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TPS61193: How can CFB be calculated for switching frequency between 500kHz and 1.8MHz?

Kazuya Nakai59
Guru 21545 points
Part Number: TPS61193

 Hello guys,

 One of our customer is designing a evaluation circuit for TPS61193.

 They'd like to know how to calculate CFB placed between LED anode (output voltage) and FB terminal as typical application, Figure 20 on page 20 in TPS61193 datasheet (SNVSAF4B).

 The datasheet says "In boost mode a 15-pF capacitor CFB must be placed across resistor R1 when operating in 300-kHz to 500-kHz range (see Typical Application for 3 LED Strings).

When operating in the 1.8-MHz to 2.2-MHz range CFB = 4.7 pF." on page 12. But it doesn't say about CFB in 500kHz~1.8MHz switching frequency case.

 If customer uses 500kHz~1.8MHz switching frequency, how can CFB be calculated?

 Could you please tell me how to calculate CFB in the switching frequency range?

 Your reply would be much appreciated.

 Best regards,

 Kazuya Nakai.

 

 

  • 0
    TI__Expert 4565 points

    Hi Nakai-san,

    There is not any calculation tool for Cfb. 15pF and 4.7pF came from design simulation results for both extreme cases, and not optimal value for all cases for each SW freq. These 2 values are just typical values work for various apps conditions.

    Any middle value can be tried for 500~1.8MHz closed to either end value.

    For example, Cfb = 10pF can be tried for 1MHz.

    Cfb value will affect DC/DC compensation loop, but the impact can't be estimated easily. The best way is to measure phase margin of compensation loop.

    Thanks,