Lowest cost way to get VBAT to an SMT32 from auto/boat 12V?

[jnz]

I don't need it. But I kinda want it. The ability to completely unpower the STM32H7 and keep it's 4KB of backup ram powered.

The VBAT pin can be supplied from 1.2V-3.6V. The a vehicle 12V is really 14.6V nominal, 8V min before I can cut out, 18V normal max, automotive spikes questionable. Looks like the total drain on this circuit should be no more than 10uA.

I have very little space, can add little cost, and can waste very little power while sleeping/off.


What clever tricks could I get away with? Zener or other diode? I figure very large voltage divider it's great because of the large range.

[DavidAlfa]

Resistor + low power zener + 100nF cap should do the job.
Take the lowest nominal current zener you can get, ex. SZBZX84C3V3LT3G seems pretty stable at 100uA, so the resistor should be adjusted to ensure 100uA at lowest voltage (8-3.3V), ex. 39K:
8V = 120uA
12V = 223uA
14.4V = 284uA
Don't care about 14.4V, that's floating charge voltage, nominal is 12-12.5V, and a small 55Ah battery will take 28 years to discharge at 223uA

[lucazader]

A Still very low cost, but probably more relaible way would be to get a super cheap LDO.

Something Like this would work a treat:
https://www.digikey.com/en/products/detail/diodes-incorporated/AP7381-28Y-13/8537767
2.5uA Quiescent current.

There are cheaper ones down in the 4-6cent range if you dont care about where the chip comes from or whot he manufacturer is etc.

[jnz]

If the best option is an LDO, then OK, but that won't mean JUST an LDO. It'll mean LDO and at least a cap.

I'm looking at 0402 sized parts here, so trying to decide between space and placement costs.

100uA is about the most I want to "spend" on this. The power off draw of a new vehicle is approx 10mA/h so adding 1% to that is fine with me.

[eugene]

Not being a smart ass, but how about using a dedicated battery? A large coin cell costs about the same as a voltage regulator, is simpler to implement, and would probably keep the memory powered for years before it needs to be replaced.

[jnz]

I don't have the footprint for a battery. STM32H7, total board size 20x40mm, and a lot of other stuff going on. Besides.... I have a battery! It's just 12V.

[Sal Ammoniac]

Not being a smart ass, but how about using a dedicated battery? A large coin cell costs about the same as a voltage regulator, is simpler to implement, and would probably keep the memory powered for years before it needs to be replaced.

+1
A separate battery is a better way to ensure the memory retains its state than powering Vbat from the vehicle’s battery because what happens if someone leaves the headlights on and drains the battery?

[Sal Ammoniac]

I don't have the footprint for a battery. STM32H7, total board size 20x40mm, and a lot of other stuff going on. Besides.... I have a battery! It's just 12V.

Do you have room for a small two-pin connector? If so, you can mount the battery off-board and run two wires to the connector on the board.

[lucazader]

Super small and cheap LDOs are available.
We ahve used this one before, comes in a 1x1 package.
https://www.digikey.com/en/products/detail/on-semiconductor/NCP115AMX330TCG/9829207

Its pretty cheap if you have high volumes. the biggest downside for this particular part is the Quiescent curent is 95uA, much higher than the previously linked LDO.
I am sure there are lots of other tiny LDOs out there for cheap with very small packages.

[SiliconWizard]

https://www.onsemi.com/products/power-management/linear-regulators-ldo/ncv8730

[thm_w]

Not being a smart ass, but how about using a dedicated battery? A large coin cell costs about the same as a voltage regulator, is simpler to implement, and would probably keep the memory powered for years before it needs to be replaced.

Generally a battery costs more than a cheap LDO or zener (<5c), and you want to avoid shipping products with batteries in them, if possible.

[jnz]

It doesn't matter at all if the battery goes dead. Just saves some boot up time and I don't need to interrogate for our other components again. The user won't notice a difference, but if I can keep some of the RAM alive, with little effort while shutting the entire board down completely, I'm happy to do it.

My only issue with the LDOs is you need to add $0.30 or so to the LDO cost. Two extra caps and placement of three parts. Although it doesn't look like I can get away from that.

[SiliconWizard]

My only issue with the LDOs is you need to add $0.30 or so to the LDO cost. Two extra caps and placement of three parts. Although it doesn't look like I can get away from that.

I'm curious what kind of design you are dealing with. Something with a STM32H7 (so somewhat powerful, not a cheapass gadget), for automotive environments, but for which a couple tens of cents would matter?

Regarding cost and PCB area, I'm all the more curious as a 12V input from a vehicle will require some significant protection if you don't want to fry your circuit at some point, and this protection will take more room and probably cost than a small LDO with two 0402 caps. But there's probably some info we're missing there.

[NiHaoMike]

For such a small current, a resistor and zener would be the way to go. Immunity to voltage spikes is only limited by the voltage rating of the resistor.

[jnz]

I should know this, but it seems that zeners that are really stable at low currents are sold as voltage references the Ti LM4040 seems to use an external resistor but depending on the voltage seems like it’s only two parts placement and then I don’t need to find a super specific zener.

Supply chain has changed the way I build things