caused by the
protection diodes at any common mode is simply:
IOSaddr = ID1 – ID2
(2)
100
REVERSE CURRENT (𝛍A)
150°C
125°C
10
the supplies. If, for example, the positive supply cannot sink a
significant amount of current, the overvoltage current can force
the positive supply voltage to increase.
One way to prevent this is to use back-to-back Zener diodes
from the positive input to ground, as shown in Figure 8. When
the Zener voltage of either D1 or D2 is exceeded, the diode
shunts the overvoltage current to ground, protecting the power
supplies. This configuration prevents charge pumping during
overvoltage conditions, but Zener diodes have higher leakage
current and capacitance than small-signal diodes. In addition,
Zener diodes have a soft-knee characteristic in their leakage
current profile. This, as described previously, adds an additional
CMRR penalty over the amplifier’s common-mode range. For
example, the BZB84-C24 is a back-to-back Zener diode pair
with a working voltage between 22.8 V and 25.6 V. The reverse
current is specified as 50 nA max at 16.8 V, but the manufacturer
doesn’t specify what the leakage is closer to the Zener voltage.
Also, to achieve a sharper breakdown characteristic, Zener diodes
are generally made of more highly doped diffusions than their
small-signal cousins. This causes a relative increase in parasitic
capacitance, which translates to increased distortion (especially
at higher amplitudes) and increased instability.
100°C
1
VCC
75°C
RFB
50°C
0.1
ROVP
VIN
D2
25°C
D1
0.01
0
10
20
30
40
50
60
VEE
Figure 8. Precision op amp with external protection using
a current-limiting resistor and two Zener diodes.
70
CONTINUOUS REVERSE VOLTAGE (V)
Early Integrated Overvoltage Protection
Figure 7. 1N5711 reverse current vs. continuous
reverse voltage.
From Equation 2, the VOS penalty can be derived at the extremes
of the common-mode range as follows:
VOSpenalty = IOSaddr × ROVP
(3)
Using 60 nA as the leakage of the 1N5711 at 30 V, and a 5-kΩ
protection resistor, VOS at each extreme would be increased by
300 µV, causing an additional 600 μV ∆VOS over the entire inputvoltage range. In data sheet terms, an op amp with 110-dB CMRR
would suffer a 17-dB reduction. Inserting a feedback resistor to
equalize source impedance only helps when the common mode
is 0 V, and does nothing to prevent additional IOS over the full
common-mode range. Table 1 shows the same calculation for
diodes commonly used for protecting precision amplifiers. For
CMRR penalty calculations, a 5-kΩ protection resistor is assumed.
All costs are recent quotes (2011) in USD from www.mouser.com.
Another possible drawback to the method shown in Figure 6
is that the protection diodes shunt the overvoltage current into
The previous section discussed drawbacks to some commonly
used methods of external amplifier protection. Some of these
drawbacks could be avoided if the amplifier itself is designed
to tolerate a large input overvoltage. Figure 9 shows a common
integrated protection scheme used on a differential input pair.
VCC
D1
VIN+
D5
R1
D6
D2
D3
R2
VIN–
D4
VEE
Figure 9. A differential input pair with resistive overvoltage
protection (ESD protection not shown).
This circuit includes input protection resistors on both amplifier
inputs. Although overvoltage protection is generally needed on
only one input, equalizing the parasitic capacitance and leakage
Table 1. Commonly Used Protection Diodes and Their Impact on a 110-dB CMRR Precision Op Amp
1N5711
BAV99
PAD5
BAS70-04
1N914
BZB84-C24
IOSaddr (nA)
60
10
<<0.005
8
40
50
VOSpenalty (µV)
600
100
0
80
400
500
CMRR Penalty (dB)
17
6
0
5
14
16
Cost @ 1k Quantities
$0.07
$0.015
$3.52
$0.095
$0.01
$0.034
Analog Dialogue 46-02, February (2012)
3
at each input reduces distortion and offset current. Furthermore,
the diodes do not have to handle ESD events, so they can be
relatively small.
En,total = (en,op amp)2 + (en,Rovp)2 + (RS × in,op amp)2
(4)
If a 1-kΩ resistor is used to protect an op amp with 4-nV/√Hz
of noise, the total voltage noise will increase by √2. Integrating
the protection resistors doesn’t change the fact that overvoltage
protection increases the input-referred voltage noise, but
integrating R1 and R 2 with the op amp ensures that the data sheet
noise specification covers the protective circuitry.
To avoid the noise-overvoltage trade-off requires a protective circuit
that presents a low resistance when the amplifier inputs are within
the specified range and a very high resistance when the amplifier
inputs exceed the rails. This characteristic would provide improved
overvoltage protection on-demand, thus lowering the overall noise
contribution under normal operating conditions. Figure 10 shows
one circuit implementation that behaves in this way.
VCC
VIN+
J1B
J2B
J1A
J2A
VIN–
VEE
Figure 10. Input differential pair with active overvoltage
protection.
Jxy are all P-channel JFETs; they are depletion mode devices, so
the channel is the same polarity as the source and drain. When
the amplifier input levels are between the rails, J1A and J2A act
as simple resistors with resistance equal to R DSON because the
input bias currents are small enough that any potential difference
between the channel and gate won’t pinch the channel closed. If
V IN+ were to exceed the negative supply by a diode drop, current
would begin to flow through J1A , causing the drain to pinch
closed. This transition is actually J1A moving out of triode and
into the linear region. If V IN+ were to exceed the positive supply
voltage by a diode drop, J1A would act as a lateral PNP. V IN+ to
the gate would act as a forward-biased emitter-base junction,
with the other junction acting as the base-collector, standing
off overvoltage.
The current-voltage plot of Figure 11 shows the change in input
impedance of a FET-protected op amp when subjected to an
overvoltage sweep. The R DSON of the protection FET is 4.5 kΩ; as
the amplifier’s positive input is pulled above the rail, the protection
FET’s resistance increases rapidly to 22 kΩ at 30 V, limiting the
input current to 1.5 mA.
4
1.2
CURRENT (mA)
Adding resistance, either external or internal, adds to the
amplifier’s root-sum-square (RSS) thermal noise (Equation 4):
1.4
REFF = 12.5k𝛀
1.0
REFF = 22.0k𝛀
REFF = 4.5k𝛀
0.8
0.6
0.4
0.2
0
0
5
10
15
20
25
30
OVERVOLTAGE (V)
Figure 11. Effective input impedance of a FET protected
op amp when subjected to a dc overvoltage sweep.
The Benefits of Integration
Amplifiers such as the ADA4091 and ADA4096 demonstrate that
robust, overvoltage-tolerant op amp inputs can be achieved with
a minimal impact on precision (as in Figure 10). The ADA4096
provides 32-V protection, regardless of supply levels—eliminating
the need for external components that can either be inexpensive
but vastly degrade the amplifier’s precision, or precise but more
costly than the amplifiers themselves.
Figure 12 shows the ADA4096-2 in a 2-mm × 2-mm LFCSP
package next to a couple of discrete components often used
for external input protection. The ADA4096-2’s integrated
protection provides a significantly reduced PCB footprint; its
effects are included in the op amp’s specifications; and it protects
the amplifier even when power is not applied (see Figure 13). In
addition, the ADA4091 and ADA4096 have rail-to-rail inputs
and outputs (RRIO) and are free from phase-inversion over the
entire overvoltage protection range (see Figure 14). These benefits
allow system designers to worry a little less about power-supply
sequencing and latch-up.
BAS70-4-V
5k𝛀
ADA4096-2
2mm
Figure 12. The ADA4096-2 in a 2-mm × 2-mm package
occupies less space than two components commonly
used for external voltage protection.
Analog Dialogue 46-02, February (2012)
INPUT BIAS CURRENT (mA)
5
4
3
In summar y, integrated over voltage protection provides
many benefits:
1.Improved robustness and precision in analog signal chains.
2.Reduced time-to-market (TTM), shorter design time, and
reduced testing requirements.
3.Reduced bill of materials (BOM) cost.
4.Fewer components required in approved component lists.
5.Reduced PCB footprint/higher density.
6.Lower failure rates.
VCC = +15V
VEE = –15V
6
VCC = VEE = 0V
Conclusions
7
2
1
0
–1
–2
–3
–4
References
–5
LOW RDSON SERIES FET
5k𝛀 SERIES RESISTOR
–6
–7
–48 –40 –32 –24 –16 –8
0
8
16
24
32
40
48
VIN (V)
Figure 13. ADA4096-2 input OVP current limiting with and
without power.
Authors
T
CH1 10.0V
CH2 10.0V
1N914 data sheet available at www.fairchildsemi.com.
1N5711 data sheet available at www.st.com.
BAV99, BAS70-04, and BZB84-C24 data sheets available at
www.nxp.com.
PAD5 data sheet available at www.vishay.com.
JESD22-A114D standard available at www.jedec.org.
Eric Modica [[email protected]] graduated
from San Jose State University in 2002 with
a BSEE. Responsible for process models and
precision amplifier design, he has worked for
Analog Devices for nine years.
M20.0ms
T 34.20%
A CH1
–3.6V
Figure 14. ADA4096-2 on ±10-V supplies with the inputs
pulled 30 V above and below the rails.
Analog Dialogue 46-02, February (2012)
Michael Arkin [[email protected]] is
a product marketing manager for the Precision
Operational Amplifier Group. He received his BSEE
from West Coast University and an MBA from the
University of Texas. Michael has more than 15 years’
experience marketing electronics products with
companies such as ADI, TI, Pulse, and Lineage Power.
The authors would like to thank Derek Bowers and Harry Holt for
their technical contributions to this article.
5
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