Latest / Hardware Startups with Fexingo: Devices, Robotics, and Manufacturing Tech Companies / Why Hardware Startups Should Use Overcurrent Protection ICs
Transcript
- Lucas: So earlier this week I was reading through a postmortem from a hardware startup that launched a smart bike light on Kickstarter. Raised six hundred thousand dollars. First batch shipped. And within a month, they had a fifteen percent failure rate — boards smoking, batteries swelling, the whole nightmare. Luna: Classic overcurrent scenario? No fuse, no current limit, just raw battery power hitting the LED driver? Lucas: Exactly. They had a five-amp-hour lithium-ion pack, a boost converter, and a microcontroller. But they skipped the overcurrent protection IC. The BOM cost would have been about forty cents per unit. Instead, they ate sixty thousand dollars in replacements, plus the reputational hit. Luna: Forty cents against sixty grand. That's the kind of ratio that keeps me up at night. Lucas: And it's surprisingly common. I've looked at a hundred hardware startup boards over the past few years, and maybe a third of them have any form of dedicated overcurrent protection beyond a polyfuse on the input. And a polyfuse is better than nothing, but it's slow — trip time can be seconds, which is an eternity for a sensitive IC. Luna: So what's the alternative that's actually fast enough? Lucas: That's the topic today: overcurrent protection ICs — specifically eFuses and current-limiting load switches. Chips like the Texas Instruments TPS25942 or the Analog Devices LTC4365. They react in microseconds, they have programmable current limits, and they often include overvoltage protection and reverse-current blocking in the same package. Luna: Yeah, it's a small gesture that adds up. And it keeps the show ad-free, which I appreciate when we're talking about something as detail-oriented as this. Lucas: Alright. So the eFuse — it's essentially a smart switch with a current-sense circuit built in. You set the limit with a single resistor, and the chip compares the voltage drop across its internal FET. If current exceeds the threshold, it either latches off or folds back depending on the mode. Luna: And foldback is important for capacitive loads, right? When you're powering up a board with big input caps, the inrush can look like a short circuit. Lucas: Exactly. Without foldback, a simple fuse would blow on every power-up. The eFuse allows a controlled current ramp — maybe one amp for a hundred milliseconds — then settles to the operating current. That's something a polyfuse can't do. Luna: What about the cost difference? I know a forty-cent polyfuse is tempting when you're trying to hit a price point. Lucas: The eFuse IC itself might be sixty cents to a dollar twenty in moderate quantities. Plus you need a few passives — the set resistor, maybe an input capacitor. Call it a dollar fifty total. But consider the alternative: a single burnt trace on a four-layer board can cost thirty dollars to rework. A battery pack fire is a product recall. Luna: That e-scooter recall from 2023 — the one with the thermal runaway — that was traced back to a missing current limit on the battery discharge path, right? Lucas: Yeah, the Segway Ninebot Max G30 recall. The root cause was a failed FET that shorted the battery directly to the motor controller. A simple eFuse with overcurrent protection would have shut it down in microseconds. Instead, they had to recall forty-two thousand units. Luna: And that's a big company with engineers who should know better. For a startup, the margin for error is even thinner. Lucas: Right. So let's talk about a specific part. The TPS25942 from TI — it's a twenty-eight-volt, five-amp eFuse with adjustable current limit, overvoltage clamp, and reverse current blocking. You can set the limit from about two hundred milliamps to five amps with a single resistor. And it comes in a small QFN package — three by three millimeters. Luna: That's tiny. Is it hard to solder by hand? For prototyping? Lucas: It's challenging but doable with a hot-air station and a stencil. For first prototypes, I'd recommend using a breakout board from someone like Adafruit or SparkFun — they sell a carrier board for about eight dollars. Then once you validate the design, you integrate the bare IC. Luna: What about the LTC4365 from Analog Devices? I've seen that one in a lot of industrial designs. Lucas: Good part. It's an ideal diode controller with overcurrent and overvoltage protection. One feature I really like is the adjustable overcurrent timer — you can set it to ignore short transients, like motor startup current, but still trip on sustained overloads. It's a bit more expensive, maybe one dollar fifty in onesies, but it's very robust. Luna: So for a startup prototyping a robot or a drone, which approach would you recommend? Lucas: I'd say start with a load switch that has built-in current limiting — something like the TPS22965 from TI. It's a five-amp load switch with adjustable rise time and current limit. Costs about fifty cents. It's not as full-featured as the eFuse, but it'll prevent your motor driver from pulling a dead short. That alone eliminates the most common failure mode. Luna: And if you need reverse polarity protection as well? Because usb c is great, but not all cables are wired correctly. Lucas: Then look at the TPS25821. It's a usb c eFuse with overcurrent, overvoltage, and reverse current protection specifically for USB applications. It even handles the CC logic for source/sink negotiation. About seventy cents. Luna: That's almost too easy. Why don't more startups use them? Lucas: Two reasons. First, they don't know they exist — they think 'protection' means a fuse or a PTC. Second, they're afraid of the datasheet. A typical eFuse datasheet is thirty pages. But in practice, you only need to read about five: the pinout, the current-limit setting, the typical application circuit, and the layout guidelines. Luna: Layout guidelines — that's the part that bites people. If you route the high-current path too close to the sense resistor, you get false tripping. Lucas: Exactly. The key is Kelvin sensing — run separate traces from the sense resistor to the IC's sense pins, not sharing the power trace. TI has a good application note, SLVA774, that shows the layout. Four-layer board, top layer power, second layer ground, keep the sense lines short and away from the inductor if you have a switching regulator. Luna: And for a battery-powered device, should the eFuse go on the battery side or the load side? Lucas: Both, ideally. One eFuse on the battery output to protect against a load short, and another on the charger input to protect against a bad charger. But if you can only afford one, put it on the battery output. That's where the energy is stored. A dead short there can dump ten amps into a trace that was designed for two. Luna: That's the kind of advice that saves a prototype from becoming a fireball. I've seen it happen. Lucas: Me too. And it's avoidable. A dollar fifty in components and a few hours of design review. The return on investment is enormous. Luna: So for a listener who's about to do their first PCB layout for a hardware startup, what's the one takeaway? Lucas: Add overcurrent protection before your first prototype run. Pick an eFuse with adjustable current limit, set it to one point two times your maximum expected load, and lay out the sense lines carefully. It takes two hours and costs less than two dollars. It will save your board, your battery, and your reputation. Luna: And if the listener is already past prototype and into production, is it too late to add protection? Lucas: Not at all. You can add an external eFuse module inline with the power cable. There are off-the-shelf boards from Pololu and others that handle up to ten amps. It's not as elegant as an integrated solution, but it's a quick fix while you spin the next board revision. Luna: Good. Because the last thing you want is your first production run becoming a recall statistic. Lucas: Exactly. And on that note — I think the key insight for today is that protection isn't an afterthought. It's a design parameter you choose as early as your target current and voltage. The ICs exist, they're cheap, and they're well-documented. The barrier is simply knowing to look for them. Luna: And knowing which one to pick. For most startup boards, the TPS25942 or the TPS22965 will cover ninety percent of use cases. Lucas: I'd add the LT4365 for anything industrial or automotive. But for a consumer gadget, those TI parts are perfect. And with that, we'll wrap up Episode 120. Next time, we're going to talk about something I've been getting asked about a lot: how to choose the right connector for high-vibration environments. Luna: Looking forward to it. Until then, keep your boards safe and your current limited. Lucas: Cheers.