DN533 - Robust High Voltage Over-The-Top Op Amps Maintain High Input Impedance with Inputs Driven Apart or When Powered Down

Robust High Voltage Over-The-Top Op Amps Maintain High
Input Impedance with Inputs Driven Apart or When
Powered Down
Design Note 533
Glen Brisebois
Introduction
Linear Technology’s Over-The-Top® op amps have
an input stage topology that allows them to operate
closed loop well above the positive supply rail. The
inputs remain high impedance when split apart in
voltage and also when shut down or with complete
loss of power supply. They are indispensable in robust
systems, where reliability is required in the face of
uncertain power sequencing. The LT6015, LT6016 and
LT6017 extend the op amp operational input voltage
capability to 76V, and improve precision with trimmed
offset voltage of 350µV (max) over all common mode
input voltages and over temperature.
Input Topology—Theory of Operation
An Over-The-Top input stage is shown in Figure 1.
At low common modes, the PNPs Q1 and Q2 form a
conventional precision differential pair with tail current
provided by I1. The diff pair forwards its collector currents into the folded cascode pair Q7, Q8, which then
drive the output stage. As the common mode rises to
within 1V of the upper supply rail, Q9 begins to steal
the tail current away from the diff pair and passes it
through the Widlar of Q11, Q12 which then biases
up the diode connected pair Q3, Q4 which in turn
V+
+
L, LT, LTC, LTM, Over-The-Top, Linear Technology and the Linear logo are
registered trademarks of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
I3
8µA
I4
8µA
+IN
–
Q9
PNP
Q1
Q2
PNP PNP
–IN
Q3
PNP
Q6
PNP
Q11
NPN
One Useful Circuit that Gets the Feedback “Up There”
The circuit of Figure 2 is a simple 4-resistor difference
amplifier. Differential input voltages applied at VIN
appear at the output, gained up a factor of 100 with
relatively little effect from VCM, especially when the
CMRR adjust is dialed in. The op amp inputs can ride
on a common mode up to 76V above the –5V supply.
I1
16µA
1V
V–
bias up the precision common base pair Q5, Q6. The
collectors of Q5 and Q6 are paralleled into the same
folded cascode as before. So the Q1, Q2 diff pair and
the Q5, Q6 common base pair are essentially in parallel, with each pair handling a specific input common
mode range. The true power of this approach is that
Q12 (as well as all the other junctions involved) can
handle a whopping 76V. That means that the Q5, Q6
input stage is active and precise even when taken far
above V+, and the op amp remains closed loop as long
as the feedback can get up there too. Be aware that
Q5 and Q6 do not provide current gain, so the LT6015
worst case offset current of 15nA rises to 500nA in
Over-The-Top mode.
Q12
NPN
Q7
NPN
Q4
PNP
Q5
PNP
TO
OUTPUT
STAGE
Q8
NPN
R3
6k
R4
6k
dn5gb F01
Figure 1. Over-The-Top Input Stage on LT6015 Can Common Mode to 76V, Independent of Positive Supply Voltage
12/14/533
The VCM + VIN voltages applied to the input resistors can
be slightly higher due to the attenuation of the resistor
divider at the + input. Worst-case input error at high
input voltage extremes due to offset current is 500µV,
with better accuracy typical between –5V and +5V.
has protection diodes to its rails, then unpredictable
system glitches may drive the FET gate high, putting the
full VBAT voltage across R3. R5 keeps any downstream
protection diodes or short-circuits at some distance,
and should be sized for value and power, assuming
the FET can turn hard on. This is an example of circuit
considerations necessary when designing robustness
into high voltage systems.
CMRR
ADJUST
97.6k
1k
+
VIN
1k
–
+
5V
5k
+
VBAT = 1.5V TO 76V
–
–5V ≤ VCM ≤ 71V
–5V
100k
VCM
–
R1
200Ω
1%
VOUT = 100 • VIN
LT6015
0.1Ω
10W
Figure 2. Gain of 100 Differential Amplifier. The OverThe-Top Input Stage of the LT6015 Can Handle Common
Mode Input Voltages 76V Above V–, Independent of the
Positive Supply Voltage
High Side Current Sense
The circuit of Figure 3 is a precision high side current
sense amplifier that functions over a wide input common mode range and goes high impedance when its
supply vanishes. The inputs of the op amp are held
high, and feedback is level shifted “up there” through
the FET. Because the FET operates from VBAT, the
output clips as it approaches VBAT – VR1 – VDS. R1 and
R3 set the gain accuracy. One might think R2 can be
a 5% tolerance resistor, but it is added to accurately
reject DC errors created by the input bias current,
which is quite high in Over-The-Top mode, so 1% is
not a waste. R4 is there for Jim Williams, who never
wanted to see a MOSFET gate without a resistor on
it. R5 is a current limiting resistor with a cautionary
tale. If this circuit is operated at higher VBAT and
VSUPPLY, and is driving downstream circuitry which is
on lower supply, or is off, and the downstream circuitry
VSOURCE = 0.2V TO 76V
R1
200Ω
ISENSE
0.1Ω
RSENSE
LOAD
R2
200Ω
+
LT6015
dn5gb F02
LOAD
VSUPPLY = 5V TO 50V
0.1µF
R2
200Ω
1%
R4
100Ω
–
R3
2k
1%
VOUT =
BSP89 1V/A
0V TO 1V OUT
(LIMITED BY
R5 VBAT - VDS AND
330Ω VSUPPLY - VGS - VOH)
DN5GB F03
Figure 3. High Side Current Sense Amp Operates Up to 76V
Even on a Single 5V Supply. FET Provides Feedback. Full
Scale Output Is Limited by VBAT And VSUPPLY. The 330Ω
Output Resistor Is Precautionary—See Text. For Robust
Operation if VSUPPLY Goes Low or Is Removed Completely,
All Inputs Go High Impedance So VBAT Is Not Loaded
To extend the serviceability of the high side current
sense amplifier to lower input voltages, the dual LT6016
can be used, as shown in Figure 4. Taking less gain in
the first stage keeps the MOSFET source voltage low,
allowing for a lower input common mode voltage limit
down to 0.2V. Circuit gain is recovered in the second
amplifier stage.
Conclusion
The Over-the-Top LT6015 family of amplifiers provides
the industrial system designer a precision solution
for high voltage monitoring using conventional low
voltage regulated power supply rails. These amplifiers
have built in protection mechanisms for many extreme
operating conditions to ensure robust designs.
VSUPPLY
0.1µF
+
BSP89
LT6016
–
R6
100Ω
1%
VSUPPLY = 3V TO 60V
*DIODE IMPROVES OUTPUT SWING LOW
R3
200Ω
+
LT6016
–
R4
200Ω
R5
10k
1N4148*
VOUT
VOUT = RSENSE • ISENSE 1+
R5
R4
DN5GB F04
Figure 4. Extended Range High Side Current Sense Amplifier with VSOURCE Down to 0.2V
Data Sheet Download
www.linear.com/6015
Linear Technology Corporation
For applications help,
call (408) 432-1900, Ext. 3755
dn533f LT/AP 1214 111K REV A • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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 LINEAR TECHNOLOGY CORPORATION 2014
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