When referring to power-supply or voltage-drive necessities, we regularly use simplifications akin to “it is a 3.3 V microcontroller” or “this FET has a threshold voltage of 4 V.” These descriptions don’t consider that digital units function over a vary of voltages—a 3.3 V microcontroller may work usually with any provide voltage between 3.0 V and three.6 V, whereas a MOSFET with a 4 V threshold voltage may attain ample conductivity someplace between 3.5 V and 5 V.
However even these range-based specs will be deceptive. A digital IC that accepts provide voltages between 3.0 and three.6 V doesn’t cease working utterly when the VDD rail drops to 2.95 V. The truth, as Determine 1 exhibits, is extra sophisticated.
Determine 1. The provision voltage delivered to an IC corresponds to certainly one of 4 operational states. Picture used courtesy of Robert Keim
As we see above, the IC has 4 totally different operational states:
- Full efficiency: The gadget features to specification.
- Practical: The gadget features, however might not meet all datasheet specs.
- Undefined: The gadget’s habits is unpredictable.
- Inactive: The gadget is totally off.
If VDD falls beneath the decrease finish of its specified vary, the gadget exits the “full efficiency” state and is now undervoltage. Nevertheless, VDD might not fall far sufficient to render the gadget inactive, through which case the IC will proceed to function in both the “practical” or the “undefined” state. This working situation can be known as brownout.
Because of its unpredictability, the undefined state is especially regarding.
The Results of Undervoltage
Critical issues can happen if the circuit’s transistors are working beneath the voltage vary they have been designed for. Although generally an IC going haywire is merely inconvenient, fashionable life is stuffed with conditions the place our work, our well being, and even our lives depend upon semiconductor units.
With inadequate provide voltage, CMOS logic circuitry might not transition reliably between the fully-on and fully-off states. In higher-power FETs, insufficient drive voltage can lead to extreme channel resistance, inflicting the element to overheat. Low drive voltage may also forestall a bipolar junction transistor from being correctly biased.
In microprocessors or recollections, undervoltage can lead to bit errors that corrupt knowledge or lead to glitchy knowledge transmission. Any such malfunction isn’t prone to trigger direct harm to the IC, however defective management indicators generated by a processor can provoke all kinds of extremely undesirable—and even harmful—system habits. To make sure digital units function safely and reliably, it’s essential for ICs to incorporate options that defend towards undervoltage.
That is significantly essential in battery-powered techniques, for 2 causes:
- A battery’s voltage regularly diminishes because the battery discharges, so it’s a reasonably secure wager {that a} battery-powered system will sometimes encounter insufficient provide voltage.
- Undervoltage safety may help forestall battery harm as a consequence of extreme discharge.
Undervoltage safety for ICs generally takes the type of undervoltage lockout (UVLO) circuitry. Let’s check out how this works.
An Undervoltage Lockout Circuit
The target of UVLO circuitry is to detect an insufficient energy provide voltage after which “lock out” a powered gadget, making certain that the gadget stays in a managed state till the undervoltage situation is resolved. Determine 2 exhibits a fundamental UVLO circuit.
Determine 2. An instance UVLO circuit. Picture used courtesy of Analog Gadgets
On this circuit, the availability voltage is measured utilizing a resistive divider and in comparison with a set threshold voltage (VT). If the divided provide voltage falls beneath the edge, the comparator’s output transitions and deactivates the downstream circuitry by opening a change. In follow, “opening a change” will often imply driving a MOSFET into its nonconductive state.
Since this circuit is meant to detect provide voltages that might trigger different parts of the system to malfunction, the designer should make sure that the edge voltage generator and comparator can function correctly at these low provide voltages. Sometimes, the comparator in an undervoltage lockout circuit will embrace hysteresis to stop the system energy from quickly switching on and off when the facility provide voltage is close to the edge.
Incorporating Undervoltage Lockout
Although the above circuit will be carried out utilizing discrete elements, it’s quite common for ICs to incorporate UVLO performance on-chip. Let’s take a look at some examples, beginning with the TLV741P linear regulator from Texas Devices.
Determine 3 exhibits the TLV741P’s practical block diagram. Throughout undervoltage lockout, the output is related to floor by the 120 Ω pulldown resistor on the proper aspect of the schematic.
Determine 3. Practical block diagram for the TLV741P linear regulator. Picture used courtesy of Texas Devices
Determine 4 exhibits how UVLO is included within the TPS54331 buck converter, additionally from Texas Devices. This circuit permits the consumer to regulate UVLO threshold voltages by way of exterior resistors.
Determine 4. A UVLO circuit that makes use of present sources to regulate the edge. Picture used courtesy of Texas Devices
Although the C8051F310 microcontroller from Silicon Labs doesn’t use the time period “undervoltage lockout” in its datasheet, its power-on-reset circuit and VDD monitor present equal performance. They maintain the gadget in an inactive state when the availability voltage is simply too low for dependable operation.
Determine 5. Timing graph for the C8051F310’s power-on reset and VDD monitor reset. Picture used courtesy of Silicon Labs
Lastly, Determine 6 exhibits an NCP302 supervisor IC from Onsemi. These can be found for a number of totally different UVLO thresholds, starting from 0.9 V to 4.7 V.
Determine 6. Simplified circuit diagram for a supervisor IC from the NCP302 sequence. Picture used courtesy of Onsemi
I like to recommend utilizing a supervisor IC in the event you:
- Want undervoltage safety for a element that doesn’t have a UVLO function.
- Wish to implement one undervoltage threshold for a whole circuit board.
When you might additionally use a custom-made circuit constituted of discrete elements, I contemplate the built-in circuit resolution to be a better option.