LM5145: shunt resistor placement

Suzanne Tempel

Part Number: LM5145
Other Parts Discussed in Thread: LM5148

Tool/software:

I've got a buck converter where the shunt resistor is placed after the inductor. Now I would like to use the signal from this shunt resistor to limit the current. I tried it and although the ILIM pin voltage goes through ground, there is no response on the duty cycle. Is it possible to get it to work with this shunt placement?/resized-image/__size/320x240/__key/communityserver-discussions-components-files/196/7573.RigolDS19.png dark blue is the ILIM pin, light blue is the PWM for the top fet.

over 1 year ago

0 Timothy Hegarty over 1 year ago

TI__Guru 73452 points

Hi Suzanne,

The LM5145 senses current with respect to GND, either by low-side MOSFET Rdson sensing or shunt current sensing. Take a look at the LM5148 80V peak current-mode controller instead, which uses a shunt in series with the inductor for current sensing.

Regards,

Tim

0 Suzanne Tempel over 1 year ago in reply to Timothy Hegarty

Prodigy 10 points

Hi Tim,

Thank you, I know from the data sheet that it shouldn't be in series with the inductor. Is there a specific reason that it shouldn't work? I inverted the voltage from the shunt before it goes to the ILIM resistor, which means that the pin can go below GND at high current. The main difference then seems to be that it is not equal to GND during turn off of the low-side MOSFET.

Regards,

Suzanne

0 Timothy Hegarty over 1 year ago in reply to Suzanne Tempel

TI__Guru 73452 points

Hi Suzanne,

The LM5145 is designed to sense the current in the low-side FET, not the inductor. I recommend using Rdson current sensing to implement current limit with this controller – no need for the shunt. If you prefer current-mode control, take a look at the LM5148.

Tim

0 Suzanne Tempel over 1 year ago in reply to Timothy Hegarty

Prodigy 10 points

Rdson current sensing isn't working either. Maybe the problem is that I've got a gate driver in between the LM5145 and the FET, which means that I've got the SW pin connected to ground. If ILIM it needs that signal, it would explain the problem.

0 Timothy Hegarty over 1 year ago in reply to Suzanne Tempel

TI__Guru 73452 points

Suzanne,

The LM5145 has integrated gate drivers – no need for external drivers. Send on the full schematic and a completed quickstart calculator file for review.

Regards,

Tim

0 Suzanne Tempel over 1 year ago in reply to Timothy Hegarty

Prodigy 10 points

Hi Tim, I know I'm not using it completely as intended. Max output power is 31A, but it can also be down to 5mA.

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LM5145: shunt resistor placement