Detecting Over-Power Scenarios using Maxim's Quad Channel Power Monitors and Voltage Supervisors

It is inevitable to have a voltage protection circuit and a power monitoring circuit in any advanced electronics application such as tablets, smartphones, ultrabooks, notebooks, etc. Power monitors are generally used in all applications that require a measure of average power through the critical rails. These devices sample and accumulate the voltage and current of a rail to provide instantaneous power measurements. The MAX34417 is a specialized power monitor used to determine power consumption of portable systems.

Voltage monitors are supervisory ICs that monitor the voltage rails for any overvoltage or undervoltage faults. When supply voltage falls below a set threshold due to the excess current consumption by a system, the supervisory IC asserts the flag output to shut down the input power supply of the system. The MAX16143 is an example of such a single-channel supervisory circuit that monitors the supply voltage within the input tolerance setting.

These supervisory ICs do not report faults in overpower scenarios. However, for portable applications, it is very important to report overpower/energy faults and assert an output to the microcontroller to prevent damage to the rest of the circuitry. A circuit, comprising of power monitor IC (MAX34417) and supervisory IC (MAX16143), provides a solution to report under-voltage and overvoltage, as well as overpower scenarios. The following section of this application note explains this circuit in detail with two examples.

Figure 1 is a top-level block diagram for this kind of application. The resistor R_SENSE is a current- sensing element. The differential voltage between IN+ and IN- pins translates to the load current of the system. The MAX34417 calculates the average power drawn from the battery over a period based on the voltage readings at IN- pin and current readings through differential voltage at the sense resistor. Power readings can be read through I²C interface.

The supervisory IC (MAX16143) monitors the supply voltage, which is dependent on the drop across the sense resistor as given in Equation 1. In overpower scenarios, this voltage drop across R_SENSE increases and thus the input voltage to the MAX16143 drops down. As soon as this voltage reaches the undervoltage threshold, the MAX16143 asserts its reset pin, which is connected to the flag pin of the system. This flag output is an indication of an overpower scenario.

The system can differentiate between undervoltage and overpower scenarios by monitoring both the flag output and the power registered data of the MAX34417 device. In an undervoltage scenario, the flag asserts, but the power register of the monitoring channel indicates the nominal power level. Whereas, in an overpower scenario, not only does the flag assert, but also the power registered data is either above or at the saturated power level.

The input voltage to the supervisor can be calculated as in Equation 1:

V_SUP = V_BAT -(R_SENSE × I_IN)

Here, V_BAT = 12V, I_IN = 1A, R_SENSE = 100mO. As per the equation, V_SUP = 11.9V.

The input tolerance setting of the MAX16143, the supervisory IC in this example, is 1% above and below nominal voltage. In this case, the nominal voltage is 12V. Hence, the input overvoltage threshold value is set to 12.019V (1% above nominal voltage) and the input undervoltage or overcurrent threshold is set to 11.781V (1% below nominal voltage) as shown in Figure 2.

The current sense resistor R_SENSE helps the supervisory IC in detecting overpower conditions.

For example, if I_IN = 2.5A, the voltage drop across the sense resistor will be 2.5A x 100mO = 250mV, and V_SUP becomes 12 - 250mV = 11.75V. Since the supply voltage of the supervisor has fallen below the set threshold, the reset signal (flag output) is asserted.

When the current starts returning to its nominal range, V_SUP goes above 11.781V and the fault condition is deasserted.

Here is another example when V_BAT = 3.3V:

According to Equation 1, if I_IN = 1A and R_SENSE = 100mO, the voltage drop across the sense resistor will be 1A × 100mO = 100mV and V_SUP becomes 3.3V - 100mV = 3.2V.

If the input tolerance setting of the MAX16143, the supervisor is 1% then the input overvoltage threshold is set to 3.232V and input undervoltage threshold is set to 3.168V.

When there is an overpower condition, for example, if I_IN = 2A, the voltage drop across R_SENSE is 2A × 100mO = 200mV and V_SUP becomes 3.3V - 200mV = 3.1V. Since the supply voltage of the supervisor has fallen below the set threshold, the reset signal (flag output) is asserted.

Conclusion

This application note provides a solution for detecting overpower scenarios in any electronic systems. Designers can use these methodology or example to design a system with variable load currents, which require indicator output for overpower, undervoltage or overvoltage scenarios.