VDD decoupling caps maximum values

[WiPe]

Hello,

I read your white paper nWP-030 about selecting the decouping caps. But I didn't find any explanation why you also specify maximum values for the caps. Especially CVDD effective capacitance 5.5 uF max. would be a little problematic as we are using external FEM from Skyworks that requires quite large currents and a 4u7 decoupling cap is recommended very close it also. If we'll have about 5 uF effective on VDD pins of nRF527840 and ~3 uF in front of FEM,we'll go over the maximum recommended value. 

Also, we'll need to deliver power over a long cable and would like to have lots of capacitance on the nRF end of the cable, tens of uF:s, to avoid voltage drop issues on power peaks.

Our powering scheme is meant to be versatile meaning an installer person may use either VDD(+VDDH) or VDDH to supply power. Also internal regulators may be configured on the field either to LDO or SMPS mode depending on installation case and power source.

So could open little more why there are those maximum limitations and in what cases (powering schemes) they need to be respected?

[Andreas]

Hi,

 

This is the capacitive load the REG0 is designed for and is thus only relevant for the power configurations where REG0 is used (supply on VDDH). The capacitance on the main supply pin (VDDH for high-voltage config and VDD+VDDH for normal voltage config) can exceed this.

As for the FEM, note that the REG0 can deliver a maximum of 25mA when nRF52840 TX power < 0 dBm, I would assume your FEM draws a bit more than this. In this case you should not supply the FEM from the VDD rail anyway.

 

Best regards,

Andreas

[wallance.lin]

Hi

I'm using nRF52840 and also find the same issue after checking nWP-030, our design follows "7.3.4 Circuit configuration no. 4 from nRF52840 datasheet". after considering tolerance, we find

1. the maximum effective Vdd cap is 7.11uF > 5.5uF(maximum Effective capacitance in VDD),

2. the minimum capacitance of Cdec4 is 0.68uF < 0.7uF (minimum Effective capacitance in DEC4),

do you have any concern for this? and where we need pay attention or check?

[Andreas]

Hi,

 

This is outside spec, and could cause the power supply system to not behave according to spec.

 

Best regards,

Andreas

[wallance.lin]

Hi Andreas

thanks! I would like to check which one or both of them could cause power supply system issue? 
Over maximum Effective capacitance in VDD or  lower than minimum Effective capacitance in DEC4? 

[Andreas]

Hi,

 

With config 4 both VDD and DEC4 are regulator outputs and inputs, meaning both VDD and DEC4 must be withing the specified range.

 

Best regards,

Andreas

[John Man]

Hi Andreas,

Per Datasheet, CVDD is requested in the range of 2.7~5.5uF. However, the reference design (configuration No. 5 schematic) shows that the total VDD nominal capacitance is 4.7uF+1uF=5.7uF. This already violates the CVDD requirement. Can you let me know if CVDD requirement is valid or the reference schematic is valid-----they are in contradiction.

Thanks

John