TPS61096A: What is effective value of Cout when 28V is needed?

Hi Eric-san,

Is it possible to answer this question?

Thank you.

Hi Koki-san,

I think 2*pcs 10uF capacitor is suitable for 18V output which effective capacitor is about 2uF. So for 28V output, I think the suitable effective capacitor should be about 3.2uF.

Best Regards,

Eric Yue

Hi Eric-san

Thank you for quick answer and sorry for rush you.

Is 3.2uF for one Cout? Or total Cout such as two 1.6uF Caps parallelly ?

Let me confirm.

Thank you 

Koki Tsushima

Hi Koki-san,

Both is okay as long as the effective capacitor is suitable. 2*pcs will have smaller ESR but increase PCB size.

Best Regards,

Eric Yue

Hi Eric-san

Sorry for typo.

Is 1.6uF for one Cout? Or total Cout such as two 1.6uF Caps parallelly and totally 3.2uF?

But referring to your answer, that should be totally 3.2uF, right?

Thank you 

Koki Tsushima

The total capacitance is 3.2uF, one 3.2uF or two 1.6uF caps.

Best Regards,

Eric Yue

Hi Eric-san,

Thank you for your kind.

I re-calc again but I guess total effective value of Cout should be enough for 2.06uF.

I think 2*pcs 10uF capacitor So I think " suitable for 18V output which effective capacitor is about 2uF."which you mentioned is correct.

But why did you guess this may be suitable for 3.2uF effective Cap?

Will minimum of effective Cap be changed on their Voltage?

Best,

Koki Tsushima

Hi Koki-san,

The 2uF effective Cap is suitable for 18V output voltage. I think customer's application is 28V output, so the effective Cap should increase proportionally. Then 3.2uF is suitable for 28V output.

BR

Eric Yue

Hi Eric-san,

Why the effective Cap should increase proportionally?

Is it due to their stabilization?

If it is due to ripple, customer can accept it, but lower cap is better.

Sorry for taking your time.

Best

Koki Tsushima

Hi Koki-san,

Yes, the main reason is loop stability.

Hi Eric-san,

I understood that minimum effective Cout is 3.2 due to loop stability etc.

Is it possible to explain in detail? - i.e. detail chars of loop etc. or any other reasons.

It is OK to tell me we cannot share this info because of design circuit.

Thanks

Koki Tsushima

Hi Koki-san,

Please find detailed information of compensating boost control loop in below app notes. Cout will add an pole to the power stage, and directly effect loop bandwidth. So small Cout will lead to large bandwidth, which may cause system instability.

Compensating the Current-Mode-Controlled Boost Converter

Best Regards,

Eric Yue

Hi Eric-san,

Thank you for your kind.

I understood that large Cout is better for stability as it was mentioned on Equation 11.

But I do not know why effective cap should be increased when large Vout. Could you tell me?

- I understood that Cout should be increased for Vout due to DC bias chars.

And it seems to be no loop chars of TPS61096A.

Is it possible to explain for loop chars of it for those Cout? Or it is OK if you cannot share that info because it should be related design.

Thank you.

Koki Tsushima

Hi Koki-san,

Sorry that I cannot share the detailed loop design of TPS61096A. But for a typical boost converter, Vout will effect the control loop stability which can be found in the app note shared before. And if we want to counteract the effect of Vout increase, we should increase Cout to maintain the same stability. 

Hi Eric-san,

Thank you for your kind.

Customer and I understood what you said and referred to 2 Design Step on Compensating the Current-Mode-Controlled Boost Converter.

As (11), Cout＝ ΔI_TRAN / (2pai * fc * V_TRAN).

ΔI_TRAN  should be small in this case, and customer do not care for V_TRAN so much, so Cout can be lower more.

Is this understanding correct?

Or by considering above things, the min should be approx. 3.2uF ?

Best 

Koki Tsushima

Hi Koki-san,

V_TRAN is different from output voltage ripple, it is related to loop stability. As you can see equation (2) and (3) from AN, Cout will influence the power stage poles/zeroes and whole converter loop stability. Typically, output voltage ripple is ~20mV, but V_TRAN is ~200mV which is related to load current transient, it is very different, so please check whether customer do not care for V_TRAN. If Cout is not enough, the loop will be non-stable.

BR

Eric Yue

Hi Eric-san

Thank you so much for your kind.

Customer wants to use TPS61096A for load transient chars : 1% →28V*1%=280mV and ΔItran=10mA,

but is 3.2uF effective cap needed? 

(I am so sorry but is ΔItran can be 10mA? I guess customer misunderstood ΔIran and Iout. Are they difference?)

Best 

Koki 

Tsushima

Hi Koki-san,

What is customer's Iout current? I think the ΔItran can be 90% of full load current. ΔItran is the load current transient. I don't have accurate information of needed of effective cap value, but according to EVM design, I think 3.2uF will be safe enough.

BR

Eric Yue

Hi Eric-san

Iout is few mA and it is enough up to 10mA.

BR

Koki Tsushima