How to Use LEDs with Supervisory ICs for Undervoltage and Overvoltage Fault Indicators
Traditionally, supervisory ICs have been used to monitor voltage rails for undervoltage faults (UV). When a supply rail falls below a set threshold, the system is held in reset using the reset output signal to the microcontroller to prevent erratic behavior or catastrophic failure.
Recent supervisory ICs are designed to monitor rails for undervoltage faults as well as overvoltage faults (OV), thereby confining a rail within a permissible window threshold, as shown in Figure 1. It is hard to identify which event at the input caused the system reset because there is only one dedicated reset output that responds to both undervoltage and overvoltage faults.
This application note discusses the following configurations:
- OV/UV detection circuit using the MAX16132 and LEDs
- OV/UV detection circuit using latched LED status
- OV/UV detection circuit using an SR latch to latch the LED
Figure 1. MAX16132 undervoltage/overvoltage threshold window.
UV/OV Detection Circuit that use LEDs
For automotive applications that focus on advanced driver-assistance systems (ADAS), it is very important to monitor OV faults because overvoltage can damage the microcontroller and render it unreliable. OV fault events in vehicles can happen because of a variety of different transient situations. During these events, the prime focus is to save the microcontroller and activate redundant circuitry in the case of a failure.
Figure 2 shows a simple application circuit that shows undervoltage and overvoltage using blue and red LEDs. The circuit disables the faulty voltage regulator once an overvoltage event has been detected.
Figure 2. UV/OV detection circuit using LEDs.
In Figure 2, the MAX16132, a single channel supervisory IC, monitors the 3.3V at IN (VIN) with an input tolerance setting of ±7%. The +7% tolerance sets the input overvoltage threshold value to 3.531V, and the −7% tolerance sets the input undervoltage threshold value to 3.069V. The MAX9648 is a very low hysteresis comparator with a non-inverting input that is connected to a resistive divider network between the 5V rail and RESET. During an OV and UV event, the resistive divider sets a reference voltage equal to the nominal monitored voltage (VMON) which is the same as VIN at 3.3V. The same resistive ladder acts as a pullup for the RESET pin. Calculate the value of R1 and R2 with the following equation:
When there is no fault condition, VRESET is the same as VDD, but during any fault, VRESET can be up to 0.3V based on the RESET sink current. The maximum RESET sink current of the MAX16132 is 20mA.
In this case, VDD = 5V and VREF = 3.3V. Choose R2 = 10KΩ and VRESET = 0.3V for R1 = 4.24KΩ. This limits the current into the RESET pin to 330ΩA.
In Figure 2, logic A is an open drain output of the MAX9648, and logic B is an open drain output of the MAX16132 (RESET).
Add a VREF IC for Stability
The solution shown in Figure 2 is a cost-efficient solution for a system with a stable 5V rail (VDD). If the 5V rail is noisy, use the MAX6037 for a stable VREF voltage, as shown in Figure 3. The MAX6037 is a low-power, fixed, and adjustable reference output. The input for the MAX16132 is a 5V rail, and the output reference voltage of the MAX6037 can vary from 1.184V to 5V. Figure 3 shows that the MAX6037 is used as the VREF for the comparator MAX9648.
Figure 3. UV/OV detection circuit using LEDs and a VREF IC.
UV and OV Fault Cases
In Figure 3, A is the output of the comparator MAX9648, and B is the RESET output of the MAX16132. Y1 and Y2 are logic outputs of inputs A and B. Y1 is expressed as the digital OR of active-low A and B, and Y2 is the digital OR of A and B.
Y1 = active-low A + B
Y2 = A + B
Table 1. Truth Table for Input-Output Logic
Case 1: UV Fault
When the 3.3V monitoring voltage rail (VMON) drops below the undervoltage threshold point of 3.069V, RESET asserts logic B to low. When the comparator's output drives input A to high, it drives Q1 on. The blue LED lights up by sinking current into the RESET, thus indicating an undervoltage event at IN.
When the 3.3V rail (VMON) rises above the undervoltage threshold, RESET de-asserts, which pulls logic B high. As long as the 3.3V rail (VMON) is within the window thresholds, RESET has a high impedance and cannot sink the current regardless of the state of the transistors.
Case 2: OV Fault
When the 3.3V rail (VMON) goes above the overvoltage threshold of 3.531V, RESET asserts due to the overvoltage event at IN, which drives logic B low. However, the comparator’s output drives input A low. As shown in Table 1, when input A and B are at a logic low, Q2 turns on. Because RESET is low, the red LED lights up by sinking current into RESET, which indicates an overvoltage event at IN. During an OV fault, use the logic Y2 (source of FET Q2) and either turn off the LDO (or the DC-DC controller) to save the remaining system or alert the microcontroller that the OV event occurred.
Maintain Uniform LED Luminosity
The IRF7307 from Infineon® used in Figure 2 consists of a NFET (Q1) and a PFET (Q2) in the same package. For uniform LED luminosity, R1 and R2 need to be adjusted to compensate for the higher RDSON of PFET. The reset output low voltage (VOL) of the MAX16132 is a maximum of 0.3V and depends on ISINK current. The ISINKMAX of the MAX16132 is 20mA. Calculate the appropriate value of R3 and R4 by applying the following equations:
ISINK = IPULLUP+ILED
The VLED depends on the selected LED and can be up to 1V. The VRESET can be as high as 0.3V depending on the ISINK current. The ILEDMAX is the maximum permissible current through the LED. Choose ILEDMAX = 10mA and RDSON = 5Ω to have R3 = 365Ω. Figure 4 shows the variation of VMON and the corresponding behavior of different LED voltages with respect to time. During an undervoltage fault, the blue LED glows, and during an overvoltage fault, the red LED glows.
Figure 4. OV/UV faults in the circuit.
Latch the LED Status with the MAX16054
A detection circuit that uses the MAX16132 and LEDs is cost efficient, but if the fault does not persist for a long time, the LED quickly turns off without notice. To mitigate this problem, use the MAX16054 IC, which is a pushbutton on/off controller with a debouncer and built-in latch. The MAX16054 can latch the fault event once the VMON crosses the nominal voltage window. For an OV or UV event, one of the corresponding LEDs turns on after the MAX16054 debounce period and stays in the same condition until the MAX16054 CLR is pulled low. The MAX16054 helps debug the issue even after the fault is cleared from the circuit.
The circuit shown in Figure 5 supports latching of the logic outputs Y1 and Y2 when a fault occurs. Either Y1 or Y2 switches from high logic to low, which triggers the MAX16054 IC to turn on the LED. The user can press the CLR pushbutton to turn off the LED and clear the fault. The fault condition must persist for at least the duration of the MAX16054 debounce time to latch the LED on, which is typically 50ms.
Figure 5. UV/OV detection circuit with LED status latched.
Figure 6 shows that when VMON crosses the overvoltage threshold (indicated by the orange region), the signal Y2 asserts. When Y2 crosses the MAX16054 debounce period, the MAX16054 (OV latch) latches the output high. Press the CLR pushbutton to clear the fault.
Figure 6. Timing diagram of OV fault detection with latched output.
Latch the LED Status using an SR Latch
To latch the output during shorter fault conditions, replace the MAX16054 IC with an SR latch. As shown in Figure 7, the capacitors C1 and C2 give valid logic during a powerup condition. When a fault occurs, the SR latch sets the output high and turns on the LED. To clear the fault, the pushbutton must be pressed.
Figure 7. UV/OV detection circuit with LED status latched using SR latch.
Any sensitive system in an automotive environment can be protected from unwanted transients, accidental overvoltage and undervoltage circuits, and short circuits using an OV/UV detection circuit that uses LEDs and supervisory ICs such as the MAX16132 or a latched LED status using an IC or an SR latch. These solutions leverage separate indicators for both undervoltage and overvoltage faults. For any system-level debugging, previous faults in the system can be stored by latching the LED using the MAX16054 or an SR latch.