LM5166: Current limit Specification in PFM mode

Yoshwa

Part Number: LM5166
Other Parts Discussed in Thread: COT

Our customer has designed a PFM mode application of 50V input and 12V 0.45A output.

HIS, RT and ILIM pins are pulled down to GND directly.

He measured coil current to be going up to about 2A at the peak point of the ripple current on about a 0.3A load condition.

The ripple current are repeated with two and three burst pulses

The current limit value is specified up to 1375mA (MAX).

What does this mean?

over 8 years ago

0 Samuel Jaffe over 8 years ago

TI__Mastermind 25590 points

Yoshwa,

Can you share waveforms, schematic, and layout? It sounds like you're getting current overshoot or maybe your inductor is not sized properly. It's hard to say for sure without more information.

-Sam

0 Timothy Hegarty over 8 years ago

TI__Guru* 75462 points

Yoshwa,

What is the inductance value? Note that there is a current response time of 80ns once the current exceeds the nominal ILIM value. This is specified in section 7.3.5 of the datasheet. The current overshoot will be more evident at high VIN. Please use the LM5166 quickstart tool to assist with component selection for your design - this is available for download in the LM5166 product folder. Also, another option is to use the LM5166 in COT mode.

Regards,
Tim

0 Yoshwa over 8 years ago in reply to Timothy Hegarty

TI__Expert 4050 points

Tim-sann

Thanks for your comment.

Customer used WEBENCH tool and designed his circuit for VIN=35V to 60V, VOUT=12V IOUT=0.47A frequency=600kHz.

Inductor and capacitor values are as follow,

L=15uH Isat=1.9A RDC=89mΩ

Cin=4.4uF Cout=44uF

An inductor current phot is as attached, where VIN=50V, IOUT=about 300mA.

Green is SW voltage and Red is inductor current.

LM5166 VIN=50V Iout=0.3A.pdf

0 Timothy Hegarty over 8 years ago in reply to Yoshwa

TI__Guru* 75462 points

Yoshwa-san,

The inductance is too low. If you fill out the design in the LM5166 Excel quickstart calculator, you will see the cell for inductance is indicated in red.

For 60V in to 12Vout, 27uH is the lowest inductance allowed to maintain peak inductor current less than 1.6A. See equation 29 in the datasheet for clarification.

Regards,
Tim

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LM5166: Current limit Specification in PFM mode