High-Voltage, Programmable-Gain Current Monitor

A similar version of this article appeared in the June 1, 2007 issue of PET.

Telcom, LDMOS, automotive, and numerous other applications require the measurement of current flow at high voltages (high-side current). Often, a circuit operating at 5V must monitor currents at 48V. Techniques using costly high-voltage difference amplifiers and other special devices can measure such currents, but the circuit of Figure 1 does it with a standard 5V difference amp, including provisions for gain adjustment.

The difference amp (U1, MAX4198) combines gain accuracy with low-power operation, but its maximum supply voltage is only 7.5V. To overcome this limitation, the pnp transistor Q1 transforms U1's voltage output to current. Thus, Q1's current output bridges the gap between the 48V monitored current and the 5V monitor circuit. To minimize errors induced by U1's gain resistor, zener Z1 and resistor R_SHUNT clamp U1's operating voltage to a nominal 3.0V. U1's maximum operating current (I_CC) is 55µA, so the maximum value for R_SHUNT is:

R_SHUNTMAX = (V_INMIN - V_Z1MAX)/(I_CCMAX + I_Z1MIN)
= (42V - 2.7V)/(55µA + 100µA) = 249kΩ

Where I_Z1MIN is the minimum current required for a flat zener characteristic, V_INMIN is the input voltage, and V_Z1MAX is the zener tolerance.

For the connections shown in Figure 1, the voltage drop across R_OUT equals the drop across R_SENSE. For a maximum full-scale (FS) voltage of 100mV 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 25kΩ resistors between IN+ and REF. The percentage error introduced by this effect is lower for higher output currents.

On the other hand, higher current levels can create over-voltage in U2, and they waste power. A good trade-off seems to be 350µA. Thus, R_OUT = 100mV/350µA = 286Ω (287Ω is the closest standard value). At 3.0V operation, the maximum error current induced by the resistor load is 3.0V/50kΩ = 60µA, or 17%. This may seem a bit 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 pnp transistor (MMBT3906) is chosen as the level translator. Operating at 350µA and close to 45V V_CE, the transistor's power dissipation is very low (< 20mW).

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

Max voltage on pin "H" of the MAX5402 = 410µA × 12.5kΩ = 5.12V

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

The technique presented allows use of a 5V (maximum) difference amplifier in a 48V 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's important to provide separate grounds 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.