LM61440-Q1: WEBENCH Issue

The point at which the mode goes from CCM to PFM seems to depend on Vout.   

The problem occurs with the following parameters:

Vin = 24V,   Vout = 20V

Iout = 700mA,   L Ipp = 360mA

L = 4.7uH, 32mohms

C = 12uF, 2.3mohms

With 15V output, PFM mode comes in at Iout = 590mA and L Ipp = 620mA

Ray

Hi Ray,

For more in-depth information on PFM mode operation, please see this app note. 

That is correct that the PFM to CCM transition varies depending on Vout condition.

The lower the fixed output voltage Vout is the more inductor ripple current there will be for the same test conditions and component value. This means that at lower Vout, with some amount of loading the inductor current will hit the peak current command of the device faster. 

Can you provide the exported design so I can have a look? 

Regards,

Jimmy 

Here it is: https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=D01705B08BDE8B35

I am using the AFS version of the chip, but there does not seem to be an AFS version in WEBENCH.  I just wanted to get an idea of the efficiency at load currents in the neighborhood of 300-400mA with 15 and 20V outputs

Thanks,

Ray

Hi Ray,

It looks like only the "AAS" version is being used on WEBENCH designer for the LM61440-Q1 device.

Let me check if I have the EVM and test tomorrow in the lab for you.

This will be bench data testing at room temperature for 24Vin| 15V & 20Vout| 2A. 

As for enabling the "AFS" version, I will need to contact the support team to see how to enable this option for future designs.

Expect to hear back from me tomorrow EOD if I am able to find an EVM. Otherwise I will order it (3-5 business days) and can take data early next week.

Regards,

Jimmy