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Part Number: LM5121-Q1
Tool/software: WEBENCH® Design Tools
I would like to use LM5121/-Q1 in my design and use Webench to confirm. My design will have external Vin supply of 5V and it should enable me to bring down minimum input to 3V. But the webench tool only allows 4.5V minimum. Can you advise solution?
According to datasheet, LM5121/-Q1 need a minimum of 4.5 V for the device to get through start up phase and attain steady state. After attaining steady state Vin can go as low as 3 V until the point where it goes into shut down mode. Once the device goes to shut down mode, you'll need a supply of 4.5 V to restart the device again. If your input source drops to three volts and if the device goes into shut down mode (due to transients or any abnormality), It may not be able to restart after that. Hence, Webench limits users from creating designs below 4.5 V Vin.
I hope this answers your question. Please let us know if you have further queries.
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In reply to Vishwanath Tigadi:
Thank you for attending to my question. Like I said, I intend to power Vin via external source (low power boost) and does not expect it to go below 5V. If the datasheet specifically mentioned 3V minimum input (for boost, not Vin), I expect to be possible to be used down to 3V. I think it would help to reduce the 4.5V limit so the tool may still be used keeping in mind the Vin requirements.
But of course I only say this because I need it :). Just checking if you would consider.
But in case it is not possible, is my plan on supplying Vin externally going to work so I can boost the high current 3V? Am I right in assuming that voltage monitoring is done only on the Vin part and not on the other pins? If so, I will just manually compute the values for 3V input. (for boost, not Vin)
In reply to Emmanuel Roble:
Changing Webench limits may create bad designs on webench for the end users.
I'll move this thread to the applications design team who can help you with your specific concern.
Thanks for considering the usage of the LM5121 in your design. I will answer your question in two parts.
1) Is my plan on supplying Vin externally going to work so I can boost the high current 3V?
Yes, you can supply VIN externally so you can boost the 3V input voltage.
However, you will need to account for the CSP/CSN common-mode voltage, which has a minimum of 3.0V, so if your input voltage deviates below 3.0V, successful operation cannot be guaranteed.
2) Am I right in assuming that voltage monitoring is done only on the Vin part and not on the other pins?
VIN pin on the chip is the input to the VCC regulator. The reason the datasheet specifies a voltage of at least 4.5V at the VIN of the chip is that the VCC UVLO is 4.1V.
Furthermore, depending on application, voltage monitoring is also done on the UVLO pin. You will need to size those resistors accurately so that the part will turn on/off at the correct voltage.
Hope this helps.
In reply to Richard Fu22:
Thank you for attending to my concern. Regarding your answer on number 1, it will put my design on the borderline of operation. Do you have alternative part that has lower common mode voltage on that portion?
Can you advise better controller for my design? My basic specifications is below
Vout = 9V 6A
Vin = 3V - 4.2V
If non-synchronous controllers can be used in your design, the LM5155 can be a suitable controller since it does not have a common-mode voltage for the current sense amplifier. Bias can be powered externally with your boost input voltage.
Thank you for your LM5155 suggestion. I made a prototype following the layout in datasheet/evm with values taken from the LM5155 calculator tool. But on my actual test the output voltage drops at just ~2.5A output. I have attached the schematic, pcb layout and LM5155 spreadsheet calculator. Can you check if I may have missed something?
Thanks for using the LM5155 in your design! I've noted some comments below.
1) According to the LM5155 datasheet, if you connect VCC to BIAS, you can power bias in the operating region from 2.97V - 16V. Otherwise, you will need to supply a voltage greater than or equal to 3.5V. In your case, did the output drop when your bias pin voltage was lower than 3.5V?
2) The soft-start capacitance is quite big, but it should be ok.
3) Add a pull-up resistor from Pgood to a system rail.
4) UVLO voltage divider is ok for nominal value, but I would try to size different values so that you can cover the range of the threshold voltage. See step 6 of the LM5155 calculation tool. The calculated value is 40.2 KOhm, but I'm not sure why you chose 20 KOhm.
1) layout looks ok in regards to the switch node closeness. The gate driver loop could be closer, but seems ok. I will need scope shots to better assess.
1) Does the output drop only at 2.5A? Or does it drop at loads higher than that? In either case, can you take a scope shot of the SW, COMP, UVLO, and Vout?
My Vcc is actually connected to 5v external source. So i did not find the need to connect bias to vcc.
I was not able to update but the uvlo resistors are actually both 40.2k.
After fixing the comp resistors and having freshly charged batteries, am able to output 8.75V/4.5A at 3.3V input. I will check in detail if my problem occurs at near 3V and will give you update.
Do you have any updates?
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