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LM5085: AN-1878 EVB, can it have an adjustable Vout

Lauren West
Prodigy 10 points
Part Number: LM5085
Other Parts Discussed in Thread: LM5145

I need an adjustable buck converter to do the following:

Vin = 48VDC

Vout = adjustable between 12V and 36V

Iout >= 4.5A

Ideally, I would like to control Vout adjustment with an Arduino PWM output, but would settle for a trim pot. 

Based on the specifications, LM5085 looks like it's a great candidate. However, I'm not an EE and I was hoping to find an eval board to get things moving faster.

The AN-1878 Evaluation Board looked like a great candidate, because it uses LM5085, but it only offers a 5V output and is not adjustable. Can the eval board be modified to meet my needs? Is there a better solution entirely? 

  • 0
    TI__Guru 74162 points
    Hi Lauren,

    Thanks for the enquiry. Yes, you can adjust the output voltage by injecting a current into the FB node. Just apply one or two RC filter stages to your variable duty cycle PWM signal and apply the resultant DC voltage through a resistor into FB. To convert a 5V EVM, you at least would need to change the output caps to 50V rated parts and also adjust the feedback resistor divider ratio for the required VOUT setpoint.

    The LM5085 is one possibility here, although it is non-synchronous so the power diode dissipation will be quite high at 4.5A (4.5A x ~0.4V if you use a 60V or 100V Schottky diode). You can also consider the LM5145 synchronus buck controller with high-side and low-side FETs. Use the quickstart file (available by download from the product folder) and WEBENCH to assist with component selection.

    Regards,
    Tim