Monolithic Operational Amplifier Works from ±4.75V to ±70V and Features Rail-to-Rail Output Swing and Low Input Bias Current

Monolithic Operational Amplifier Works from
±4.75V to ±70V and Features Rail-to-Rail
Output Swing and Low Input Bias Current
Michael B. Anderson
Monolithic operational amplifiers have been around
since the 1960s, but this ubiquitous device still sees
steady improvements in performance. The LTC6090
precision monolithic operational amplifier takes a big step
forward by extending the supply voltage to ±70V without
compromising the features that are expected in a precision
op amp. The LTC6090 is available in a small 8-lead SO
package and a 16-lead TSSOP package. Both packages
feature exposed pads to reduce thermal resistance,
eliminating the need for a heat sink. An easy interface
to low voltage control lines and built-in thermal safety
features simplify the task of high voltage analog design.
Operational amplifiers are expected to
have low input bias current, low offset,
and low noise. The LTC6090 is no exception. Designed with a MOS input stage the
input bias current is typically 3pA at 25°C
and less than 100pA at 85°C. This makes
it well suited for high impedance applications such as a photodiode amplifier
Figure 2. LTC6090 output voltage 140VP–P 10kHz sine
wave
OUTPUT VOLTAGE SWING (V)
V+ – 0.8
V+ – 1.0
V+ – 1.2
V – + 0.8
V – + 0.6
14 | January 2013 : LT Journal of Analog Innovation
V–
0.001
0.01
SINK
0.1
1
10
LOAD CURRENT (mA)
+
8
LTC6090
1
5
200k
1%
100mW
6
4
VOUT
22.1k
1%
–3V
fast slew rate and rail-to-rail output stage
rated for ±10m A that can drive up to
200pF. An example shown in Figure 2 is a
140VP–P 10k Hz sine wave. Figure 3 shows
the output swing is well maintained as
load current is increased. And the fidelity
of the output voltage at 100VP–P extends
out to 8kHz as shown in Figure 4.
Figure 4. LTC6090 total harmonic distortion plus
noise vs. frequency
SOURCE
TA = 125°C
TA = 25°C
TA = –40°C
3
7
Figure 1. Extended dynamic range 1M
transimpedance photodiode amplifier
V+ – 0.6
V – + 0.2
–
VOUT = IPD • 1M
OUTPUT NOISE = 21µVRMS (1kHz – 40kHz)
OUTPUT OFFSET = 150µV MAXIMUM
BANDWIDTH = 40kHz (–3dB)
OUTPUT SWING = 0V TO 12V
VS = ±70V
V – + 0.4
2
–3V
Figure 3. LTC6090 output voltage swing vs load
current
V+ – 0.4
125V
PHOTODIODE
SFH213
On the output side, precision op amps are
expected to maintain precision when driving loads. Again, the LTC6090 does not disappoint. The unity gain stable output drive
capability includes a 10MHz GBW product,
V+
25µs/DIV
IPD
shown in Figure 1. The low input offset
voltage is less than 1.6mV, and the noise
is 11nV/√Hz at 10kHz. The input common mode range is to 3V of either rail or
a range of 134V across a 140V supply.
V+ – 0.2
VOUT
20V/DIV
10M
1%
TOTAL HARMONIC DISTORTION + NOISE (%)
HIGH VOLTAGE AND
HIGH PERFORMANCE
0.3pF
100
10
VS = ±70V
AV = 5
RL = 10k
CF = 30pF
1
0.1
VOUT = 100VP-P
0.01
0.001
VOUT = 50VP-P
VOUT = 10VP-P
10
100
1000
10000
FREQUENCY (Hz)
100000
design features
Operational amplifiers are expected to have low input bias current, low
offset and low noise. The LTC6090 is no exception. Designed with a MOS
input stage, the input bias current is typically 3pA at 25°C and less than
100pA at 85°C. This makes it well suited for high impedance applications.
HIGH IMPEDANCE APPLICATIONS
REQUIRE LOW LEAKAGE CIRCUITS
The low input bias current of the
LTC6090 make it an excellent choice
for high impedance applications that
require high voltage. As shown in
Figure 5, input bias current is logarithmically dependent on temperature,
doubling for every 10°C increase. In
addition, input protection devices sit
in an isolated pocket where leakage
increases as the voltage on the input pin
increases with respect to V–. In Figure 5
the input pin is held at mid-supply.
In order to maintain low input bias
current, care should be taken during
PCB layout. Special low leakage board
material can be considered. In critical applications, consider using guard
rings. The TSSOP package with exposed
pad has guard ring pins that can be used
to protect the input pins from leakage
currents. An example PCB layout of an
inverting amplifier is shown in Figure 6.
Note that the solder mask should be
pulled back over the guard ring to expose
the PCB metal. It is important that the
PCB be clean and moisture free. Consider
cleaning it with a solvent and rinsing
any residue with tap water, then baking
the board to remove any moisture. We
have also found that thoroughly washing the board using soap and tap water
(without solvent) yields good results.
INTERFACING LOW VOLTAGE
CONTROL LINES TO A HIGH
VOLTAGE OP AMP
The low voltage control lines on the
LTC6090 can be interfaced as low as
the negative supply rail, or as high as
5V below the positive supply rail. The
COM pin acts as a common to interface
to the low voltage control lines, and can
be connected to the low voltage system
ground or left to float. The output disable, OD, and overtemperature, TFLAG,
pins are now referred to the low voltage
system ground. COM, OD and TFLAG pins
are protected with diodes and resistors
as shown in Figure 7. If left floating the
COM pin will be pulled above mid-supply
by the OD pin internal pull-up resistor
to 21V when the supplies are ±70V.
THERMAL PROTECTION:
USE OD AND TFLAG
At 140V total supply voltage and
2.7m A typical quiescent current, the
LTC6090 consumes 378mW of power. Add
a load and the power can exceed a watt,
making good thermal design a priority.
Both packages, the SO and TSSOP, feature
an exposed pad on the bottom of the
Figure 7. The low voltage interface configured to
automatically disable the output stage when the
junction temperature of the die reaches 145°C
LTC6090
V+
2M
10k
OD
Figure 6. PCB guard ring example layout
Figure 5. LTC6090 input bias current versus junction
temperature
V+
OUT
2M
10k
COM
1000
2M
INPUT BIAS CURRENT (pA)
±70V
100
V–
10k
500Ω
±5V
10
+IN
TFLAG
10k
1
R2
–IN
0.1
0
20
40
60
80 100 120
JUNCTION TEMPERATURE (°C)
140
C1
R1
30k
V–
January 2013 : LT Journal of Analog Innovation | 15
An important feature designed to protect
the LTC6090 from exceeding 150°C junction temperature shuts down the output
stage when the junction temperature gets
too high. This is accomplished by connecting the overtemperature pin to the
output disable pin. The overtemperature
pin, or TFLAG pin, is an open drain pin that
pulls low when the junction temperature
of the die reaches 145°C. The 5°C built-in
hysteresis releases the TFLAG pin when
the junction temperature reaches 140°C.
The output disable pin, or OD pin, is an
active low pin that turns off the output
stage and lowers the quiescent current
of the device to 670µ A when pulled low
with respect to the COM pin. When these
two pins are tied together, the LTC6090
is disabled if the junction temperature
of the die reaches 145°C. Note that these
pins can float and be tied together.
An additional thermal safety feature
shuts off the output stage when the
junction temperature of the die reaches
approximately 175°C. The 7°C of hysteresis enables the output stage when it
returns to approximately 168°C as shown
Figure 8. LTC6090 thermal shutdown hysteresis plot
3.0
2.5
SUPPLY CURRENT (mA)
package, which is internally connected to
the negative supply rail, V–, and must be
connected to the negative power plane.
Connect as much PCB metal as practical to
the exposed pad—the thermal resistance of
the package is proportional to the amount
of metal soldered to the exposed pad. In a
best case scenario the thermal resistance,
qJA , of the SO package is 33°C/W. For 1W of
power, the junction temperature of the die
increases 33°C above ambient temperature.
2.0
1.5
1.0
0.5
0
162 164 166 168 170 172 174 176 178
JUNCTION TEMPERATURE (°C)
in Figure 8. Note that Figure 8 shows
the junction temperature. This feature
is intended to prevent the device from
thermal catastrophic failure. Operating
the LTC6090 above its absolute maximum junction temperature of 150°C can
reduce reliability and is discouraged.
CONCLUSION
The LTC6090 features the high performance specs of a low voltage precision
amplifier, but with the ability to work
with ±70V for high voltage applications.
These features include high gain, low input
bias current, low offset and low noise for
a precision front end. A rail-to-rail output
stage can drive a 200pF load capacitor and
±10m A of load current, making this part
suitable for precision high voltage applications such as high impedance amplifiers.
Easily interfaced control lines for disabling
the output and a thermal shutdown function are simple to implement. Small 8-lead
SO and 16-lead TSSOP packages both have
exposed pads to reduce thermal resistance,
eliminating the need for a heat sink. n
(LTC3115-1 continued from page 13)
In addition, many devices must remain
operating for a period of time after bus
failure in order to initiate a controlled
shutdown. The LTC3115-1 application
shown in Figure 9 is a 24V rail restorer
application that maintains a clean and
well-regulated 24V output rail from a
noisy input supply rail, which can fluctuate above and below the regulation target.
In addition, as shown in the waveforms
of Figure 10, this supply is able to maintain regulation of its 24V output through
momentary interruptions in bus power.
CONCLUSION
The flexibility and high efficiency of the
LTC3115-1 make it perfectly suited to
meet the demanding needs of the next
generation of automotive electronics and
16 | January 2013 : LT Journal of Analog Innovation
portable devices, especially those operated from multiple power sources. Its
internal power switches and programmable switching frequency minimize the
power solution footprint, supporting
the increasing demand for miniaturization of electronic devices in the portable
and automotive arenas. Low Burst Mode
operation and shutdown quiescent currents prolong battery life and facilitate use
in always-active automotive applications.
The LTC3115-1 is ideal for noise-sensitive
applications, given its low noise, fixed
frequency PWM mode, which produces a
predictable and well controlled EMI spectrum with switching edges that can be
synchronized to a system clock. Internal
soft-start minimizes inrush current during
start-up and an internal divider in the
control path reduces the impact of input
voltage variations, and makes the loop
easier to compensate in applications with
widely varying input voltages. A programmable input undervoltage lockout
allows the input voltage at which the
part is enabled to be set by the user, and
provides for independent control of the
hysteresis. The LTC3115-1 also features
complete disconnect of the output from
the input in shutdown, and is fully protected with output short-circuit protection and overtemperature shutdown. n