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# TPS54240-Q1: Design Problem/Question with the Operating Frequency of the IC

Part Number: TPS54240-Q1
Other Parts Discussed in Thread: TPS54240,

Hello,

I am designing a power supply using TPS54240-Q1 with 28 V nominal input voltage and 10 V output voltage. I used WEBENCH to design my circuit at first, however, solution size is larger than what I would like to have. Also WEBENCH does not let me choose the frequency of the device according to the datasheet. The IC can have a switching frequency as high as 2.5 MHz, and I would like to have my device operate around 1.5 MHz range for this operation. I have done calculations with respect to the selected switching frequency, but I am not sure since WEBENCH does not allow me to design the TPS54240 in such frequency. Below, I specify the requirements of the supply and would really appreciate if you can help me with the questions I have.

• Input voltage = 18 - 32 V (absolute max.-min.), 28 V nominal
• Output Voltage = 10 V
• Output current = 210 mA nominal, 500 mA maximum
• Output Voltage Ripple = 1% of Vout

For 1.5 MHz operating frequency, output inductor value is calculated as 33 uH. Output capacitor value is calculated as 1.2 uF and it passes all the equations given in the datasheet. My questions are;

• Can I trust the calculations made with respect to the datasheet and how realistic is my solution?
• For input capacitance, which bypass capacitors should I be using to filter the input properly with respect to the given operating conditions?
• Is there a way I can design the circuit on WEBENCH with the desired 1.5 MHz frequency?

Thank you.

• Hello,

I am not sure to the reason why the frequency is limited to 400kHz. I will need to have this corrected.

A single 10uF should be sufficient, with a low ESL 220nF placed closely to the the device.

For small-size applications, I would recommend synchronous or better yet, sychronous module solution.

Please find below suggestions and let me know what you think.

https://www.ti.com/product/TPSM5601R5H

www.ti.com/.../LM61440

www.ti.com/.../LM62440-Q1

• Thank you for your answer and I really appreciate the device suggestions. Noted these devices for future purchase and use in applications. I had no idea such devices of TI existed with integrated inductors, it will be really helpful for the designs I am working on.

I have one more question if you don't mind answering. Do you think that the 1.2 uF output capacitance that I calculated from the datasheet is a viable option for this IC? I am having second thoughts about my calculations since output capacitance values used in this IC is usually around 47 uF range or more, however switching frequency decreases the size of output capacitance (as expected). Your opinion on this topic will be really appreciated.

• 1.2uF effective output cap may enable  a marginally stable design...the transient response will be not-so-good. The calculation is probably for the theoretical bare minimum cap.

47uF would be good starting point for low ripple and good transient response/stability

• Thanks for the information, will change my design accordingly.