Diode and limit in one chip
The CH213 datasheet describes itself as a Schottky barrier diode plus a self-resetting fuse, and that is exactly what it is — at a fifth of the drop, reacting in twenty microseconds instead of a hundred and fifty milliseconds, in a three-pin package.
The last two pages have been building one part. A polyfuse limits current and is slow. An ideal diode blocks the wrong direction and has no limit. Put both on a board and you have two components, a quarter of a volt of drop, and a fault response measured in tenths of a second.
The CH213 is both of them in a SOT23-3. Three pins: in, out, ground. No resistor, no enable, no configuration at all.
What it costs to be in the way
At 5 V the datasheet gives the on-resistance as 160 mΩ and the drop directly, by current:
| Through it | It drops |
|---|---|
| 0.1 mA | 15 mV |
| 100 mA | 20 mV |
| 200 mA | 32 mV |
| 500 mA | 80 mV |
| 1000 mA | 160 mV |
A Schottky at 500 mA drops about 400 mV. A polyfuse in series with it adds another 75. So 80 mV against 475, and the CH213 is also doing the reverse blocking and the polarity protection.
It draws 3.6 µA to run.
The limit, and the fold
A plain current limit holding 1.3 A across the whole of a 5 V rail would be burning 6.5 W in a SOT-23. It would not survive. So the CH213 does what a bench supply does when you short it: the harder you pull the output down, the less current it allows.
Drag the load resistance from a hundred ohms down to a short and watch the operating point walk the curve. Three of its points are datasheet specifications and the fold between the last two is this figure's interpolation of the sentence that describes it:
- Above about 3.7 Ω the chip is just 160 mΩ of MOSFET, and the drop is the current times that.
- Below that, it becomes a current source at about 1.3 A.
- Below 0.95 V on the output, it decides this is a short and folds back to about 250 mA.
It gets there in about 20 µs, and then it sits at 250 mA into a dead short, cool, until you fix it. Compare that with a polyfuse's 150 ms and the 0.8 W it burns off while it holds itself open.
The foldback is the difference between surviving a short and surviving one indefinitely. 250 mA at 5 V is 1.25 W of fault, of which the chip is dissipating most. The DFN6 package spreads it better than either SOT-23 (140 °C/W against 220), and the over-temperature cut-out at 130 °C is there for the case where nothing spreads it enough.
Which of the four to buy
The honest summary is that there is no best one, and the question that picks between them is what your load actually needs.
More than 1.3 A? The CH213's limit is fixed and you cannot move it. Use an SY6280 and choose your own.
Need to switch it on and off? The CH213 has no enable pin. SY6280 again.
Need the lowest possible drop and nothing else? The LM66100's 79 mΩ beats the CH213's 160, and it goes down to 1.5 V in where the CH213 stops at 2.2.
Want everything, once, in three pins? This chip. It is the right default for a USB-powered project under an amp.
The one it shares with every part in this chapter
Reverse polarity is handled by the chip turning off, not by the chip standing in the way of a large current. With the input connected backwards there is a few milliamps of leakage and the switch is open, so the load behind it is safe. The chip itself is rated to −6.5 V on that pin. Beyond that, nothing here helps you, and neither does anything else on a hobby bench.
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