NE555 vs TLC555: when the CMOS 555 is a drop-in, and when it isn't

The rise and fall times are measured at slightly different loads (15 pF on the NE555, 10 pF on the TLC555), so read them as a guide, not a race. The pinout drawings at the top of this page show both 8-pin parts and the TLC555's 14-pin TSSOP.

Most "can I swap it?" problems come from pin 3. The NE555's output stage is built to push current out as well as pull it in, up to 200 mA each way. The TLC555's output sinks well but sources only about 10 mA.

• An LED from the output to ground lights while the output is high. Either chip handles a few milliamps. Above about 10 mA, the TLC555 is past its typical source rating.
• An LED from the supply to the output lights while the output is low. That is fine on both. The TLC555 sinks 8 mA with only 0.21 V typical across it at 5 V.
• A relay coil, buzzer or small motor straight on pin 3 is an NE555 job. On a TLC555, put a transistor or a logic-level MOSFET between pin 3 and the load. The BC547 vs 2N2222 vs 2N3904 and IRFZ44N vs IRLZ44N pages cover the choice.

There is one pinout catch. The 8-pin TLC555 packages match the NE555, but the 14-pin TSSOP (PW) version puts the pins in different places: GND on 1, TRIG on 3, OUT on 5, VDD on 14. Check the package code before you lay out a board.

1. A blinker on a battery

You build a 1 Hz LED blinker and want to know how long the battery lasts with the LED off. Take the chip's own current only, at 5 V with no load, output low: 3 mA typical and 6 mA maximum for the NE555, 0.18000000000000002 mA typical and 0.35 mA maximum (at 25 °C) for the TLC555. For every 1,000 mAh of battery capacity:

• NE555: 1,000 ÷ 3 = 333 hours typical (about 14 days), 1,000 ÷ 6 = 167 hours worst case.
• TLC555: 1,000 ÷ 0.18000000000000002 = 5,556 hours typical (about 231 days), 1,000 ÷ 0.35 = 2,857 hours worst case.

That is about 17 times longer on the TLC555. Two things shrink the gap in a real circuit. First, the LED and the timing resistors draw their own current, and the LED usually dominates. Second, a CMOS chip uses more power as the frequency goes up. TI gives the formula PD = VDD² × f × (CPD + CL), with CPD = 115 pF at 5 V. At 1 Hz that term is a few nanowatts and you can ignore it. At 189 kHz it is about 0.6 mW, the same as another 118 µA from the supply (see example 5).

2. Running from a 3.3 V rail

Your board has an ESP32 or another 3.3 V microcontroller, and you want a hardware watchdog pulse from a 555 on the same rail.

• NE555: not rated. Its recommended minimum is 4.5 V.
• TLC555C: rated from 2 V, so 3.3 V is well inside.
• TLC555I: rated from 3 V. It works, with 0.3 V to spare, so a sagging rail can push it out of spec.
• TLC555Q and TLC555M: rated from 5 V. They are the automotive and military grades and won't do this job.

So buy the C grade (or the I grade if the board must work below 0 °C and the rail is solid), and read the full part number on the reel, not just "TLC555".

3. An LED on pin 3 at 5 V

You want the LED to light while the output is low, with about 8 mA through it. For this example take a red LED with a 2.0 V forward drop (read yours from its datasheet). TI gives the TLC555's output-low voltage at 8 mA as 0.21 V typical, 0.4 V maximum.

• Resistor: (5 − 2.0 − 0.21) ÷ 0.008 = 349 Ω. The next E24 value up is 360 Ω.
• Current with 360 Ω: (5 − 2.0 − 0.21) ÷ 360 = 7.75 mA typical, (5 − 2.0 − 0.4) ÷ 360 = 7.22 mA worst case.

The NE555 gives 0.15 V typical, 0.4 V maximum at the same 8 mA (7.92 mA and 7.22 mA through 360 Ω), so the same resistor works for both chips. Turn the LED around (output to ground, lit while high) and the NE555 still copes easily, but the TLC555 is close to its 10 mA typical source rating. Stay at a few milliamps there, or use the sink connection. The LED resistor calculator does this sum for other LEDs.

4. A 10-minute delay

You want a one-shot that holds a pump on for 10 minutes. Monostable timing is tw = 1.1 × R × C on both chips. With a 100 µF capacitor, R = 600 s ÷ (1.1 × 100 µF) = 5.45 MΩ.

On an NE555 at 5 V this fails. TI puts the largest timing resistance at about 3.4 MΩ on 5 V and about 10 MΩ on 15 V. That ceiling is given for the astable circuit, but it comes from the THRES pin's input current, which is the same pin here. You can see why:

• Near the end of the timing interval, the current through R into the capacitor is (VCC − ⅔VCC) ÷ R = VCC ÷ (3R).
• At 5 V with 5.45 MΩ, that is 5 ÷ (3 × 5.45 MΩ) = 306 nA.
• The NE555's THRES pin can take up to 250 nA of that, which leaves about 56 nA to finish charging the capacitor. In that worst case the capacitor creeps up to the threshold and the 10-minute delay stretches to about 23 minutes (Figure 4). Any leakage in the capacitor on top of that can stop it timing out at all.
• At 15 V the charging current is 917 nA, and up to 250 nA of it (27 %) still goes into the pin, so the delay can run to about 12 minutes.

On a TLC555 the same pin takes at most 75 pA, about 0.02 % of the 306 nA. At that level the chip's input current no longer matters. A 100 µF electrolytic's own leakage current now decides the accuracy, so check the capacitor's datasheet, or use a smaller film capacitor with a bigger resistor: 10 µF needs 54.5 MΩ, where the charging current at 5 V is 31 nA and the TLC555 still takes only 0.25 % of it.

5. A 190 kHz clock

You need a clock of roughly 190 kHz. Take RA = 1 kΩ, RB = 3.3 kΩ and C = 1 nF:

• f ≈ 1.44 ÷ ((1 kΩ + 2 × 3.3 kΩ) × 1 nF) = 1.44 ÷ 7.6 µs = 189 kHz.
• tH = 0.693 × 4.3 kΩ × 1 nF = 2.98 µs; tL = 0.693 × 3.3 kΩ × 1 nF = 2.29 µs.

The NE555 is out. TI says to stay at or below 100 kHz and points to the TLC555 for anything faster. Its edges also eat into the period: the period is 5.28 µs, and 300 ns maximum for each of the rise and the fall is 11 % of it.

The TLC555 handles it. Its worst-case rise plus fall (75 + 59.99999999999999 ns) is under 3 % of the period. Two cautions from TI's own datasheet:

• The simple formulas hold below 100 kHz. Above that, propagation delay and the discharge switch's on-resistance add to each half-cycle, so the real frequency comes out lower than 189 kHz. TI gives a fuller formula (equation 8). In practice, build it, measure it, and trim RB.
• Dynamic power rises with frequency. At 5 V, 189 kHz and a 10 pF load: 5² × 189 kHz × (115 + 10) pF = 0.59 mW, or about 118 µA on top of the static current.

• Choose the TLC555 for battery-powered timers, 3.3 V boards, long delays with large resistors, small timing capacitors, clocks above 100 kHz, or monostable pulses shorter than 10 µs (it goes down to 1 µs).
• Choose the NE555 when pin 3 has to drive a relay, a buzzer or a string of LEDs directly, when the supply runs above 15 V, or when you are repairing an existing NE555 circuit that already drives a heavy load.
• To swap an NE555 for a TLC555 in an existing circuit, check that the supply is 15 V or less, that the output never sources more than a few milliamps, and add the 1 µF electrolytic TI asks for next to the 0.1 µF ceramic.
• To swap a TLC555 for an NE555, check that the supply is at least 4.5 V, that RA + RB stays under 3.4 MΩ on 5 V (10 MΩ on 15 V), that the frequency is under 100 kHz, and that the extra 3 to 10 mA of supply current is acceptable.

• Tie RESET (pin 4) to the supply if you don't use it. TI says so for both chips. On a TLC555 driven from a microcontroller, add a 10 kΩ pull-up so the pin never floats.
• Fit a capacitor from CONT (pin 5) to ground. TI's TLC555 layout uses 0.01 µF. Both datasheets say decoupling CONT can improve operation.
• Bypass the supply right at the chip: 0.1 µF ceramic for the NE555, 0.1 µF ceramic plus 1 µF electrolytic for the TLC555.
• The TLC555 is rated for ±1000 V human-body ESD, the NE555 for ±500 V. Neither is meant to take a static zap on an exposed pin. TI's TLC555 sheet shows series resistors and TVS diodes for pins that leave the board.
• Need a timer on a 3.3 V rail? Order the C or I grade. The Q and M grades start at 5 V.

For a new design on 3.3 V or 5 V, we'd start with the TLC555C and drive anything heavier than a few milliamps through a transistor, or hang LEDs from the supply so the output sinks. We'd keep the NE555 for 12 V boards where pin 3 drives a relay or buzzer directly, and for repairs where the old circuit relies on its 200 mA output.

The NE555 page has the full pin descriptions, the NE556 dual and a 1 Hz flasher worked out. The 555 astable and 555 monostable lessons explain the timing, and the 555 LED flasher system walks through a complete build. Other pages that compare parts the same way: LM358 vs 741 vs TL072 and 7805 vs LM317 vs AMS1117.