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LM3671 step-down DC-DC converter: sigh.

A project log for Motorola 68000 computer

A mc68k-based computer motherboard, some expansion cards, and a very basic multitasking OS, all built from scratch.

StuartStuart 04/30/2017 at 00:090 Comments

The following is a rant. If you're not in the mood for reading a rant, please don't bother reading on.

So: the LM3671 dc-dc converter. According to its data sheet, "...only three tiny surface-mount external components required (one inductor, two ceramic capacitors)". Astonishing! Compared with an LDO, the cost of an extra inductor buys you a switching converter with much better efficiency. Unless, of course, it doesn't.

Some background: I'm building a video card for the MC68000 system. I'm using the design of the video card as an opportunity to validate some tech that I'm going to use in an MC68030-based system that's coming along soon. The video card will attach to one of the expansion sockets on the MC68000 motherboard; these provide +5V power. The video card uses a LM3671MF-3.3 and a LM3671MF-1.2 to develop the 3.3V and 1.2V supplies required by the card. Easy! So I thought, anyway...

The LM3671 features an enable pin, EN, which enables or disables the regulator. On all the regulators I've ever worked with, such a pin can be strapped to the input voltage rail in order to force the regulator into a permanently-on state - this is the required behaviour in the majority of cases, after all.

There's an interesting throwaway sentence on page 18 of the LM3671 data sheet: "... It is recommended to set EN pin low to turn off the LM3671 during system power up and undervoltage conditions when the supply is less than 2.7V...." This appears to be the only concrete recommendation in the datasheet regarding the EN pin.

It turns out that this "recommendation" is actually rather more of a requirement. In my design, the EN pin is connected directly to the input voltage supply - a regulated +5V - because I want the LM3671 to be operational all the time the +5V supply is present.

It appears that this arrangement is not suitable for the LM3671-1.2 and the LM3671-3.3. What I see, at the output of both regulators, is a rough triangle wave, sawing between 0V and about 2V. The regulators draw an effectively unlimited amount of current in this state, and present an output that is not only unregulated, but swings far above the rated output voltage. I have reproduced this result on multiple LM3671 devices, using a pcb layout which is nearly identical to the layout suggested in the device's data sheet. I am not the only person to have encountered this problem.

So: my conclusion is that I will definitely not be using this device in future, and I will be suspicious in the extreme of any device making similar claims regarding the number of external components required to make an effective regulator. It's a shame, as the device promises so much; I guess the rule is, as ever: if it seems too good to be true, it probably is. Caveat emptor.

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