Robust High Voltage Over-The-Top Op Amps Maintain High Input Impedance with Inputs Driven Apart or When Powered Down
Linear Technology » LT6015, LT6016, LT6017
Glen Brisebois, Linear Technology
Design Note 533
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 76 V, and improve precision with trimmed offset voltage of 350 μV (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 1 V 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 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 76 V. 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 15 nA rises to 500 nA in Over-The-Top mode.
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 76 V above the –5 V supply. 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 500 μV, with better accuracy typical between –5 V and +5 V.
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 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.
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 0.2 V. Circuit gain is recovered in the second amplifier stage.
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.
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