LM22670: Border during internal frequency(500kHz) and RT frquency

Kuramochi Tadahiko

Part Number: LM22670

Hi,

I received the following question from my customer.

If RT resistance is set larger and larger(to few MΩ), what ohms is that switching frequency changes from RT frequency to internal frequency(500kHz) as below?

They are recognizing that the mode is set during start up of the regulator.

Please tell me for their reference.

Sorry for my odd question.

Best Regards,

Kuramochi

over 6 years ago

0 Katelyn Wiggenhorn over 6 years ago

TI__Genius 11040 points

Hello,

The internal oscillator can be adjusted from 200kHz to 1MHz with the RT resistor. The figure in the datasheet shows how to modify the RT resistor for the possible sync frequency range. The device was designed for the specified RT range.

If the RT resistor is too high, it will not pull down the RT pin and activate the sync mode. Consequently, the device will operate at the internal switching frequency of 500kHz.

Please let me know if anything remains unclear or if you have any additional questions.

Best Regards,
Katelyn Wiggenhorn

0 Kuramochi Tadahiko over 6 years ago in reply to Katelyn Wiggenhorn

Guru 28715 points

Katelyn-san,

Thank you for your reply.
Do you know the boundary during RT frequency and internal frequency(500kHz)?
Best Regards,
Kuramochi

0 Katelyn Wiggenhorn over 6 years ago in reply to Kuramochi Tadahiko

TI__Genius 11040 points

Hi Kuramochi-san,

I am reaching out to the designer for this part, and I will provide feedback on your question.

Best Regards,
Katelyn

0 Katelyn Wiggenhorn over 6 years ago in reply to Katelyn Wiggenhorn

TI__Genius 11040 points

Hi Kuramochi-san,

I have received feedback on this question.

The RT pin will have a voltage between 0.65V - 0.95V after start up. The RT pin must be pulled down with more than 1.7uA. Consequently, a resistor larger than 500kOhm should not be used for margin. Above a resistance of 540kOhm, the current will not be less than 1.7uA.

Best Regards,
Katelyn

Power management

Power management forum

LM22670: Border during internal frequency(500kHz) and RT frquency