TPSM63606: Switching Frequency Discrepancy

Soo Cheng Yeap

Part Number: TPSM63606

Hi,

Per TPSM63606 datasheet page 18 equation 6, the switching frequency calculation is based on VIN and VOUT. For example, when VIN = 12V and VOUT = 7.5V, the calculated switching frequency is ~700kHz.

However if I refers to page 16 Table 6-1, the suggest switching frequency is 1.2MHz to 1.6MHz which is way off from what I calculated.

I also try to generate the same setting in Webench and Webench shows that the Rt is 9.76kohm or translated to 1.31MHz. In this case, Webench result is tally to Table 6-1 but not correlated to Equation 6.

Kindly verify on this.

Thanks.

Regards,

Soo Cheng

over 1 year ago

+1 Timothy Hegarty over 1 year ago

TI__Mastermind 49732 points

Hi Soo,

The idea is to keep the inductor ripple less than 30-50% of the rated Iout of 6A given the integrated inductor of ~2uH. This limits inductor core loss to an acceptable level.

Equation 6 calculates the Fsw for a given Vin and Vout assuming a ~2A pk-pk ripple current. Thus, the Fsw for your 12Vin, 7.5Vout condition is lower given the low Vin. In contrast, the table provides a general recommendation over a range of Vin conditions (e,g, 24V nominal). Note the ripple current increases with Vin, so Fsw should be higher to accommodate that.

If you expect Vin-max in your application to be 12V, then go ahead and use 700-800kHz. This will reduce FET switching losses while still maintaining good performance for the integrated inductor.

Regards,

Tim