LM2623: How to design external components

Hi Kuramochi,

Thanks for reaching out.

Does the input voltage come from a Supercapacitor and it will start working at 2.7V? The reason I ask this question is lots of our devices have a minimum startup input voltage requirement.

Hi Liu-san,

Thank you for your reply.

The input voltage source is not a supercapacitor but battery.

Best Regards,

Kuramochi

Hi Kuramochi,

Thanks for the information.

You can copy the below schematic for your application. All you need to do is changing the feedback resistors RF1 and RF2 values to set the output voltage at 6.5V.

Hi Liu-san,

Thank you for your advice.

Could you tell me how you calculated inductance?

My customer need to verify by themselves.

Best Regards,

Kuramochi

Hi Kuramochi,

Understood.

Unfortunately LM2623 switching frequency is not easily to calculate. That's why in datasheet 8.2.2.2 Choosing the correct C3 capacitor explains the reason. As the switching frequency has a relationship with inductor ripple and inductance, there is no clear calculation equation for L.

Another solution is using TPS61021A with a discrete charge pump circuit to output 6.5V/10mA. TPS61021A is much newer compared to LM2623 and has detailed design steps in datasheet.

Liu-san,

Thank you for your advice.

I understand.

I have an additional question.

My customer is requesting fsw=300kHz and Cout = 20000uF.

Can it be realized by LM2623?

Best Regards,

Kuramochi

Hi Liu-san,

I have additional questions.

You selected 4.7uH before.

How did you judge that 4.7uH is no problem on this condition?

Best regards,

Kuramochi

Hi Liu-san,

Could you check also the above question?

Best Regards,

Kuramochi

Hi Kuramochi,

The inductance is selected based on the required inductor ripple that you need.

If customer wants a fixed frequency, I would suggest them use the TPS61021A + discrete charge pump solution.

Liu-san,

Thank you for your advice.

>The inductance is selected based on the required inductor ripple that you need.

Can inductor ripple be calculated by VIN*ton/L?