BQ24640: Minimum recommended PAD size for RVA ( S-VQFN-N16) package?

Hi Jing,

This Iñaki Larequi, the person who contacted Anthony regarding the BQ24640 footprint request.

I would like to explain a little bit more the design where the BQ24640 is used and its working conditions. The BQ24640 works as charger in an ultracapacitor bank (composed of 4 ultracaps in series, thus maximum voltage is 10.8V) from a DC voltage of +12V. This part is used in a power board with base cupper thickness of 105um. Our pcb manufacturing suppliers have defined us a minimum separation of 0.3mm between conducting elements for reliable massive production. To accomplish this separation in the BQ24640 footprint, the only way is to reduce the pad width to 0.2mm.

The BQ24640 charges the bank at a constant current of 1Amp and once the bank voltage arrives to maximum value, the BQ24640 maintains the charge with a low current of 0.5Amp. The ultracap bank only charges at power-up of the whole system, and it only works again if the grid is lost and comes back. Therefore, the number of charging process is quite low (less than 1 per day as average).

I´m aware of the impact in solder reliability if the pad width is reduced to 0.2mm. However, the working conditions for the BQ24640 are not very demanding. In such situation, do the BQ24640 team still recomend not to reduce the pad width to 0.2mm?
If the feedback is to maintain a minimum pad width of 0.24mm, then I must find an alternative part for the ultracap charger. Having a fast look in TI portfolio, the part LM5085 could be an alternative. Any other part suggestion according to the working conditions I described above?

Thank you

Hi Jing,

I select 105um as base copper thickness for my power design. That means the final thickness in external layers is around 140um once the pcb is manufactured. Therefore, my pcb suppliers have recommended me to keep at least 300um of distance between conducting elements. In the VQFN footprint of the BQ26640, the only way to accomplish that requirement is to reduce the pad width down to 200um. That´s why I contacted TI to get a feedback about the feasibility of reducing the pad width.

From your answer I understand that TI suggests not to reduce the pad width from the value (280um) defined in the recommended footprint. At this moment, we are manufacturing some prototypes of these board where the pad width was reduced to 250um. I will check then in a couple of weeks, depending on the results I could decide if I face the risk of reducing further more the pad width.

In the meantime, I would like to find an alternative. It´s true that the LM5085 is a buck controller, but it offers an external current limit (through the PMOS rdson or through an external resistance). From my understanding, if a supercap bank is connected at the output of this controller, the charging current of the capacitors would be the selected current limit. It´s clear that the LM5085 would be have less features compared to BQ24640 as a supercap charger but it could work with that role. In my design, I could accept a single charging current for the supercap bank. The supercap bank is an energy back-up when the grid is lost, and the charging time is not very critical as it happens at power-up of the whole equipment. In fact, in my current design I define the value of 1Amp for the maximum charging current. That current means that the charging time would be less tan 5 minutes, an acceptable time for us. In this working conditions, do you see the LM5085 as an alternative for the BQ24640?

Best regards.

Hello Inaki,

I am not very familiar with LM5085. My only concern would be when the supercap is completed depleted, would LM5085 able to charge it up. You might want to start a separate post on LM5085 on this.