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LM5121: How can tRES be calcurated?

Kazuya Nakai59
Guru 21605 points
Part Number: LM5121

 Hello guys,

 One of my customers is designing a schematic and BOM using LM5121 for their new products.

 For the design, they want to know tRES (Hiccup Mode Off-time). Do you have any equation to calcurate tRES?

 I know the total time of tRD and tRES can be calcurated by equation 36 in 8.2.2.14 Restart Capacitor CRES

on page 36 in the device datasheet. But the equation is not for tRES only because it contains tRD.

 Could you please give me any equation for tRES only?

 Your reply would be much appreciated.

 Best regards,

 Kazuya Nakai.

 

 

  • 0
    TI__Guru* 78400 points
    Hi Nakai-san,

    For total tRES as shown in Figure 27, it is simply the t1 + 8x t_ramp, where t_ramp stands for the triangle ramp time cycle, and t1 the time duration for CRES fist rises from the 1.2V to 2.0V thresholds:

    t1 = CRES x 0.8V /10uA

    t_ramp = CRES * 2V /10uA +CRES x 2V/5uA

    So, total tRES = CRES x 0.8V /10uA + 8 x CRES * 2V /10uA + 8x CRES x 2V/5uA

    Hope this clarifies.

    Thanks,
    Youhao