Original question:
TPS7A47: Two TPS7A47 in parallel and residual phase noise contributions
I am trying to get 3.3Vout, 1.75A out of 2 TPS7A47, 12Vin.
Based on the Rballast calculations
VoutNom=3.3; Vref=1.4; Iout=1.75; N=2; Efb=0; Evref=VoutNom*.005; Evout=Evref+2*(1-Vref/VoutNom)*Efb; RB=(VoutNom*2*Evout)/(2*1-Iout) Voutmin=VoutNom*(1-Evout)-(Iout/N)*RB*(1+.001*RB)
This gives a Rb of 0.4356Ohms, and Voutmin of 2.86V, do I adjust Voutnom to get the correct Voutmin? Are my calculations correct?
Hi Tyler,
The equation in the app note has a mistake. Eout = maximum tolerance of output voltage and the unit is in percent. For example, for a 2.5% output accuracy, Evout = 0.025. For a 3.3V output, the output could be off by 2.5% x 3.3 = 82.5mV.
For your case, RB=(VoutNom*2*Evout)/(2*1-Iout) = 3.3 x 2 x 0.025 / (2 - 1.75) = 0.66 Ohms.
Voutmin=VoutNom*(1-Evout)-(Iout/N)*RB*(1+.001*RB) = 3.3 x ( 1-0.025) - (1.75/2) x 0.66 x (1+0.001x0.66) = 2.63V.
This drop is too big to make the parallel LDO solution to work, one thing you could do it to tie the two NR/SS pins together to reduce the output difference between the two devices.
There is another post that one customer did some analysis with various Rb that may be helpful for you to take a look.
https://e2e.ti.com/support/power-management/f/196/t/832277
Regards,
Jason Song
Hi Tyler,
Understood. We also have the next-gen of TPS7A83/84/85 comings (samples would be available next month), but again, they only take up to 6.5V Vin. Currently, the LDOs that can provide 2A and can take 12V Vin are as the following:
The lowest noise is around 13uVrms. Do you think any of these devices would work for your application?
Regards,
Jason Song
