Power Electronics

Shut Down a Current-Sense Amp by its Supply Pins

Unlike traditional op amps, the high-side current-sense amplifier does not include an internal electrostatic discharge (ESD) protection diode from each input pin to its power-supply pin. As a result, it can operate at common-mode voltages well above the level of its VCC supply. Furthermore, pulling the VCC pin of a typical current-sense amplifier to ground places the part in shutdown mode — in which it draws no quiescent current, only leakage current. Thus, the VCC pin of a current-sense amplifier can serve as a shutdown pin.

Consider a typical battery-operated device in which a power source, for example, a low-dropout voltage regulator (LDO), powers several ICs on a circuit board that includes a MAX4173 current-sense amplifier. To extend battery life by saving power, the system frequently turns off the LDOs and current-sense amplifiers, as simulated in the Fig. 1 circuit.

Pull the VCC Pin to Ground

To simulate the signal otherwise obtained by placing a current-sense resistor in the power-supply line, a 20-mVPK-PK signal, offset by 20 mV, rides on a 10-V common-mode input voltage. The loss of VCC is simulated by a 0-V to 5-V square wave at the VCC pin. During 5-V intervals at VCC, the amplifier operates in its active mode, but during 0-V intervals it goes into shutdown mode.

Because the amplifier gain is 50, the expected output is 50 × (20 mVPK-PK + 20 mV), that is, a 1-VPK-PK sine wave offset by 1 V (Fig. 2). As expected, the amplifier is active when 5 V is applied and produces the expected output. When VCC goes to 0 V, the output also goes to 0 V. The part shuts down and draws no supply current.

Connect an NMOS Transistor

Another way to shut down a current-sense amplifier is to connect an NMOS transistor in the ground path (Fig. 3) and drive it with logic-level signals capable of turning the transistor on and off. When the transistor is on, the amplifier operates normally. When it's off, the amplifier shuts down because its ground is floating.

The output waveform for the Fig. 4 setup demonstrates the expected behavior, amplifying the input signal during the 5-V intervals and floating close to VCC during the 0-V intervals. During the shutdown intervals, the leakage current measured at the VCC pin is just 4 µA because of the 1-MΩ input impedance of the measurement scope. When the scope probe is absent, only the NMOS transistor's leakage current is drawn from VCC. Input current on the RS+/RS- pins is just 0.3 µA.

Thus, one can easily put the MAX4173 in shutdown mode either by pulling its VCC pin to ground, or by opening its ground connection using an NMOS transistor. Similar results can be expected from other current-sense amplifiers as well.

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