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I'm trying to build something designed in Webench for the 1st time
design's public link: https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=D70B2B0A4BFC193A
Schematic attached
Question - the schematic doesn't show an RT pin, but the datasheet does (and it says not to leave it floating or tie to GND). The schematic shows a MODE_SYNC pin, tied to GND, but the datasheet doesn't mention this pin.
Is the takeaway that I shouldn't trust Webench or am I missing something?
Is it a case of "it's useful for selecting the best IC and high level operating conditions (e.g. Fsw) for your requirements but you need to select component values using the datasheet's detailed design process... ie the generated schematic and BOM is useless"?
Or it can't even be trusted for selection process?
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I don't see a RT pin in the datasheet : https://www.ti.com/lit/ds/symlink/lm64460-q1.pdf
As for MODE / SYNC , see pages 17 and 18.
But I really have to wonder why you'd decide to make a 5v 0.7A dc-dc converter with a chip that costs 5$ in small quantities. It's a chip designed for up to 36v, with up to 6A switching current, not for circuits which are meant to output less than 1A of current.
You have much better cheaper regulators for 1-2A of current, they may be slightly less efficient at maybe 92-95% instead of 97% but they'll cost under 1$
A cheap 30 cents AP62250 will do up to 18v in, up to 7v out, 2.5A out ... with around 90% efficiency : https://www.digikey.com/en/products/detail/diodes-incorporated/AP62250WU-7/12349212
The datasheet contains everything you need, example schematic, recommended values, even how to layout the circuit.
Richtek RT6224D is up to 18v, up to 5v out, up to 3A, and will go up to around 95% efficiency : https://www.digikey.com/en/products/detail/richtek-usa-inc/RT6224DHGJ6F/6205448 (note theres also a RT6224 but that one runs at lower switching frequency)
MP1477 is another good example : https://www.digikey.com/en/products/detail/monolithic-power-systems-inc/MP1477GTF-Z/7361360
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I don't see a RT pin in the datasheet : https://www.ti.com/lit/ds/symlink/lm64460-q1.pdf
It turns out digikey's LM64460 page links to the wrong datasheet (the LM63460) which is different by 1 digit
But I really have to wonder why you'd decide to make a 5v 0.7A dc-dc converter with a chip that costs 5$ in small quantities. It's a chip designed for up to 36v, with up to 6A switching current, not for circuits which are meant to output less than 1A of current.
Fair question. It's mainly because $5 is nothing in this context. This would be low volume production where the cost of operating the devices over 10+ years is going to overwhelm the parts, manufacturing and (TBD) even dev cost. These are remote devices that'll be costly to do battery service on, and which have to be compact (constrained battery) and can't do much energy harvesting, so a few percent power savings is significant. This was top of the pack out of Webench.
But maybe I'm underestimating the design effort and one of your (much appreciated) suggestions will be better. -
My average current draw will be ~150mA @ 5V, whereas 0.7A is my max draw (during very low duty-cycle Rx/Tx)
I looked at the RT6224D for example, efficiency at 12V input at 150mA is only 85% whereas Webench's solution claims 97.5%. I've found many datasheets show reduced efficiencies at light loads currents and the LM64460 seems an exception. FWIW it boasts "diode emulation" and "switching frequency reduction" as technologies to boost light load performance. -
You can get higher efficiency with regulators optimized for lower currents and which run or can be configured to run at lower switching frequencies.
I gave you links to very cheap and high switching frequency as you save money in both the chip and the cost of inductors and everything.
There are better chips that cost a bit more, for example this one's around 2$... see MP9841 or MPQ9841 (the automotive qualified version) : https://www.digikey.com/en/products/detail/monolithic-power-systems-inc/MP9841GL-Z/14645652
See graphs at page 8, you have 12v->5v @500kHz and 12v->5v @1 Mhz and also 12v->5v at ultra light load (<10mA) graphs : https://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MP9841GL/document_id/9095/
It can be configured to as low as 300kHz and will be nearly 95% efficient when running at 500kHz
Also, yes you may get 97% peak efficiency but you'd pay something like the 9$ for it - those extra 5-7$ you'd pay could be used somewhere else, depending on your design. For example 5$ may double the battery capacity, which would give your product more battery life than that extra 3-5% efficiency could give.