Tool/software: WEBENCH® Design Tools
I am designing using TPS65400, to make question easier lets focus on SW3 .
My parameters are Vin=5V, Vout =1.8V, Iout 1.8A, Cout =2x 10uF 1mohm ESR
From webench I am getting Rfb1=20k, RfB2=16k, Rcomp=9.09kohm, Ccomp=1.8nF, Ccomp2=39pF
Using eqation for Rc from data sheet page 24 Table 3 Rc=(2*pi*fc*Co*V0)/(Gm*Gmps*Vref), and assuming as data sheet says Gm=120uA/V, Gmps=10A/V and Vref =0.8V I am getting Rc=27kOhm while webench calculates 9.090kOhm.
Then of cource since Rc is wrong all othe calculations ( Cc and Croll) are wrong.
But even assuming Rc=9090ohm, acc to same data sheet Cc=(Rl*Co)/Rc= (1*0.00002)/9090=2.2nF not 1.8nF given by webench?
Also Cc2=Croll=(Resr*Co)/Rc=(0.0005*0.00002)/9090=1.1pF not 39pF given by webench?
Could you please explain the discrapencies and advise what equations should be used for calculations.
Also is there a chance to run simulations (especially bode plot) in webench for TPS65400 , I can not find options to run them in the menu?
Thanks Sebastian
Since Rc i sused in all othe requations all ot
Hi Sebastian,
The equations used for compensation used are same as in datasheet for this device. The differences are in the parameter values being shown in the datasheet (Fig 17 on page 24) with Gm=120uA/V and gmps=10A/V and what is actually there in the electrical characteristics table 7.5 on page 8 where Gm=133uA/V. The gmps value is actually different for output 3/4 as compared to outputs 1/2 as they have different power stages and the numbers we have are gmps(outputs 1,2) = 14.49 and gmps(outputs 3,4) = 7.5. Along with this, the output capacitance value might be different than 20uF as we also do DC bias voltage derating based on manufacturer data when we have it in our database otherwise we use linear derating. With the linear derated model, for a 5V output, a 10V rated output cap with nominal value of 44uF will have 22uF value.
In order to simulate bode plot, you need to click on a single channel and then click "Sim" tab. Please see attached screenshot below:
Regards,
Amod
Hello Amod, thak you for your answer.
I tried using parameters you have suiggested:
Vin=5V, Vout =1.8V, Iout 1.8A, Cout =2x 10uF dereated to 14uF ( webench value ) 1mohm ESR, Gm=133uA/V, Gmps=7.5A/V, fc =50KHz
Rc=(2*pi*fc*Co*V0)/(Gm*Gmps*Vref) gives 9.9kohm still not 9.090 kohm but I am not sure if this not close enough so happy to accept this.
To check the correctness I jump to another channel SW1 , where I wantto have 1V at 2.6 A so parameters are:
Vin=5V, Vout =1V, Iout 1.8A, Cout =2x 10uF dereated to 14uF ( webench value ) 1mohm ESR, Gm=133uA/V, Gmps=7.5A/V, fc =50KHz
Rc=2.9k ohm not 3.48kohm given by Webench
What do you think where the difference can come from?
I guess something is still not right also looking at Cc2=Croll=(Resr*Co)/Rc=(0.0005*0.000014)/9090=7.7pF not 39pF
On the Bode plot the sim button is not available:
Sebastian
Hi Sebastian,
I can take a look at the detailed calculations. If you can please share your project and send the public link here, i can look at your design details for these calculations.
For the simulations, once you click on the channels on the left and then see the Sim button on the top?
See step below is to click on any channel and then you should observe the sim tab. Bode plot simulation is the first one in the dropdown. Please share your screenshot if you are unable to see this same type of screenshot. Please use the "Attach file" feature to get your screenshot through.
Regards,
Amod
Hi Amod,
Just noticed my screenshot (with missing sim button) was not attached:
Also sharing the project is not a problem at all :
https://webench.ti.com/wb5/OpenPublicSharedProject.jsp?id=E2E34E576E1BDE4A&s=c
Thanks in advance
Sebastian
I can see the screenshot somehow does not get through, so trying to attach it as file.
Hope this works
Sebastian
Hi Sebastian,
Thanks for sharing the design. I ran the calculations again and found slight differences in gmps and derated Cout value which may not show up in the UI due to rounding. I am sharing the details of the values below in the screenshot:
The snapped values are the ones being used in the actual design.
I am not sure why the simulation tab is not visible for you once inside a "channel" view. If you can please confirm that you see for a new design, then it might help debug this issue. Another fallback option is to use the PSPICE average model under: www.ti.com/.../toolssoftware
Regards,
Amod
Hi Sebastian,
I am unable to reproduce the simulation button issue for TPS65400.
Kindly note that TPS65400 is PMU IC (Power Management Unit) consisting of 4 Buck channels. Please find below snapshot from Data-sheet FYR.
WEBENCH also has implemented this device in a similar way where "PMU1 Total" represents total PMU IC whereas "PMU1_CH1", "PMU1_CH2", "PMU1_CH3" and "PMU1_CH4" are the individual channels [BUCK1, BUCK2, BUCK3 and BUCK4] of TPS65400.
Currently there is no way to run electrical simulation for TPS65400 at system level [i.e PMU1_Total]. Instead you can run Bode simulations for individual channels. Just select any of the channels (Ch1 is selected) in below image and select on "Sim" option [see below image]
Somehow for your shared project, we are unable to get "Sim" Option even for individual channels.
Could you please try recreating new project from http://www.ti.com/product/TPS65400 and let us know if you see "Sim" option for individual channels ? Alternatively you can use below shared project for your input/output requirements.
https://webench.ti.com/wb5/OpenPublicSharedProject.jsp?id=C7C0A630F6BEA174&s=e
Thanks & Regards,
Harish
