DRV3901-Q1: VREG capacitor calculation and inrush current control

Vishwas H R

Part Number: DRV3901-Q1

Tool/software:

Hello,

I have referred to excel for calculating the Vreg capacitor using the link: DRV3901-Q1: External Components - Motor drivers forum - Motor drivers - TI E2E support forums

I could see the formula for min cap on Vreg is not correct.

I have following questiong:

1. The formula should be C = (2*Pout*time)/(Vinitial^2 - Vuvlo^2). So, if i consider Vreg is 8V and diode drop between Vreg and Vdrv 0.5V. Then C = (2*1.75*7.5*0.5/1000)/(7.5^2-5^2) = 420uF. Vuvlo I considered 5V beacause minimum voltage required to operate the device is 5V at Vrdv pin. Since, it is constant current I have considered Pout to be Iout*Vout because all the drop will be across load irrespective of resistance value.

2. Since we are using the high bulk capacitor do we need inrush current limiter? I feel it it is not needed because I already have regulator before the Pvdd supply which is having soft start of 5ms and also diode between Pvdd will limit.

6 months ago

0 Keerthivasan Kumanan 6 months ago

TI__Mastermind 18725 points

Hi Vishwas,

1) The excel calculator is based on the following equation:

ΔQ/ΔV = C

Your expression that was derived from energy should be equivalent. I will look into why it is giving you a different answer tomorrow. Here is the updated version of the calculator.7446.VREG_CAP (2).xlsx

2) The regulator would work. Another way to limit the inrush current is to just add a series resistance.

Best,

Keerthi

0 Vishwas H R 6 months ago in reply to Keerthivasan Kumanan

Intellectual 330 points

Hello Keerthi,

1. Sure, please check and let me know.

2. Where to add series resistance and how to choose the value? I think series resistance is not feasible because when it is fired let's say 1.75A will flow which will result in high voltage drop and also lot of power will be dissipated.

0 Keerthivasan Kumanan 6 months ago in reply to Vishwas H R

TI__Mastermind 18725 points

Hi Vishwas,

The other alternative to a series resistance is to use some kind of LDO with soft start or. Please see this app note on the different ways:

Managing Inrush Current (Rev. A)

Best,

Keerthi

0 Vishwas H R 6 months ago in reply to Keerthivasan Kumanan

Intellectual 330 points

Hi Keerthi,

Ok, thanks. Did you check why the capacitor values are different using two different formula?

0 Keerthivasan Kumanan 6 months ago in reply to Vishwas H R

TI__Mastermind 18725 points

Hi Vishwas,

I am thinking it might be an assumption that you made with how you calculated Pout. I haven't been able to verify this yet. This calculator has been validated, so let me double check if there are any captures.

Best,

Keerthi

0 Vishwas H R 6 months ago in reply to Keerthivasan Kumanan

Intellectual 330 points

Hello keerthi,

Did you check why there is difference?

Thank you,

0 Keerthivasan Kumanan 5 months ago in reply to Vishwas H R

TI__Mastermind 18725 points

Hi Vishwas,

No, I haven't gotten the chance, let me check and get back to you tomorrow.

Best,

Keerthi

0 Vishwas H R 5 months ago in reply to Keerthivasan Kumanan

Intellectual 330 points

Hello Keerthi,

Any update?

Thank you,

0 Keerthivasan Kumanan 5 months ago in reply to Vishwas H R

TI__Mastermind 18725 points

Hi Vishwas,

I can get to the other formula by doing the following:

Q = C*ΔV

1/2*ΔV*i*t = 1/2*C*(ΔV*2)

P*Δt =C*(ΔV*2)

But again Vishwas, they should give the same answer. However, the calculator that has been provided has been validated on bench. If needed, please get an EVM and just verify both results experimentally.

Best,

Keerthi