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 more specific question:  We are using the Fanstel BT840X modeule, which to my understanding is essentially your NRF52840 + Skyworks 66112 amplifier.  According to Fanstel the amplifier is powered off VDD, and they have dongle where the power is 5V to VDDH + DC/DC setup feed back to VDD.  However, looking at Skyworks datasheet it can draw as much as 110mA in bursts, so I'm trying to find out why that works, if 25mA is max in that configuration??

[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,

 

I do not know more about the Fanstel module or dongle than I can see in the datasheet, but if this is true that would be a reason for concern. The REG1 stage can only supply up to 25mA externally for all cases where the radio is disabled. If the radio is enabled, which it has to be for the SKY66112-11 to draw upwards of 100 mA, this is per the spec lowered for higher output power. The regulator delivers the same, meaning it can deliver more than the REG0 spec allows, but as the output is shared with the radio this is to ensure the external supply does not pinch off the radio supply.

I do not see this working very well, so there has to be another regulator or something powering the FEM.

 

Best regards,

Andreas

[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