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??

[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