Comparator with hysteresis (Schmitt trigger)

01

Choose speed, supply current and supply range

TI's TLV3201 and TLV3202 are single and dual comparators listing 40 ns propagation delay with 40 µA quiescent current per channel, an input common-mode range that extends 200 mV beyond either rail, 1 mV input offset, push-pull outputs with about 40–60 mA short-circuit current, a 2.7–5.5 V supply and −40 to +125 °C operation, in SC70, SOT-23, SOIC and VSSOP packages, for inspection, test, high-speed sampling and telecom equipment. [1]

TI's TLV1805 is a high-voltage comparator listing a 3.3–40 V supply, 135 µA quiescent current, 250 ns propagation delay, 500 µV input offset, rail-to-rail inputs with phase-reversal protection, a high peak-current push-pull output, shutdown with high-impedance output and power-on reset, in SOT-23-6, for over- and under-voltage detection, OR-ing and smart-diode controllers, MOSFET gate driving and PLC and motor monitoring. [2]

Editorial note: the two parts show the speed-versus-current trade — 40 ns at 40 µA on a 5 V rail versus 250 ns at 135 µA on up to 40 V — so start from the delay your signal edge needs and the rail the comparator must sit on. [1] [2]

Give the supplier the supply range, propagation delay at your overdrive, quiescent current, input common-mode range, offset and drift, hysteresis (internal value or none), output type (push-pull or open-drain) and drive, shutdown or latch needs, channel count, temperature range, package, ESD rating and quantity.

02

Set hysteresis and the switching thresholds

The TLV1805 datasheet says it has 14 mV of internal hysteresis, designed to reduce sensitivity to noise, and that the actual trip point combines the set threshold, the internal offset and the hysteresis window. It adds that external hysteresis can be added to improve response to noisy or slow-moving inputs, and gives the thresholds for a resistor network from the output: VA1 = VCC × R2 ÷ ((R1 ∥ R3) + R2) and VA2 = VCC × (R2 ∥ R3) ÷ (R1 + (R2 ∥ R3)). [2]

The TLV3201 lists 1.2 mV of input hysteresis, and its datasheet also has a section on adding external hysteresis. [1]

Editorial calculation: with VCC = 5 V and R1 = R2 = R3 = 10 kΩ, R1 ∥ R3 = R2 ∥ R3 = 5 kΩ, so VA1 = 5 × 10 ÷ 15 ≈ 3.33 V and VA2 = 5 × 5 ÷ 15 ≈ 1.67 V — a hysteresis window of about 1.67 V. [2]

03

Find suppliers

The companies in the manufacturer list below show this product on their own websites. Confirm the legal seller, factory and scope for your order.

Ask any supplier for propagation delay at your overdrive and load, hysteresis over temperature and the output stage type. [1] [2]

04

Check delay at your overdrive and the open-collector output terms

Diodes Incorporated's AZV331 datasheet shows why one propagation-delay figure is not enough. This 2.5 V to 5.5 V single comparator, with a 60 µA typical supply current, lists its delay at two input overdrives. At VCC = 5 V, high-to-low delay is 600 ns at 10 mV overdrive and 200 ns at 100 mV, and low-to-high delay is 450 ns and 300 ns. At VCC = 2.7 V the high-to-low figures are 1000 ns and 350 ns, and the low-to-high figures 500 ns and 400 ns. These are stated for TA = 25 °C, VEE = 0 V, RL = 5.1 kΩ connected to VCC and VCM = 0, so ask any supplier for delay at the overdrive your signal actually gives. [3]

The AZV331 has an open collector output, so it needs a pull-up resistor; the datasheet's figures are measured with 5.1 kΩ to VCC. At 5 V it lists output sink current of 10 mA minimum and 84 mA typical at VO ≤ 1.5 V, a typical saturation voltage of 200 mV at ISINK ≤ 4 mA, and an input voltage range (VCM) of −0.1 to 4.2 V. Editorial calculation: 5 − 4.2 = 0.8 V, so on a 5 V rail the inputs must stay at least 0.8 V below the supply. [3]

The same table gives input offset voltage of 1.7 mV typical and 7 mV maximum at 25 °C, rising to 9 mV over the full temperature range, with 5 µV/ºC average drift, and the absolute maximum ratings list ESD of 4000 V human body model and 300 V machine model. Specify the guaranteed maximum offset at your temperature range, not the typical value. [3]

05

Compare a rail-to-rail part with internal hysteresis and check its ordering terms

SG Micro's SGM8741 product page describes a single comparator with a 45 ns propagation delay specified at 10 mV overdrive, 155 µA typical supply current at VS = 3 V, 0.9 mV typical offset and a 2.7 V to 5.5 V supply range. Its input common-mode range runs from −0.1 V to (+VS) + 0.1 V, and the output swings to within 200 mV of the rails with 4 mA output current, without an external pull-up or pull-down resistor. The page says the part has internal hysteresis for reducing sensitivity to noise when the input moves slowly, is compatible with CMOS and TTL logic, and has continuous short-circuit protection to both rails on any input or output pin. [4]

The SGM8741 is rated for −40 °C to +85 °C and comes in SOT-23-5 and SC70-5 packages; the listed applications include zero-crossing detectors, threshold detectors and line receivers. That temperature range is narrower than the −40 °C to +125 °C of TI's TLV3201, so state the ambient range in the request before comparing prices. [4] [1]

Editorial calculation: the SGM8741 lists 155 µA at 3 V for 45 ns, and the TLV3201 lists 40 µA per channel for 40 ns, so 155 ÷ 40 ≈ 3.9 times the supply current for a similar delay class. The two figures come from different datasheets and test conditions, so request both at the same supply and overdrive before ranking them. [4] [1]

The ordering table on the same page shows that the two package options of this part carry different moisture-sensitivity levels, and it publishes reference lead times by volume. Treat these as listed reference figures and confirm them in the quotation. [4]

06

Trace what each block controls

This is a functional reference built from the sources cited; it is not one maker's specification.

07

Fill the specification fields with values for your circuit

A comparator quotation needs values for your circuit. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

08

Send the signal, threshold and supply data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Signal: [range; edge rate; noise]
Thresholds: [rising; falling]
Hysteresis: [internal; external]
Speed: [delay at overdrive]
Supply: [V; µA]
Output: [push-pull; open-drain; load]
Package/temperature: [package; range]
Quantity: [units]

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