WEBENCH® Tools/TPS54240: TPS54240

Hello Ruslan, 

Answering your questions here:

1. What will the really output voltage Vout, if Vin really = 12V and TPS54240 will be configured at Vin = 13,2-36V, Iout=2A, Vout = 12V?

a. As it is written in data-sheet - "When the voltage from BOOT to PH drops below 2.1 V, the high-side MOSFET is turned off using a UVLO circuit which allows the low-side diode to conduct and refresh the charge on the BOOT capacitor. Since the supply current sourced from the BOOT capacitor is low, the high-side MOSFET can remain on for more switching cycles than are required to refresh the capacitor, thus the effective duty cycle of the switching regulator is high", so in real-world scenario duty cycle will be somewhat less than 100%. And your maximum attainable output voltage can be calculated from Eq 8 from this App Note. Mostly, Vin to Vout voltage drop will be dependent on Iout * HS_rdson drop. 

To improve the dropout performance of non-synchronous regulators, you can use a leaker resistor from PH to ground parallel to the diode. Please read this App Note for complete understanding. In this scenario, you will have to take the hit on the buck's efficiency.

2. Can I legally use TPS54240 in this mode for a long time, if Vin really = 12V and TPS54240 will be configured at Vin = 13,2-36V, Iout=2A, Vout = 12V?

a. Yes.

3. What will the really output voltage Vout, if Vin really = 7V and TPS54240 will be configured at Vin = 13,2-36V, Iout=2A, Vout = 12V?

a. The output voltage will be equal to Vin minus Iout * HS_rdson. And in addition to that, the effect of the duty cycle is slightly less than 100 will also be seen on the output.

4. Can I legally use TPS54240 in this mode for a long time, if Vin really = 7V and TPS54240 will be configured at Vin = 13,2-36V, Iout=2A, Vout = 12V?

a. Yes. The output voltage will be less than 7V.

5. Must I use buck-boost converter for my task necessarily?

a. Yes, If you want 7V to 12V conversion in your application. Any question related to boost convertor, you might have to file a separate E2E thread and direct it to boost the team.

Hope this helps!

Please let me know if you have more questions.

Otherwise, please close this thread, so that TI won't keep monitoring this thread any longer. Thanks!

Best Regards,

Ankit Gupta

Application Engineer.

Hello Ankit,

Thank you very much for your answers. I understood full.

We used TPS54340 DC/DC converter in our old design too. It has a good cost too and is widely used. us. Us I understand from TPS54340 datasheet to improve drop out, the TPS54340 is designed to operate at approaching 100% duty cycle too. As I understand as duty cycle is approaching 100% then minimum input supply voltage Vin min is near the output voltage Vout of the application too.

I have some questions about TPS54340 operate too:

1) I tried to simulate TPS54340 at Vin = 12 – 36V, Vout = 12V, Iout max = 3A mode in webench designer. Webench designer allows to set Vin min = 12,4V only. What will the really output voltage Vout, if Vin really = 12V and TPS54340 will be configured at Vin = 12,4-36V, Iout=3A, Vout = 12V? Can I legally use TPS54340 in this mode for a long time, if Vin really = 12V and TPS54340 will be configured at Vin = 12,4-36V, Iout=3A, Vout = 12V?

2) If 12 – 36V primary power source is absent, then reserved secondary power source is 7V. What will the really output voltage Vout, if Vin really = 7V and TPS54340 will sets at Vin = 12,4-36V, Iout=3A, Vout = 12V? Can I legally use TPS54340 in this mode for a long time, if Vin really = 7V and TPS54340 will be configured at Vin = 12,4-36V, Iout=3A, Vout = 12V? I understand, that is TPS54340 buck converter and Vin really = 7V less then Vout = 12V. But Vin really = 7V and Vout really = 7V is normal for my task, although TPS54340 will be configured at Vin = 12,4-36V, Iout=3A, Vout = 12V.

Dear Ankit, I think, that your answers will are similar TPS54240 chip, but I want to receive official confirmation just in case please.

Best regards,

Ruslan.

Hello Ankit,

Thank you very much for your answers. I understood full.

We used TPS54340 DC/DC converter in our old design too. It has a good cost too and is widely used. us. Us I understand from TPS54340 datasheet to improve drop out, the TPS54340 is designed to operate at approaching 100% duty cycle too. As I understand as duty cycle is approaching 100% then minimum input supply voltage Vin min is near the output voltage Vout of the application too.

I have some questions about TPS54340 operate too:

1) I tried to simulate TPS54340 at Vin = 12 – 36V, Vout = 12V, Iout max = 3A mode in webench designer. Webench designer allows to set Vin min = 12,4V only. What will the really output voltage Vout, if Vin really = 12V and TPS54340 will be configured at Vin = 12,4-36V, Iout=3A, Vout = 12V? Can I legally use TPS54340 in this mode for a long time, if Vin really = 12V and TPS54340 will be configured at Vin = 12,4-36V, Iout=3A, Vout = 12V?

2) If 12 – 36V primary power source is absent, then reserved secondary power source is 7V. What will the really output voltage Vout, if Vin really = 7V and TPS54340 will sets at Vin = 12,4-36V, Iout=3A, Vout = 12V? Can I legally use TPS54340 in this mode for a long time, if Vin really = 7V and TPS54340 will be configured at Vin = 12,4-36V, Iout=3A, Vout = 12V? I understand, that is TPS54340 buck converter and Vin really = 7V less then Vout = 12V. But Vin really = 7V and Vout really = 7V is normal for my task, although TPS54340 will be configured at Vin = 12,4-36V, Iout=3A, Vout = 12V.

Dear Ankit, I think, that your answers will are similar TPS54240 chip, but I want to receive official confirmation just in case please.

Best regards,

Ruslan.

Hello Ruslan,

TPS54340 has a known issue while operating at the deep dropout scenario. It has internal SS DAC, TPS54240 does not have this DAC, due to which there is a known issue that Vout drops greater than 1V. Please see the attachment, if you want to understand the issue in more detail.

TPS54340 VOUT DROPOUT ISSUE 01122017.pptx

This issue got improved in TPS45341, where internal SS DAC got changed back to analog SS. For your application where it seems like low dropout operation is quite critical, I would recommend using TPS54341.

And if you are using TPS54341, then all my answers are the same as I have mentioned in my earlier reply.

Hope this helps!

Please close the thread if you think your questions have been answered. Thanks!

Best Regards,

Ankit Gupta

Application Engineer

Hello Ankit,

Thank you very much for your answer. I understood full.

I have a last question. TI company has many buck DC/DC converters: TI Home > Power management > DC/DC switching regulators > Step-down (buck) regulators > Buck converters (integrated switch).

What should i pay attention in hide filters (and then in datasheets), when I will to choose buck DC/DC converter in big site table for my application? Our device must have function, when minimum input supply voltage Vin min is near the output voltage Vout, and if Vin<Vout then device must have low dropout legally function (as I asked about TPS54240 above: questions 1 and 2). In unfortunately, I can’t ask about each chip individually…

Best regards,

Ruslan.