AN-1387: Practical Design Considerations in Applying the ADA4177 Family of Input Overvoltage Protection Operational Amplifiers PDF

AN-1387
APPLICATION NOTE
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Practical Design Considerations in Applying the ADA4177 Family of Input
Overvoltage Protection Operational Amplifiers
by Eric Modica and Michael Arkin
INTRODUCTION
Analog Devices, Inc., has a long history of innovation in
operational amplifiers across its precision and high speed
product lines. Some innovations are aimed at reducing power
consumption while maintaining or even improving speed and
noise; others are aimed at improving precision by reducing
offset, thermal drift, and even the rejection of power supply and
common-mode voltage variations.
Recent innovations have also begun to address environmental
factors unrelated to the normal operation of the amplifier.
Examples include the integration of electromagnetic
interference (EMI) rejection and overvoltage protection (OVP)
into the amplifier front end.
The rejection of external noise sources can include the effects
of electromagnetic and radio frequency interference of closely
located switching devices or wireless communication signals
from WiFi, hand held radios, and mobile communications
equipment such as cellular phones. The integration and
specification of EMI filtering elements has become a feature on
many amplifier designs, and one in which Analog Devices has
been very active.
Similarly, the protection of the input of an operational amplifier
from voltages above the positive supply rail and below the
negative supply rail have been a target for such innovation.
Analog Devices has lead the market in introducing OVP
amplifiers since the release of the OPx91 family in 1994. This
was the first-ever amplifier with integrated OVP introduced
to the industry, offering up to 10 V of protection from excess
current during an overvoltage event. The release of the ADA4091
family of op amps in 2008 raised the OVP performance level to
25 V. Then in 2011, the release of the ADA4096 family brought
the level to 32 V of OVP, which is still the standard for
integrated protection today.
In 2014, the release of the ADA4177 family (ADA4177-1,
ADA4177-2, ADA4177-4) brought the Analog Devices
integrated OVP solution to a low noise, precision op amp for
the first time. It also added an additional feature to the OVP
solution to prevent the input current during an OVP event from
pumping up the positive voltage rail as well as adding integrated
EMI filters to the mix.
The ADA4177 family sets a new standard for robust operation
for operational amplifiers. This application note explores the
application of the OVP feature as it applies to the ADA4177 and
provides guidance on ways that the new OVP can allow users to
extend the protected range while preventing overcurrent to the
inputs and limiting self heating.
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AN-1387
Application Note
ADA4177 CURRENT LIMITING VS. NO CURRENT
LIMITING
PROTECTING POWER SUPPLIES DURING
OVERVOLTAGE EVENTS
Figure 1 shows a test circuit used to measure the input current
of the ADA4177 during an overvoltage event. The amplifier is
placed in unity gain, and the positive input is swept 15 V above
and below the supplies while measuring the input bias current.
One common method of protecting op amp inputs from
overvoltage is to use small signal or Schottky diodes connected
from an input pin to both the positive and negative power
supply; this method is shown schematically in Figure 3. The
turn-on voltage of the Schottky diode is 0.4 V, which is
approximately 0.2 V lower than that of a small signal diode.
This relative difference prevents the internal ESD diodes of the
op amp from turning on when an overvoltage event occurs.
VIN
13819-001
A
V–
Figure 1. Circuit to Test Overvoltage Current Limiting
Figure 2 shows the results of this measurement for both the
ADA4177 as well as those of a standard precision op amp tested
with 5 V power supplies. Notice that at 20 V, the ADA4177
input current is one third that of the standard op amp. If the
user wants to limit the input current further, an external series
resistor can be added. Adding this resistor increases the input
noise of the system by the rms summation of the resistor
thermal noise and the amplifier input noise. The ADA4177
noise specification includes the minimal contribution of the
internal overvoltage circuitry.
Adding the ROVP resistor provides an additional current limiting
element at the expense of increased thermal noise. A detailed
analysis of this solution and its limitations is provided in the
technical article, “Robust Amplifiers Provide Integrated
Overvoltage Protection”.
VCC
RF
D2
VOUT
ROVP
VIN
D1
13819-003
V+
30
VEE
Figure 3. ROVP, D1 and D2 Providing Input Overvoltage Protection
This solution functions by routing current away from the inputs
of the op amp. However, when the current is injected into the
supplies, the appropriate solution then depends on the application
and circuit involved. If the supply is a low dropout (LDO) regulator,
VCC and VEE are likely only designed to route current in one
direction. Figure 4 shows a typical conceptual schematic of a low
dropout regulator where the output voltage is determined by
10
JFET
PROTECTED
AMPLIFIER
–10
VSY = ±5V
–20
–30
–20
RESISTIVE
PROTECTED
AMPLIFIER
–15
–10
VOUT = VREF ×
–5
0
VIN (V)
5
10
15
20
R1 + R2
MP1
VIN
Figure 2. ADA4177 Input Current Limiting vs. an Unprotected Op Amp with
500 Ω Discrete Resistors
R2
VOUT
VREF
R1
R2
GND
13819-004
0
13819-002
INPUT BIAS CURRENT (mA)
20
Figure 4. Conceptual Schematic of a Low Dropout Regulator
Rev. 0 | Page 2 of 4
Application Note
AN-1387
12.5
MP1 is a series pass PMOS transistor that is designed to
increase the amount of current that the supply can source. It is
inferred that VOUT is not designed to sink current. Therefore, if
the overvoltage protection shown in Figure 3 injects current
into the supply, that current passes through the R1 and R2
divider, which raises the supply linearly with overvoltage.
10.0
7.5
5.0
IB (mA)
If the overvoltage occurs with the supplies powered up, the
supply voltage can exceed the intended operating voltage of the
system. If the overvoltage occurs with the system powered
down, the OVP current can unintentionally power up the
system. The ADA4177 has internal circuitry that prevents a
positive overvoltage from pumping the supplies.
–7.5
–10.0
–15.0
–50
–40
–30
–20
–10
0
10
20
30
40
13819-006
VSY = ±15V
–12.5
50
VCMI (V)
J1B
Figure 6. Input Current During Positive and Negative Overvoltage Conditions
Figure 5. Conceptual Schematic of the ADA4177 Input Protection
Figure 5 shows a conceptual schematic of the positive input on
the ADA4177. If VIN exceeds VCC, the current-limiting FET, J1B,
sources the overvoltage current into the emitter of QP1. This
current is divided down via the current gain (or β) of QP1 such
that the overvoltage current is routed to the negative supply
rather than to the positive supply.
If all inputs are likely to experience simultaneous overvoltage
for long periods of time (>500 ms), it is important to use a
limiting resistor in series with the inputs. This resistor not only
extends the overvoltage range of the device, but also shares the
power burden during an overvoltage event. Figure 7 shows the
power dissipation of the ADA4177-4 during a 32 V overvoltage
event applied to only two positive inputs and with all four
positive inputs simultaneously. Figure 7 shows a plot of the
power dissipation of the ADA4177 with two and four inputs
overvoltaged by 32 V vs. an additional input series resistance.
1.0
EXTENDING THE INPUT OVP PROTECTION RANGE
AND MINIMIZING SELF-HEATING DURING OVP
EVENTS WITH CURRENT LIMITING RESISTORS
POWER DISSIPATION (W)
0.8
The ADA4177 inputs are equipped with current-limiting JFETs.
These JFETs limit current into the amplifier during an overvoltage
or differential fault, and improve the robustness and reliability
of the device. However, to keep the input noise to a minimum
during normal operation, these FETs must be large.
With a θJA of 158 W/°C, the temperature increase is approximately
12°C. If the ADA4177-4 is being used with a possibility of all
control inputs being exposed to an overvoltage condition for an
indefinite time, power dissipation can quickly raise the junction
temperature above the maximum of 150°C.
4 INPUTS,
32V OF OVERVOLTAGE
0.4
0.2
2 INPUTS,
32V OF OVERVOLTAGE
The engineering tradeoff associated with this is that the current
limit may not be as low as is needed for all applications. As
shown in Figure 6, the positive input sinks approximately 7.5 mA
with 10 V of overvoltage (OV). The power dissipation (PD) is
calculated as follows:
PD = 10 V × 7.5 mA = 7.5 mW
0.6
0
0
2
4
SERIES RESISTANCE (kΩ)
6
13819-007
VEE
13819-005
J1A
0
–2.5
–5.0
VCC
VIN+
IB–
IB+
2.5
Figure 7. Power Dissipation of the ADA4177-4 During Overvoltage
Figure 8 shows the temperature rise of the ADA4177-4 during
the same overvoltage event using an assumed θJA of 158 W/°C
to calculate the increase in die temperature; it is plotted in the
same way as the power dissipation shown in Figure 7.
Rev. 0 | Page 3 of 4
AN-1387
Application Note
200
REFERENCES
TEMPERATURE INCREASE (°C)
ADA4177-4 product page and data sheet
ADA4177-2 product page and data sheet
150
ADA4177-1 product page and data sheet
Modica, Eric and Michael Arkin. “Robust Amplifiers Provide
Integrated Overvoltage Protection.” Analog Dialogue, 46-02,
February 2012.
100
4 INPUTS,
32V OF OVERVOLTAGE
50
REVISION HISTORY
2 INPUTS,
32V OF OVERVOLTAGE
0
0
2
4
6
SERIES RESISTANCE (kΩ)
13819-008
12/15—Revision 0: Initial Version
Figure 8. Temperature Rise of the ADA4177-4 During Overvoltage
For example, inserting a 2 kΩ resistor in series with the positive
input halves the power dissipation of the ADA4177 during an
overvoltage event and adds only approximately 1 nV/Hz of
noise to the system during normal operation. Adding this resistor
limits the temperature rise during overvoltage events. Without
this resistor, the temperature rise can reach approximately 150°C
when all four inputs are exposed to overvoltage; with the external
resistor, the temperature rise is limited to approximately 75°C.
Similarly, if two inputs are exposed to overvoltage, the
temperature rise can reach approximately 70°C without the
resistor; with the resistor, the temperature rise is limited to 40°C.
In addition, the overvoltage protected range is increased from
32 V to 50 V because the external resistor shares some of the
overvoltage burden.
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AN13819-0-12/15(0)
Rev. 0 | Page 4 of 4
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