An Explanation of Undervoltage Lockout

When referring to power-supply or voltage-drive requirements, we often use simplifications such as “this is a 3.3 V microcontroller” or “this FET has a threshold voltage of 4 V.” These descriptions don’t take into account that electronic devices operate over a range of voltages—a 3.3 V microcontroller might work normally with any supply voltage between 3.0 V and 3.6 V, while a MOSFET with a 4 V threshold voltage might attain sufficient conductivity somewhere between 3.5 V and 5 V.

But even these range-based specifications can be misleading. A digital IC that accepts supply voltages between 3.0 and 3.6 V doesn’t stop working completely when the V_DD rail drops to 2.95 V. The reality, as Figure 1 shows, is more complicated.

 

Figure 1. The supply voltage delivered to an IC corresponds to one of four operational states. Image used courtesy of Robert Keim

 

As we see above, the IC has four different operational states:

1. Full performance: The device functions to specification.
2. Functional: The device functions, but may not meet all datasheet specifications.
3. Undefined: The device’s behavior is unpredictable.
4. Inactive: The device is completely off.

If V_DD falls below the lower end of its specified range, the device exits the “full performance” state and is now undervoltage. However, V_DD may not fall far enough to render the device inactive, in which case the IC will continue to operate in either the “functional” or the “undefined” state. This operating condition is also referred to as brownout.

Due to its unpredictability, the undefined state is particularly concerning.

 

The Effects of Undervoltage

Serious problems can occur if the circuit’s transistors are operating below the voltage range they were designed for. Though sometimes an IC going haywire is merely inconvenient, modern life is filled with situations where our work, our health, or even our lives depend on semiconductor devices.

With insufficient supply voltage, CMOS logic circuitry may not transition reliably between the fully-on and fully-off states. In higher-power FETs, inadequate drive voltage can result in excessive channel resistance, causing the component to overheat. Low drive voltage can also prevent a bipolar junction transistor from being properly biased.

In microprocessors or memories, undervoltage can result in bit errors that corrupt data or result in glitchy data transmission. This type of malfunction isn’t likely to cause direct damage to the IC, but faulty control signals generated by a processor can provoke all sorts of highly undesirable—or even dangerous—system behavior. To ensure electronic devices operate safely and reliably, it’s crucial for ICs to include features that protect against undervoltage.

This is particularly important in battery-powered systems, for two reasons:

1. A battery’s voltage gradually diminishes as the battery discharges, so it’s a pretty safe bet that a battery-powered system will occasionally encounter inadequate supply voltage.
2. Undervoltage protection can help prevent battery damage due to excessive discharge.

Undervoltage protection for ICs commonly takes the form of undervoltage lockout (UVLO) circuitry. Let’s take a look at how this works.

 

An Undervoltage Lockout Circuit

The objective of UVLO circuitry is to detect an inadequate power supply voltage and then “lock out” a powered device, ensuring that the device remains in a controlled state until the undervoltage condition is resolved. Figure 2 shows a basic UVLO circuit.

 

Figure 2. An example UVLO circuit. Image used courtesy of Analog Devices

 

In this circuit, the supply voltage is measured using a resistive divider and compared to a fixed threshold voltage (V_T). If the divided supply voltage falls below the threshold, the comparator’s output transitions and deactivates the downstream circuitry by opening a switch. In practice, “opening a switch” will usually mean driving a MOSFET into its nonconductive state.

Since this circuit is intended to detect supply voltages that would cause other portions of the system to malfunction, the designer must ensure that the threshold voltage generator and comparator can operate properly at these low supply voltages. Typically, the comparator in an undervoltage lockout circuit will include hysteresis to prevent the system power from rapidly switching on and off when the power supply voltage is near the threshold.

 

Incorporating Undervoltage Lockout

Though the above circuit can be implemented using discrete components, it’s very common for ICs to include UVLO functionality on-chip. Let’s look at some examples, starting with the TLV741P linear regulator from Texas Instruments.

Figure 3 shows the TLV741P’s functional block diagram. During undervoltage lockout, the output is connected to ground by the 120 Ω pulldown resistor at the right side of the schematic.

 

Figure 3. Functional block diagram for the TLV741P linear regulator. Image used courtesy of Texas Instruments

 

Figure 4 shows how UVLO is incorporated in the TPS54331 buck converter, also from Texas Instruments. This circuit allows the user to adjust UVLO threshold voltages by means of external resistors.

 

Figure 4. A UVLO circuit that uses current sources to adjust the threshold. Image used courtesy of Texas Instruments

 

Though the C8051F310 microcontroller from Silicon Labs doesn’t use the term “undervoltage lockout” in its datasheet, its power-on-reset circuit and V_DD monitor provide equivalent functionality. They hold the device in an inactive state when the supply voltage is too low for reliable operation.

 

Figure 5. Timing graph for the C8051F310’s power-on reset and V_DD monitor reset. Image used courtesy of Silicon Labs

 

Finally, Figure 6 shows an NCP302 supervisor IC from Onsemi. These are available for several different UVLO thresholds, ranging from 0.9 V to 4.7 V.

 

Figure 6. Simplified circuit diagram for a supervisor IC from the NCP302 series. Image used courtesy of Onsemi

 

I recommend using a supervisor IC if you:

• Need undervoltage protection for a component that doesn’t have a UVLO feature.
• Want to implement one undervoltage threshold for an entire circuit board.

While you could also use a customized circuit made from discrete components, I consider the integrated circuit solution to be a better choice.