Power capacity of DC/DC vs. LDO

[Jchristf]

In the nrf52840, what is the external supply capability of the Dc/Dc converter?

We are looking to take advantage of internal regulator to generate 3.3v from a ~4V li-ion battery on VDDH.

Initially we used the internal LDO, but now realize that 25mA limitation (we need ~100mA).

We are wondering if the Dc/Dc configuration will give us greater current capability than the 25mA from LDO?

thanks

Jacob.

[Andreas]

Hi,

 

Unfortunately, the 25mA constraint applies to both LDO mode and DCDC mode. Also this is only when the radio is not enabled, when the radio is enabled the constraint is 2mA.

 

Best regards,

Andreas

[Jchristf]

A related question: Given that we need to run IO (and therefore VDD) at 3.3V and the li-ion power is 3.8v nominal,  is it worth the trouble to use the VDDH of Nrf52840 or are we better off regulating down to 3.3V outside and feed to VDD?  It depends on the efficiency of the Dc/Dc in 52840...?

[Andreas]

Hi,

 

It depends on your budget and preference. Using an external regulator to supply VDD with 3.3V would probably be the most efficient, depending on the regulator of course I would expect vendors that specialize in such products to have better performance than the REG1 stage, especially with such a small drop. If you are on a tight budget, and can possibly accept slightly higher overall current consumption, then using VDDH as main supply is a fair choice. Note though that there will need to be a 0.3V drop over REG0 (from VDDH to VDD), meaning if your battery/source drops below 3.6V then VDD and thus GPIO level will also drop below 0.3V.

 

We do not have a characterized efficiency of the regulator, but you can take a look at the compounded current consumption scenarios. Is0 and Is4 are the same, just supplied with VDD = 3V and VDDH=5V, respectively.

 

Best regards,

Andreas