8.2.4 System Power Supply Monitor Design Guidelines

The VMON_VSYS pin provides a way to monitor a system power supply. This system power supply is typically a single pre-regulated power source for the entire system and can be connected to the VMON_VSYS pin via and external resistor divider circuit. This system supply is monitored by comparing the external voltage divider output voltage to an internal voltage reference, where a power fail event is triggered when the voltage applied to VMON_VSYS drops below the internal reference voltage. The actual system power supply voltage trip point is determined by the system designer when selecting component values used to implement the external resistor voltage divider circuit.

When designing the resistor divider circuit the designer must understand various factors which contribute to variability in the system power supply monitor trip point. The first thing to consider is the initial accuracy of the VMON_VSYS input threshold which has a nominal value of 0.45 V, with a variation of ±3%. Precision 1% resistors with similar thermal coefficient are recommended for implementing the resistor voltage divider. This minimizes variability contributed by resistor value tolerances. Input leakage current associated with VMON_VSYS must also be considered since any current flowing into the pin creates a loading error on the voltage divider output. The VMON_VSYS input leakage current can be in the range of 10 nA to 2.5 µA when applying 0.45 V.

Note:

The resistor voltage divider shall be designed such that the output voltage never exceeds the maximum value defined in the Recommended Operating Conditions section, during normal operating conditions.

Figure 8-5 presents an example, where the system power supply is nominally 5 V and the maximum trigger threshold is 5 V - 10%, or 4.5 V.

For this example, the designer must understand which variables effect the maximum trigger threshold when selecting resistor values. A device which has a VMON_VSYS input threshold of 0.45 V + 3% needs to be considered when trying to design a voltage divider that doesn’t trip until the system supply drops 10%. The effect of resistor tolerance and input leakage also needs to be considered, but the contribution to the maximum trigger point is not obvious. When selecting component values which produce a maximum trigger voltage, the system designer must consider a condition where the value of R1 is 1% low and the value of R2 is 1% high combined with a condition where input leakage current for the VMON_VSYS pin is 2.5 µA. When implementing a resistor divider where R1 = 4.81 KΩ and R2 = 40.2 KΩ, the result is a maximum trigger threshold of 4.517 V.

Once component values have been selected to satisfy the maximum trigger voltage as described above, the system designer can determine the minimum trigger voltage by calculating the applied voltage that produces an output voltage of 0.45 V - 3% when the value of R1 is 1% high and the value of R2 is 1% low, and the input leakage current is 10 nA, or zero. Using an input leakage of zero with the resistor values given above, the result is a minimum trigger threshold of 4.013 V.

This example demonstrates a system power supply voltage trip point that ranges from 4.013 V to 4.517 V. Approximately 250 mV of this range is introduced by VMON_VSYS input threshold accuracy of ±3%, approximately 150 mV of this range is introduced by resistor tolerance of ±1%, and approximately 100 mV of this range is introduced by loading error when VMON_VSYS input leakage current is 2.5 µA.

The resistor values selected in this example produces approximately 100 µA of bias current through the resistor divider when the system supply is 4.5 V. The 100 mV of loading error mentioned above can be reduced to about 10 mV by increasing the bias current through the resistor divider to approximately 1 mA. So resistor divider bias current vs loading error is something the system designer needs to consider when selecting component values.

The system designer must also consider implementing a noise filter on the voltage divider output since VMON_VSYS has minimum hysteresis and a high-bandwidth response to transients. This can be done by installing a capacitor across R1 as shown in Figure 8-5. However, the system designer must determine the response time of this filter based on system supply noise and expected response to transient events.

Figure 8-5 System Supply Monitor Voltage Divider Circuit

VMON_1P8_SOC pin provides a way to monitor external 1.8 V power supplies. This pin must be connected directly to their respective power source. An internal resistor divider with software control is implemented inside the SoC for each of these pins. Software can program each internal resistor divider to create appropriate under voltage and over voltage interrupts.

VMON_3P3_SOC pin provides a way to monitor external 3.3 V power supplies. This pin must be connected directly to their respective power source. An internal resistor divider with software control is implemented inside the SoC for each of these pins. Software can program each internal resistor divider to create appropriate under voltage and over voltage interrupts.