AppNote POR / BOD

General introduction

The embedded design of industrial equipment used in applications such as automation that operate in harsh environments require understanding the effect of faults on power supply systems. Typically, the AC input voltage is universal and common DC output voltages are 5, 12 and 24 V with 24 V the most popular for industrial applications. On the AC side, faults in the power supply can be caused by power failures or mains fluctuations. On the DC side, load-side faults that can cause a voltage brown-out on the 24 V power supply are much more severe. In industrial applications, a shared power supply has been often common practice for different types of loads in the system that need to interact such as microelectronic circuits, electromechanical components, input/output modules, etc.

The permissible voltage range and interruption time of power supply systems are specified in the EN 61131-2. The voltage range for 24 V DC systems is -15%/+20%. The different types of loads within a system usually require isolated or separated voltage rails that are typically generated using types of DC/DC converters, which are much more efficient compared to linear voltage regulators. Sensitive are loads for microelectronic circuits because the interruptions of the supply voltage could cause a reset or loss of function. The purpose of using voltage monitoring and reset control circuits in embedded designs is making sure that the SoC operates properly within its specification limits when the power supply is turned on. The primary objectives are:

• Ensuring that the chip powers up and down correctly
• Monitoring the power supply voltage during operation

The netX 90 has an integrated power-on reset (POR) circuit that coupled with the brown-out detection (BOD) function enables monitoring voltages higher than 3.3 V such as a 24 V DC power supply. The following example outlines the power up sequence of the netX 90 based on a reference design using a DC/DC converter with a wide input voltage range that makes the 3.3 V output voltage less prone to input voltage fluctuations. The example concludes with a discussion about monitoring the input voltage versus monitoring the output voltage of a DC/DC converter.

Integrated POR circuit

Figure 1 depicts the power-up sequence of the netX 90 that references the NXHX 90-JTAG board design, which uses the XR7620x from MaxLinear as a 40 V synchronous step-down COT regulator for the 3.3 V power supply. The XR7620x has a wide input voltage range V_IN and both the output voltage V_OUT and the soft-start time T_SS are adjustable by external passive components.

When the external 3.3 V power supply for the V_DDIO voltage is turned on and reaches the POR_LH threshold level, the integrated DC/DC converter for the V_DDC voltage is switched on. The output voltage of the on-chip DC/DC converter rises slew rate controlled due to an internally defined soft start-up ramp time that reduces the current needed to charge the output capacitor before the actual core of the chip is released from reset. The POR threshold range of the netX 90 is below the lower end of the V_DDIO voltage range, which is typical for integrated POR circuits. This enables the chip to operate anywhere within the specified chip voltage range, which is required by certain applications such as the PCIe mini card formfactor. The POR issues a reset when the supply voltage drops below the POR_HL threshold level, which is typically in the range of ± 2.5% due to process variations. The characterization of the values is given in the electrical specification of the netX 90 (see netX 90 TRG).

The integrated power-up/down control logic of the netX 90 ensures that the on-chip DC/DC converter operates within its specification limits and releases the core of the chip when the V_DDC voltage is stabilized. With the release of the core, the Mask ROM code takes control over the step-by-step startup sequence by configuring the chip and the actual software bootup sequence. The absolute maximum time required to ensure that the 3.3 V supply rises between the minimum POR threshold and the minimum specified V_DDIO operating voltage is ≤ 5ms. After this time, plus a tolerant safety margin, the Mask ROM Code starts latching input pin bootstrap options and the device must operate within its V_DDIO specification limits.

The embedded hardware designer must assure that the 3.3 V supply continues to rise above the POR threshold level and settles within the specified output voltage range V_OUT of its power supply. In the example below, the recommended capacitor C_SS by MaxLinear for the XR7620x is 47 nF, which gives a typical soft-start time T_SS of 2.82 ms. The charge current I_SS that determines the soft-start time T_SS is characterized by a parameter range due to process variations. The slowest soft-start time T_{SS_MAX} is 4.73 ms when taking into account the maximum values from the XR7620x datasheet. As illustrated in Figure 1, the worst case time the XR7620x takes to rise the 3.3 V supply from POR_LH_MIN above VDDIO_MIN is 0.44 ms, which is significantly below 5 ms with more than a tenfold safety margin.

Figure 1: Power-up example

Integrated BOD function

The BOD is an analog input pin with a special ESD structure that can be driven while V_DDIO is not supplied and can therefore be used to monitor voltages higher than 3.3 V as long as the absolute maximum ratings are not violated. The examples in Figure 2 depict two possible BOD threshold setups to monitor either the V_OUT voltage or the V_IN voltage of the XR7620x. The voltage level that needs to be monitored has to be adjusted for the BOD threshold level by using an external resistor voltage divider because the BOD pin is not high voltage tolerant. The maximum analog input value of the BOD pin is 3.6 V. The values of the resistor voltage divider and the maximum input voltage that is monitored determine whether an optional diode clamping circuit is required.

The Example 1 on the left is assuming that a shared 24 V DC power supply with at least two types of loads occur in the system. A load-side fault on a typical higher voltage input/output system could cause a voltage brown-out on the 24 V DC power supply. For instance, a trapped sensor cable for data and power could cause a very low-ohmic load or connecting a load with a large input capacity could have a similar effect. If a brown-out occurs and the 24 V DC power supply drops below BOD threshold input voltage, which is in the example adjusted to the absolute minimum voltage limit specified in the EN 61131-2, the BOD function creates an interrupt request (IRQ) signal to both Cortex-M4 cores. If the BOD interrupt is enabled by one of the two processor cores, preferably the application segment, the Cortex-M4 services the interrupt signal by calling a user-defined IRQ software handler.

In industrial applications, a common shared power supply has been often standard practice for different types of loads in the system that need to interact and require isolated or separated voltage rails generated using types of DC/DC converters. Therefore, BOD monitoring the input voltage of the DC/DC converter, instead of the output that generates the 3.3 V single supply voltage for the netX 90, enables a more robust design approach with a much better controlled response time, e.g. to bring the system into a standstill state that does not cause any hazards. The reaction time is determined by the pulse length needed to pass the glitch filter of the BOD input and the interrupt latency of the Cortex-M4 for the interrupt service routine (ISR). Assuming the ISR is located in the on-chip SRAM with single cycle CPU access at 100 MHz, the interrupt latency according to Arm is 12 cycles, plus a possible additional 17 cycles for Cortex-M4 with Floating Point Unit (FPU) implemented.

As discussed in the section before, the POR threshold range of the netX 90 is below the lower end of the V_DDIO voltage range, which is typical for integrated POR circuits. Embedded system architects that are concerned about the effect of brown-out on the 3.3 V supply, because they bear the risk of using the same voltage rail for switching external loads without isolation, could apply the BOD function to monitor the 3.3 V output voltage of the DC/DC converter. As illustrated in Figure 1, if a brown-out occurs, the netX 90 could theoretically face a 3.3 V supply voltage that drops slightly below the lower end of the specified V_DDIO voltage range but remains briefly above the POR threshold level without encountering a reset. The Example 2 on the right is assuming that the BOD adjusted threshold input voltage lies between the lower end of the power supply voltage range and the lower end of the specified chip voltage range. However, this design approach requires tight power supply tolerances, where glitches and noise must be minimized, that would normally not expose sensitive loads to external loads.

Figure 2: BOD examples