LM5161: Minimum switching frequency or maximum Ron value

K. Sasaki14

Part Number: LM5161

I would like to know recommended minimum switching frequency or maximum Ron value or maximum ON time.

I think Ton period becomes longer as switching frequency is lower or Ron is larger.
There is concern that boot voltage become lower than UV threshold if Ton period is long. So I would like to know the above information.

Best Regards,
Kohei Sasaki

over 4 years ago

0 Iven Su over 4 years ago

TI__Expert 8245 points

Hi,

Yes, you are right, but we actually don't have a very accurate minimum switching frequency to recommend. It will not only affect boot voltage drop but many designs of the circuit. Maybe you can use the model to have some simulations but I think 300kHz is fine, since you really don't need a lower frequency to make your components very big size. Thanks.

0 K. Sasaki14 over 4 years ago in reply to Iven Su

Guru 10200 points

Su-san,

Thank you for your reply.

Let me ask additional question about high duty operation.

I think Fsw becomes lower when Vin drops and min Toff time is not achieved with both DCM and FPWM mode.
In this case, High side FET cannot turn-on until Vout level drops if Ton time is too long and boot voltage drops.
Is there possibility that this phenomenon occurs with both DCM and FPWM mode?

Best Regards,
Kohei Sasaki

0 Iven Su over 4 years ago in reply to K. Sasaki14

TI__Expert 8245 points

Hi,

Actually when Vin drops on-time will become larger (see Formula - 3 on the datasheet), then the Fsw will keep nearly constant. Thanks.

0 K. Sasaki14 over 4 years ago in reply to Iven Su

Guru 10200 points

Iven-san,

In FPWM mode, does Fsw value become lower if Vin voltage drops below Vout setting value?

Best Regards,
Kohei Sasaki

0 Iven Su over 4 years ago in reply to K. Sasaki14

TI__Expert 8245 points

Hi,

In FPWM mode, the device is still on COT control so normally Fsw value won't become lower when dropout. Thanks.

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LM5161: Minimum switching frequency or maximum Ron value