Questions about DC/DC conversion factor in nRF51822

[Leo]

Hello,

I have some questions about the DC/DC conversion factor in nRF51822.

1. In the datasheet nRF51822_PS v3.1, Figure 11 shows the graph about conversion factor vs. VDD. As Vdd is decreasing at the same Idd, conversion factor is increasing. As Idd is decreasing at the same Vdd, conversion factor is increasing. but these are contrast to the explanation of the page 43 in the nRF_Series_Reference_Manual v3.0.

2. As I know, In the normal DCDC operation, as Vin decrease at the same load current, efficiency will increase and Iin will decrease, then conversion factor will decrease. Please help me on understanding the operation of DCDC in nRF51822.

Thanks, Leo

[Stefan Birnir Sverrisson]

Hi

1. For the text in the nRF51 Series Reference Manual:

   The conversion factor (FDCDC) will decrease with decreasing supply voltage (VDD) if the current drawn through the DC/DC converter (Radio power) is kept constant. The conversion factor (FDCDC) also decreases with decreasing current consumption (IDD), for a given voltage (VDD).

   I aggree with you, it is incorrect. The conversion factor should increase in both cases, i.e. the conversion factor (F_DCDC) should decrease (DCDC efficiency should increase) as you increase supply voltage (VDD) and/or load current (IDD).

2. The image in figure 11 in the PS is correct, as you increase VDD, efficiency of the DCDC increases. Also, as load current increases, DCDC efficiency increases. For better understanding, look at this thread.