High-Side Current Monitor Uses Low-Voltage Amplifier

Jun 1, 2007 12:00 PM
By Greg Sutterlin and Cicie Wang, Field Applications Engineers, Maxim Integrated Products, Sunnyvale

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Telcom, LDMOS, automotive and many other applications require the measurement of current flow at high voltages (high-side current). Often, a circuit operating at 5 V must monitor currents at 48 V. Techniques using costly high-voltage difference amplifiers and other special devices can measure such currents, but the circuit in the figure performs the same function with a standard 5-V difference amplifier (U1), including provisions for gain adjustment.

The difference amp combines gain accuracy with low-power operation, but its maximum supply voltage is only 7.5 V. To overcome this limitation, the p-n-p bipolar transistor Q2 transforms U1's voltage output to current. Thus, Q2's current output serves as a level shifter between the 48-V monitored current and the 5-V monitor circuit. To minimize output-current errors caused by U1's internal gain resistors, zener diode Z1 and resistor R_SHUNT clamp U1's operating voltage to a nominal 3.0 V. U1's maximum operating current (I_CC) is 55 µA, so the maximum value for R_SHUNT is found with the following formula:

R_SHUNT(MAX) = (V_IN(MIN)- V_Z1(MAX))/(I_CC(MAX) + I_Z1(MIN)) =(42 V - 2.7 V)/(55 µA + 100 µA) = 249 kΩ,

where I_Z1(MIN) is the minimum current required for a flat zener characteristic, V_IN(MIN) is the input voltage, and V_Z1(MAX) is the zener tolerance for Z1.

For the connections shown in the figure, the voltage drop across R_OUT equals the drop across R_SENSE. For a maximum full-scale voltage of 100 mV across R_SENSE, you choose R_OUT to set the magnitude of the corresponding full-scale output current. This R_OUT selection involves a tradeoff, however. Higher current minimizes the effect of error current induced by the voltage drop across the two 25 kΩ resistors within U1 between the IN+ and REF terminals. The percentage error this effect introduces is lower for higher output currents.

However, higher current levels can create over-voltage in U2 and they waste power. A moderate value that avoids these extreme conditions is approximately 350 µA. Thus, R_OUT = 100 mV/350 µA = 286 Ω (287 Ω is the closest standard value). At 3-V operation, the maximum error current introduced by U1's resistors is 3 V/50 kΩ = 60 µA, or 17%. The actual value will be less due to the V_BE (about -0.7 V) of Q1. This may still seem relatively high, but a simple calibration routine can subtract most of the effects of this error, yielding a negligible effect on the calibrated result. A common p-n-p bipolar transistor (MMBT3906) is chosen as the level shifter. Operating at 350 µA and close to a V_CE of 45 V, the transistor's power dissipation is below 20 mW.

The 10-kΩ digital potentiometer U2 adjusts the current monitor's gain. U2 has a simple up/down interface and a low value of nominal end-to-end resistance (10 kΩ). Worst-case variations in the initial tolerance, supply voltage, and temperature yield a maximum end-to-end resistance of 12.5 kΩ. Adding a 60 µA error current to the 350µA full-scale signal current selected above yields the maximum output current from Q1: 350 µA + 60 µA = 410 µA. Multiplying this current by the maximum end-to-end resistance yields the maximum voltage that will develop on pin “H” of the MAX5402 = 410 µA × 12.5 kΩ = 5.12 V.

U2 specifies an absolute maximum voltage of 6 V at pin H, so this full-scale signal current is sufficiently low to avoid over-voltage at pin H.

The technique presented allows use of a 5-V (maximum) difference amplifier in a 48-V application, and the circuit can be modified as required for lower or higher common-mode voltages. The use of Q1 to transform the signal voltage to a current allows easy gain adjustment with a digital potentiometer. The digipot shown (MAX5402) can divide the full-scale signal magnitude by factors as high as 32. Such gain adjustment is useful for automotive battery monitoring, and other applications in which the monitored current varies over a wide dynamic range.

It is important to provide separate ground paths for the digital potentiometer and the op amp. It is also important not to connect these grounds to earth ground. Otherwise, that connection places the digital potentiometer in parallel with the op-amp circuit, causing the resistance at V_OUT (relative to ground) to vary with the voltage at V_IN.