Datasheet AD8212 (Analog Devices) - 10

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AD8212 Data Sheet HIGH VOLTAGE OPERATION USING AN EXTERNAL
In this mode of operation, the supply current (IBIAS) of the
PNP TRANSISTOR
AD8212 circuit increases based on the supply range and the The AD8212 offers features that simplify measuring current in RBIAS resistor chosen. For example the presence of common-mode voltages greater than 65 V. This if is achieved by connecting an external PNP transistor at the V+ = 500 V and RBIAS = 500 kΩ output of the AD8212, as shown in Figure 23. The VCE break- down voltage of this PNP becomes the operating common-mode IBIAS = (V+ − 5 V)/RBIAS range of the AD8212. PNP transistors with breakdown voltages then, exceeding 300 V are inexpensive and readily available in small IBIAS = (500 – 5)/500 kΩ = 990 µA packages. In high voltage operation, it is recommended that IBIAS remain
BATTERY RSHUNT
within 200 µA to 1 mA. This ensures that the bias circuit is
1 8
turned on, allowing the device to function as expected. At the
AD8212
same time, the current through the bias circuit/regulator is
R1 R2
limited to 1 mA. Refer to Figure 19 and Figure 21 for IBIAS and
D
V+ information when using the AD8212 in a high voltage
LOA
configuration.
A1
When operating the AD8212, as depicted in Figure 23, Transistor Q2 can be a FET or a bipolar PNP transistor. The latter is much less expensive, however the magnitude of IOUT
Q1
conducted to the output resistor (ROUT) is reduced by the amount of current lost through the base of the PNP. This leads to an error in the output voltage reading.
OUTPUT CURRENT
The AD8212 includes an integrated patented circuit, which
COMPENSATION BIAS
compensates for the output current that is lost through the base
CIRCUIT
of the external PNP transistor. This ensures that the correct
5 2 3 6
transconductance of the amplifier is maintained. The user can
Q2
opt for an inexpensive bipolar PNP, instead of a FET, while maintaining a comparable level of accuracy.
VOUT ROUT RBIAS OUTPUT CURRENT COMPENSATION CIRCUIT
004 The base of the external PNP, Q2, is connected to ALPHA 05942- Figure 23. High Voltage Operation Using External PNP (Pin 6) of the AD8212. The current flowing in this path is mirrored inside the current compensation circuit. This The AD8212 features an integrated 5 V series regulator. This current then flows in Resistor R2, which is the same value regulator ensures that at all times COM (Pin 2), which is the as Resistor R1. The voltage created by this current across most negative of all the terminals, is always 5 V less than the Resistor R2, displaces the noninverting input of Amplifier A1 supply voltage (V+). Assuming a battery voltage (V+) of 100 V, by the corresponding voltage. Amplifier A1 responds by driving it fol ows that the voltage at COM (Pin 2) is the base of Transistor Q1 so as to force a similar voltage (V+) – 5 V = 95 V displacement across Resistor R1, thereby increasing IOUT. The base emitter junction of Transistor Q2, in addition to the Because the current generated by the output compensation Vbe of one internal transistor, makes the collector of Transistor Q1 circuit is equal to the base current of Transistor Q2, and the approximately equal to resulting displacements across Resistor R1 and Resistor R2 result 95 V + 2(V in equal currents, the increment of current added to the output be(Q2)) = 95 V + 1.2 V = 96.2 V current is equivalent to the base current of Transistor Q2. This voltage appears across external Transistor Q2. The voltage Therefore, the integrated output current compensation circuit across Transistor Q1 is has corrected IOUT such that no error results from the base 100 V – 96.2 V = 3.8 V current lost at Transistor Q2. In this manner, Transistor Q2 withstands 95.6 V and the This feature of the AD8212 greatly improves IOUT accuracy and internal Transistor Q1 is only subjected to voltages well below al ows the user to choose an inexpensive bipolar PNP (with low its breakdown capability. beta) with which to monitor current in the presence of high voltages (typically several hundred volts). Rev. C | Page 10 of 13 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION NORMAL OPERATION (7 V TO 65 V SUPPLY (V+) RANGE) HIGH VOLTAGE OPERATION USING AN EXTERNAL PNP TRANSISTOR OUTPUT CURRENT COMPENSATION CIRCUIT APPLICATIONS INFORMATION GENERAL HIGH-SIDE CURRENT SENSING MOTOR CONTROL 500 V CURRENT MONITOR BIDIRECTIONAL CURRENT SENSING OUTLINE DIMENSIONS ORDERING GUIDE