
Current Sensing on a Negative Voltage Supply Rail, using a Precision Instrumentation Amplifier
By: 
Akshay Bhat, Senior Strategic Applications Engineer 

Abstract: Applications like ISDN and telecom systems need a negative voltage, currentsense amplifier. This application note describes one method for designing a negativerail, currentsense amplifier. The design is quite flexible and can be easily changed for monitoring different negative rails. The MAX4460 singlesupply instrumentation amplifier is used to demonstrate the design.
Introduction
Highside currentsense amplifiers are used principally for monitoring the current from a positive supply rail. Applications like ISDN and telecom power supplies, however, require currentsense amplifiers that operate at negative rail. This application note describes one method for designing a negativerail, currentsense amplifier.
Application Example
Figure 1. Block diagram of a telephone centralexchange, powersupply system.
Figure 1 shows a block diagram of the powerdistribution network in a typical telephone exchange. A rectifier converts the AC at the power mains to DC, and the DC output from the rectifier is used to charge a 48V leadacid battery. The battery powers the user telephones through the telephone line. The battery polarities are connected so that the line voltage is negative (48V). A negative line voltage helps to reduce the corrosion from electrochemical reactions occurring on a wet telephone line. A telecom network also uses several DCDC converters to derive intermediate powersupply rails from the 48V DC input. The intermediate power supply rails power the switches, radios, routers, ATX computers, and other electronic equipment in the telephone exchange. A currentsense amplifier oversees the system health by monitoring the 48V powersupply current.
Circuit Description
Figure 2. Negativerail, currentsense amplifier using the MAX4460.
The circuit in Figure 2 shows an implementation of the negativerail, currentsensing block. It uses an instrumentation amplifier like the MAX4460 or the MAX4208 and some discrete components.
The zener diode, D_{1}, protects the instrumentation amplifier from overvoltage damage while providing sufficient supply voltage for its operation. The current to be monitored flows to the negative supply through the sense resistor, R_{SENSE}. The instrumentation amplifier must have a single supply and operate with a groundsensing capability.
The MAX4460's output provides the gate drive for MOSFET M_{1}. Negative feedback ensures that the voltage drop across resistor R_{3} equals V_{SENSE}, the voltage across R_{SENSE}. Consequently, R_{3} sets a current proportional to the load current:
I_{OUT} = (I_{LOAD} × R_{SENSE})/R_{3} = V_{SENSE}/R_{3}(Eq. 1)
R_{2} is chosen so that the output voltage lies within the desired range of the following circuit, typically an ADC. The drainsource breakdown voltage rating of the MOSFET must exceed the total voltage drop between the two supply rails (+125V in this case). An additional opamp buffer can be used at V_{OUT} if the ADC does not have a highimpedance input.
If the sense current increases above the rated value during a fault condition, then the output voltage goes negative. Diode D_{2} protects the ADC from damage by limiting the negative voltage at output to one diode drop.
Design Steps
The above design can easily be adapted to add highvoltage, negative supply, currentsense monitoring capability. This flexibility is illustrated by choosing 120V as the negative rail. By using the following straightforward steps, one can design a currentsense amplifier for a different supply rail.
1. Specify the Zener Regulator
It is important to bias the zener on a point in its transfer characteristic that gives a low dynamic resistance (i.e., well into its reverse breakdown region) to prevent PSRR errors. Figure 3 shows a plot of the zener current versus the zener voltage for a standard zener diode configured in reverse bias. Data show that the zener voltage is not wellregulated close to the breakdown voltage. A general rule then is to select the bias point to be about 25% of the maximum current specified by the power rating. This bias point gives a low dynamic resistance without wasting too much power. The bias point is set to the desired value by choosing the resistor, R1, based on the following equation:
I_{R1} = (V_{CC} + V_{NEG}  V_{Z})/R_{1} = I_{S} + I_{Z}(Eq. 2)
Where:
V_{CC} = Positive railsupply voltage
V_{Z} = Regulated zener voltage
V_{NEG} = Absolute value of the negativerail voltage
I_{S} = Supply current for MAX4460
I_{Z} = Current through the zener diode
R_{1} must have a suitable power rating and be able to withstand the large voltages across it. Alternatively, one can use a seriesparallel combination of lower wattage resistors to ease these constraints.
Figure 3. 1N750 Zener diode transfer characteristic, V_{Z} = 4.7V.
2. Select the Power Transistor
The nchannel MOSFET, or JFET, must have a drain to source breakdown voltage rating greater than V_{NEG} + V_{CC}. This is an important constraint if the negative supply voltage is high.
3. Choose R_{SENSE}
Select R_{SENSE} so that the fullscale, sense voltage across R_{SENSE} is less than or equal to 100mV.
4a. Select R_{3}
There is considerable flexibility in choosing R_{3}. A good selection is influenced by the following two observations:
 As R_{3} is reduced, Equation 1 implies that for a fixed gain, the dissipated power increases.
 The thermal noise and leakage current of the FET set the upper limit on the selected value of R_{3}.
4b. Select R_{2}
The ratio of resistors R_{2} and R_{3} equals the voltage gain of the resulting currentsense amplifier. The output voltage is given as:
V_{OUT} = V_{CC}  I_{OUT} × R_{2}(Eq. 3)
From Equations 1 and 3 we get:
V_{OUT} = V_{CC}  (V_{SENSE} × R_{2}/R_{3})
Differentiating with respect to V_{SENSE}:
Voltage gain, A_{v} = R_{2}/R_{3}(Eq. 4)
The negative sign represents the inverting relationship between the output voltage and the input sense voltage. From Equation 4, R_{2} can thus be determined.
Results
Figure 4 plots the resulting typical output voltage as a function of the sense voltage. The following typical parameters can be inferred for the currentsense amplifier:
Input referred offset voltage = (5  4.9831)/49.942
= 338µV
Gain = 49.942
Figure 4. Output voltage variation with variation in sense voltage at T = +25°C.
Conclusion
This application note demonstrates the use of a precision, instrumentation amplifier like the MAX4460 for current sensing of a negative voltage. The described circuit can be easily redesigned for monitoring different negative rails by following the design steps listed above.
A similar article appeared in the August, 2007 issue of Power Electronics Technology magazine, a Penton Publication.
Related Parts 
MAX4208 
UltraLow Offset/Drift, Precision Instrumentation Amplifiers with REF Buffer 
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MAX4460 
SOT23, 3V/5V, SingleSupply, RailtoRail Instrumentation Amplifiers 
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MAX9918 
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MAX9919 
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MAX9920 
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APP 4050: Dec 21, 2007
APPLICATION NOTE 4050,
AN4050,
AN 4050,
APP4050,
Appnote4050,
Appnote 4050


