TPS5402: TPS5402

Hi Sarma,

Could you help share your layout and some waveforms for analysis, like VIN + VOUT+ SW + Inductor current ?

and may I ask this is for new project and why select this part ? As we had TPS54202/TPS62933 , next gen of this part may be also fit solution.

BRs,

YZ

Below is the output voltage waveform:

Switching frequency is designed at 250kHz using a resistance of 110k. This buck regulator circuit is designed to provide 5V, 1.2A output to drive the ESP32 Micro-controller and display. The designed circuit works fine till 0.6A loading condition. But beyond that, Vout starts dropping drastically although the Vin is maintained at 15V.

Please help me out in solving this problem.

Hi Sarma,

I didn't see any problem on your waveform, could you help share the waveform based on 15Vin-5Vou-1.2A ? pls include SW + VOUT+ Inductor current +Vin in the waveform. Pls share your layout for review,too.

Or you can send them to my mail young-zhao@ti.com

BRs,

YZ

I am unable to increase the loading upto 1.2A. The output voltage starts dropping as soon as I increase the loading to 0.6A.

That what i want to check at this condition, As need this abnormal wavefrom for analysis, a good waveform tells us nothing regarding this application.

BRs,

YZ

Fig.: Vin, Vout and IL at 0.5A loading condition.

Fig.: Vin, Vout and IL at 0.6A loading condition.

Fig.: Vin, Vout and IL at 0.7A loading condition.

The inductor used is 27uH, switching frequency is set at 250kHz using 110k resistor.

Please look at the above waveforms and kindly let me know if any other information required. 

Hi Sarma,

It looks your inductor shrink a lot at higher loading, is it possible to try another inductor with lager Isat current ? and to see if can work for heavy loading ?

BRs,

Young

Thank you for your feedback. I will try using an inductor of higher Isat and let you know the results.

Sarma,

Got it, pls let us know if any udpate.

BRs,

YZ

Hello Mr. Young Zhao,

I tried using a higher value of inductance keeping the switching frequency constant at 250KHz. Using these, the loading current beyond which the output voltage starts dropping has now increase to 0.9A from 0.6A (previous case). To overcome this, I redesigned the circuit by changing the switching frequency to 70kHz and using an inductor of 94uH (based on the calculations given in the datasheet of TPS5402). Even under this condition, the output voltage starts dropping beyond 0.9A loading condition. Also, I have noticed in the inductor current waveform that the switching frequency is not constant at 70kHz. It keeps fluctuating in the range of 60 to 75kHz.

Please suggest me some solution.

Hi Sarma,

May i check the PN# of inductor you changed? Not sure hte inductor value changed or same inductor value with larger current capability?

From your last wavefrom, you can check that OCP was triggered at this momment. t. The TPS5402 will increase the off time during short circuit /OC conditions by lowering the switching frequency. The switching frequency is divided by 8, 4, 2, and 1 as the voltage ramps from 0 V to 0.8 V on the VSENSE pin. You can refer table 1 in the datasheet.

You can select a larger inductor value or larger Isat to keep the derated inductor value at the reasonable level.

As no zoom in wavefrom, say the rising time of inductor is 2us~3us, the derated inductor is only 8uH ~13uH.

BRs,

Young

The part number of the inductor used is MHE0640470MTB.

You can find the details of the inductor in this link: https://www.atec-group.com/storage/system/product/atwspec/Inductive/Shielded_SMD_Power_Inductor/MHE/MHE0640-T.pdf

There are several inductors of different inductance value. Out of these, I used 7 numbers of 47uH inductor in series to get a total inductance value of around 336uH. The same has been highlighted in yellow in the screenshot povided below.

Please let me know if you need any further information.

I currently do not have inductor with higher Isat. Do you suggest any series, parallel combination of inductors to solve this issue?

I was initially using 27uH inductor (value obtained as per the calculation of the datasheet for Vin 15V, Vo 5V, switching frequency 250kHz, Io 1.6A, LIR 0.3). I later on changed to 336uH using series connection of 7 numbers of 47uH inductors. Do you think the value of the inductor obtained initially, i.e., 27uH is appropriate?

Hello Mr. Young Zhao

I tried using three different values of inductors with higher Isat. The switching frequency for all the three cases is 250kHz. The problem still persist, i.e., the output voltage of the buck converter circuit remains regulated at 5V for loading condition upto 6A. Beyond 6A, the output voltage starts reducing drastically till it falls to 0. The waveforms of the inductor current obtained for all the three cases are shown below:

Fig. 1: Inductor Current for 27uH Inductor designed using PQ2020 ferrite core for Ipeak taken as 6A.

Fig. 2: Inductor Current for 27uH Inductor with Isat of 4.2A. The inductor was ordered online.

Fig. 3: Inductor Current for 23uH (parallel connection of inductors of 46uH and Isat of 5A each). The inductor was ordered online.

Please suggest me some solution.

Regards,

Upasana Sarma

Hi Sarma,

what do you mean by"the output voltage of the buck converter circuit remains regulated at 5V for loading condition upto 6A", the loading up to 6A ? is it true ?

TPS5402 is only with 1.7A DC loading capbility, it would trigger OCP (over current protection) when increase loading and then reduce Fsw.

Could you help confirm the loading ? and address the issue again ?

BRs,

Young

Hi Mr Young,

I am extremely sorry for the confusion. I meant to write "loading condition upto 0.6A" instead of 6A. Upto 0.6A loading condition, the output voltage remains regulated at 5V I but beyond that, the output voltage starts decreasing.

BRs,

Dr. Sarma

Also, could you please specify that peak current beyond which OCP is triggered. Because as per datasheet, what I could understand is that there is a peak inductor current limit of 2.5A. Is this correct? 

If yes, then in my board, the peak current does not even go beyond 1.5A until now. Please let me know your suggestions in this.

Hi Sarma,

Could you help capture the SW + Inductor current at >=0.6A when Vout drop ? Want to check what's happening again, it's wired to me.

and is it possible to measure the bode plot at 0.5A ? seems there is double pulse from below waverfrom ? could you help double check SW pin wavefrom ?

and if it is possible , pls share your latest sch and layout to my mail box for review young-zhao@ti.com

BRs,

YZ

Sure, I will update you with the waveforms soon.

BRs,

Dr. Sarma

Hello Mr. Young,

There are no pins specified in the name of SW in the datasheet. There are ROSC and SS pins. Could you please specify exactly which pin waveform do you want?

BRs,

Dr. Sarma

Hi Upasana,

The waveform required by Young is captured at LX pin, which is required SW waveform.

BRs

Zixu

Hi,

Fig: SW and inductor current waveforms obtained using 27uH inductor with Isat of 5A approx for 0.6A loading condition.

I have now changed the inductor to 230uH of Isat 5A and tested it. I obtained the following waveforms which are shown below. With this inductor, I could upto 0.8A loading condition and get regulated output voltage. But beyond 0.8A, the problem still exists.

Fig 1: SW and inductor current waveforms at 0.5A loading condition.

Fig 2: SW and inductor current waveforms at 0.6A loading condition.

Fig 3: SW and inductor current waveforms at 0.7A loading condition.

Fig 4: SW and inductor current waveforms at 0.8A loading condition.

BRs,

Dr. Sarma

Hi Sarma,

Appreciate your help on this waveform sharing, 2 input from my side

1.Loop is not stable at >0.6A loading, you can see the double pulse of SW pin. Pls follow this app to confirm the external compensation Designing Type III Compensation for Current Mode Step-Down Converters (Rev. A)* Make sure to use the parameters from TPS5402, gmps = 9 A/V, gmea = 92E-6 and vref = 0.8 V.

2. High SW spike was observed , seems this layout is not good enough. Is it possible to share us the sch and layout for review ?

and in the end, if this is a new project not re-use TPS5402, I would highligh that we have newer solution TPS54202,TPS62933 which can provide better performance and supply.

BRs,

Young

Hi Mr. Young,

Thank you so much for your suggestions. I will try designing the compensator. Also please find the attached sch and pcb layout of the buck converter. Please let me know your suggestions to improve the same.

Hi Sarma,

I will check the layout and feedback to you later.thanks for understaning .

BRs,

YZ