Creating a Bidirectional Current-Sense Amplifier from Two Unidirectional Current-Sense Amplifiers
IntroductionMany current-sense applications require bidirectional current-sensing capabilities. For instance, when a laptop is plugged into an AC power line, the AC adapter supplies power to the laptop and charges the battery. The battery's charge current is monitored to ensure that the battery does not overheat and that the total input power drawn from the AC adapter does not exceed UL-mandated limits. Similarly, the battery discharge current is monitored for fuel-gauging/active power management on the load device when the AC adapter is not available and battery capacity needs to be conserved. Figure 1 shows a typical current-sense amplifier implementation in a battery-charging/-discharging application.
Figure 1. The typical battery-charging/-discharging circuit used in laptop applications.
Building a Bidirectional Current-Sense AmplifierThe MAX4172, MAX4173, and MAX4073 are three popular unidirectional current-sense amplifiers that feature current outputs. The MAX4172 is available in a µMAX® package and offers precision 1.6mV maximum offset voltage (0.75mV VOS for SOIC version) and ±2% maximum gain-error specifications over the industrial temperature range. The MAX4173 is available in a tiny SOT23 package; it features 3mV maximum offset and an internal 12kΩ load resistor to convert its current output to a voltage output. The MAX4073 is similarly available in a tiny SC70 package. All three devices also have high signal bandwidths, making them attractive solutions within analog control loops.
The voltage-input, current-output feature of these devices enables one to leverage them in a wide variety of clever circuits. One can, for example, use two unidirectional current-sense amplifiers—such as MAX4172 or MAX4173—to build a fast bidirectional current-sense amplifier (Figure 2).
Figure 2. Two circuits showing bidirectional current-sense applications created from two unidirectional current-sense amplifiers.
Part A is active when VRS+ > VRS- (i.e., when the battery is supplying the load current) and Part B is active when VRS- > VRS+ (i.e., when the battery is being charged by the AC adapter). A general purpose op-amp combines the output currents from the two amplifiers into an appropriate output voltage. VREF sets the output voltage at zero current (zero-sense voltage). The output voltage increases above VREF when Part A is active and decreases below VREF when Part B is active. By using Ra ≠ Rb in the MAX4172 circuit, different gains can be used for positive (discharging) currents and negative (charging) currents.
The MAX4173 circuit uses a 1nF capacitor in its feedback to stabilize the control loop. Since the GND pin of Part B is modulated by the output of the op-amp, the VCC applied to the MAX4173 should be at least 3V above the maximum output of MAX4230. This value ensures sufficient supply-voltage headroom for proper operation of the device.
Transfer characteristics for both MAX4172 and MAX4173 circuits are provided in Figure 3 and Figure 4, respectively.
Conditions for MAX4172 circuit are:
VBAT = 8V
VCC = 3.3V for MAX4172 and MAX4230
Ra = Rb = 1kΩ
VREF = 1.2V
Figure 3. The transfer characteristic of a MAX4172 circuit.
Conditions for MAX4173 circuit are:
VBAT = 8V
VCC = VBAT for MAX4173
VCC = 5V for MAX4230
VREF = 1.5V
Figure 4. The transfer characteristic of a MAX4173 circuit.
Maxim also offers integrated bidirectional current-sense amplifiers: