iTechGuides is reader-supported. When you buy through links on our site, we may earn an affiliate commission. As an Amazon Associate I earn from qualifying purchases. Learn more
Hysteresis gives a comparator-based undervoltage lockout (UVLO) or overvoltage lockout (OVLO) separate trip points for rising and falling voltage. That gap prevents repeated switching when noise or a load-induced voltage sag makes the input hover around a single threshold. Choose the feedback topology to match the protection function, calculate both trip points, and budget for real component errors before relying on the circuit.
Why a single lockout threshold can chatter
A comparator without hysteresis changes state at one nominal threshold. If the supply rises slowly with noise, or dips under load, its sensed voltage can cross that point repeatedly and make the load switch turn on and off.
A battery or other source with appreciable resistance makes this worse: turning on the load pulls the source voltage down, which can trigger shutdown; removing the load lets the voltage recover, which can trigger startup again. Hysteresis creates a voltage band with distinct rising and falling thresholds. It prevents chatter when the disturbance is smaller than that band, but it does not fix an unsuitable operating window or eliminate the need to test source and load behavior.
What’s actually slowing this PC down?
Pick the symptom - the matching free tool is one click away.
Start with the required operating window
Set the lower and upper limits from the system’s valid operating range, source impedance, load behavior, and expected disturbances—not from a universal recommended band. For UVLO, the rising threshold is where the system may start and the falling threshold is where it must shut down. For OVLO, define the corresponding limits around the voltage at which the system may resume after an overvoltage and the voltage at which it must disconnect.
#1 Best Overall
- The LM393P is a dual differential input voltage comparator designed for operation from a single supply over a wide voltage range. The common-mode input voltage range includes ground and these devices have open collector outputs
- Single supply or dual supplies, wide range of supply voltage: maximum rating: 2V to 36V
- Low supply-current drain independent of supply voltage: 0.4 ma; Low input bias current: 25 na; Low input offset voltage: 2 mv
- The LM393P contains two independent voltage comparators that are designed to operate from a single supply over a wide voltage range. Dual supplies can also operate as long as the voltage difference between the two supplies is within 2 V to 36 V and V CC is at least 1.5 V higher than the input common-mode voltage
- The LM393P with two independent voltage comparators and are designed for use with a single supply over a wide voltage range. The quiescent current is independent of the supply voltage, and these outputs can be connected to other open collector outputs for a line to line relationship
Before choosing a circuit, check whether the comparator’s output polarity and available control signal can create the needed feedback in the relevant state. Positive feedback is the principle, but a feedback resistor that works for one UVLO arrangement can have the wrong effect in an OVLO arrangement.
Choose a hysteresis method
Use a comparator’s built-in hysteresis
Some comparators provide distinct input thresholds internally. For example, Analog Devices illustrates comparator thresholds of VT + 100 mV and VT − 100 mV. With a divider whose top and bottom resistors are RT and RB, that example corresponds to a 200 mV × (RB + RT)/RB band at the supply input. These are illustrative values, not a universal comparator specification; use the selected device’s data sheet and circuit conditions.
Feed the UVLO divider tap from the switch output
In the illustrated UVLO topology, resistor RH connects the divider tap to the power-switch output. With the switch off and its output near 0 V, RH is effectively in parallel with RB; with the switch on, it is effectively in parallel with RT. The state-dependent divider ratios create separate trip points:
The Tool Desk
Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Vrise = VT × ((RB || RH) + RT) / (RB || RH)
Vfall = VT × (RB + (RT || RH)) / RB
Here, A || B means A × B / (A + B). If the comparator has intrinsic hysteresis as well, use the appropriate rising or falling comparator threshold in the corresponding calculation.
In Analog Devices’ example, VT = 1 V, RT = 10 × RB, and RH = 100 × RB produce an 11.1 V rising threshold and a 10.09 V falling threshold—a 1.01 V input-referred band. The same feedback arrangement does not work for OVLO: when a rising input turns the switch off, feedback pulls the comparator input in the direction that tends to turn it back on.
Rank #2
- 10Pcs LM358P Operational Amplifier IC LM358N LM358 DIP-8 Dual Operational Amplifier
- 10Pcs LM393P LM393 Dual Differential Comparators DIP8
- 10Pcs NE555 Timer IC NE555P Pulse Generator DIP-8
- 5Pcs LM339N DIP-14 Low Power Quad Voltage Comparators IC LM339
- 5Pcs LM324N LM324 Quad Om-Amp DIP-14 Quadruple Operational Amplifier 14-Pin IC
Switch a resistor in or out of the divider
A comparator-controlled transistor can connect or disconnect a resistor in parallel with, or in series with, a divider leg. The changing divider ratio sets the hysteresis band. Depending on comparator polarity and control behavior, switched-resistor arrangements can serve UVLO or OVLO.
| Switched-resistor example | Rising threshold | Falling threshold | Input-referred band |
|---|---|---|---|
| Parallel resistor example from Analog Devices | 11.1 V | 11 V | 100 mV |
| Series resistor example from Analog Devices | 11 V | 10.091 V | 909 mV |
These are source examples, not recommended design values. Include the transistor’s on-resistance if it is not negligible compared with the switched resistor.
Crashes, No Sound, or Screen Glitches?
Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteWindows Errors? Fix Them Before They Spread
Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallUse a controlled current
A controlled current source can replace the switched shunt resistor. In the convention used by Analog Devices, current IH is enabled below threshold. The rising threshold is:
Vrise = VT × (RB + RT) / RB + IH × RT
The falling threshold is the original divider threshold:
Vfall = VT × (RB + RT) / RB
The resulting input-referred hysteresis is IH × RT. Analog Devices names the LTC4417 and LTC4418 prioritized controllers as examples of this method. A controller’s pins and state behavior determine whether it fits; do not assume current injection, switched resistors, and output feedback are interchangeable.
Rank #3
- LM311P is a high-speed voltage comparator with strobed operation and open-collector output
- High-speed comparison applications analog-to-digital converters and precision timing circuits
- Excellent noise immunity with strobe capability allowing controlled timing of comparison operations
- High-speed comparator with strobe input and open-collector output for flexible interface
- Precision measurement systems high-speed analog circuits and conversion applications
Calculate the nominal UVLO and OVLO thresholds
Basic two-resistor UVLO
For a comparator reference VT and a divider with RT above RB, the ideal rising UVLO threshold is:
Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsVUVLO = VT × (RB + RT) / RB
This assumes negligible comparator input bias current and no added hysteresis network. Analog Devices’ example uses VT = 1 V and RT = 10 × RB, giving an 11 V threshold.
Shared divider for UVLO and OVLO
A three-resistor string—RT, RM, and RB—can feed both comparators. With the comparator input polarities arranged for the lower and upper limits, the ideal thresholds are:
VUVLO = VT × (RB + RM + RT) / (RB + RM)
VOVLO = VT × (RB + RM + RT) / RB
An AND gate can combine the comparator outputs so the system is enabled only between the two limits. One shared string uses less divider bias current than two independent two-resistor strings, while separate strings make the thresholds more independently adjustable. The shared arrangement still requires comparator polarities and output logic that keep the switch on only within the intended window.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Budget for threshold errors
Divider equations give nominal values, not guaranteed trip points. Real thresholds can shift with reference inaccuracy, comparator input offset, leakage or bias current, resistor tolerance, intrinsic hysteresis, and the voltage or resistance of the switching element. Check these effects against the full valid operating window rather than adding a generic safety margin.
Rank #4
- BOJACK high quality 12 Values 120 Pcs IC Assortment Kit
- Packed Sorted accordingly in A Plastic Storage Case
- Huge IC Assortment contains: LM324N, LM339N, ULN2003AN, ULN2803APG, LM358P, LM386, LM393, NE5532P, NE555P, UA741CN , JRC4558D, PC817
- Including 12 pcs DIP8 socket, 4 pcs DIP14 socket, 2 pcs DIP16 socket, 2 pcs DIP18 socket
- Contains various usefull IC types such as: Photocouplers,Timers,Osciallators, Dual Opamps, Quad Opamps, Comperators, Audio Amplifiers, Echo Processors, Current Mode Controllers, Darlington Arrays, Voltage Charge Pump
Input leakage and divider current
For a simple divider, Analog Devices expresses the nonideal input threshold as:
(VT ± VOS) × (RB + RT) / RB ± ILK × RT
where VOS is comparator offset and ILK is input leakage. The article offers a rule of thumb: divider current at the trip point equal to 100 times input leakage keeps leakage-caused input-threshold error below 1%. It is guidance for that analysis, not a universal design requirement. Another check is ILK × (RB || RT) < VOS, which makes leakage error smaller than offset error.
In an Analog Devices LTC4367 worked example, the stated maximum pin leakage is ±10 nA and comparator threshold offset is ±7.5 mV around 500 mV. Allowing 3 mV leakage error leads to RB || RT < 300 kΩ. For an 11 V input threshold, the example calculates RB = 309 kΩ and RT = 6.49 MΩ, yielding 1.62 µA divider current, or 162 times the 10 nA leakage. These are the article’s example values; verify current specifications in the relevant device data sheet before applying them.
Comparator and switch checks
- Confirm the reference accuracy, comparator offset and leakage limits, and any built-in hysteresis across the relevant operating conditions.
- Check comparator input common-mode and supply ranges, output type and drive capability, and propagation behavior.
- Verify that the comparator output can control the switch or feedback element with the required polarity and state behavior.
- Check switch drive requirements. An N-channel MOSFET may require a gate voltage above the supply and a charge pump; a P-channel MOSFET has reversed gate polarity.
- Include switched-resistor transistor on-resistance and other nonideal switch states if they affect the divider ratio.
The Analog Devices article’s worked component figures are not a substitute for current manufacturer specifications. Texas Instruments’ TLV1805 product folder and data sheet provide one comparator example whose application material discusses hysteresis; they do not validate a completed protection design.
Verify the circuit under real startup and fault conditions
After calculating nominal thresholds, evaluate the circuit at component and operating extremes. Check startup and shutdown with the actual source impedance and load, including the voltage recovery that follows disconnection. Validate noise, temperature, reference and resistor tolerances, comparator limits, and switch behavior. Confirm that the system starts and remains on where intended, shuts down at the required limit, and does not re-enable improperly during a fault.
The equations and example values here follow Pinkesh Sachdev’s Analog Devices article, “Adding Hysteresis for Smooth Undervoltage and Overvoltage Lockout”. The inspected article page does not display a publication date, so its numerical examples are identified as examples rather than dated recommendations.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
