Precision Circuit Monitors Negative Supply Current

Sep 1, 2005 12:00 PM
By Ken Yang, Editor, Senior Member of the Technical Staff, Maxim Integrated Products Inc., Sunnyvale

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Supply-current monitoring is a necessary feature in high-reliability systems for which excessive current can cause damage or compromise safety. Such systems avoid overload faults by monitoring their power supply and shutting it down before the fault occurs. However, most current-monitoring ICs are designed for positive-voltage supplies. For negative supplies, the circuit of Fig. 1 monitors load current and provides a proportional output voltage.

Voltages at the inverting and noninverting terminals of op amp IC1A are forced to be equal by an active-feedback current mirror. V_R1 = V_SENSE, and therefore:

Three alternatives are now possible. You can convert the output current (I_R1) to voltage by connecting resistor R_O to ground, to V_CC or to an inverting amplifier. Connecting R_O to ground (GND) eliminates the need for a positive supply. In that case, the output voltage is negative and proportional to load current:

(R_O connected to GND).

You can connect R_O to V_CC for applications that require a positive output voltage, but the output will be referenced to V_CC:

(R_O connected to V_CC).

To reference the positive voltage output to ground, you must use an inverting amplifier (IC1B) as shown in Fig. 1:

(R_O connected to an inverting amplifier).

Note that R_O does not affect output voltage for the inverting-amplifier case, but that resistor is usually needed for stability. R_G can be optional, but it also provides stability by isolating the op amp from the capacitive load of the MOSFET gate. Finally, resistor R_C provides compensation for the op amp's input bias current.

Fig. 2 shows measurement error versus load current for the Fig. 1 circuit. To ensure accurate current measurements, the resistors (except for R_G and R_C) should have a tolerance of 1% or better. R_SENSE must be rated to dissipate the power associated with high load currents.

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