Shut Down a Current-Sense Amp by its Supply Pins

Nov 1, 2007 12:00 PM
By Arpit Mehta, Strategic Applications Engineer, and Prashanth Holenarsipur, Product Definer, Maxim

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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 V_CC supply. Furthermore, pulling the V_CC 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 V_CC 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 V_CC Pin to Ground

To simulate the signal otherwise obtained by placing a current-sense resistor in the power-supply line, a 20-mV_PK-PK signal, offset by 20 mV, rides on a 10-V common-mode input voltage. The loss of V_CC is simulated by a 0-V to 5-V square wave at the V_CC pin. During 5-V intervals at V_CC, 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 mV_PK-PK + 20 mV), that is, a 1-V_PK-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 V_CC 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 V_CC during the 0-V intervals. During the shutdown intervals, the leakage current measured at the V_CC 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 V_CC. 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 V_CC 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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