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TPS923652: Formula confusion & output capacitor

Elan
Prodigy 80 points
Part Number: TPS923652
Other Parts Discussed in Thread: TPS92641, TPS923655, , TPS922053

I'm trying to replicate the data sheet calculations in section 9.2.2.2.1, but I'm unable to. The data sheet says: 

VIN(max) = 24 V, VOUT = 12 V, ILED = 2 A, fSW = 1.2 MHz, choose KIND = 0.5, the calculated inductance is 4.4 μH. A 4.7-μH inductor is chosen.

When I plug this into the formula, I get (24 * 12) / ((24 + 12) * 0.5 * 2 * 1200000) = 6.67 uH. What am I doing wrong?

Second question is around the output capacitor. I'm coming from a Shunt FET design where minimizing or eliminating the output capacitance helped achieve higher dimming ratio and dimming-to-dark. With this new chip, is this not the case? Or if I want to achieve the 10000:1 dimming ratio should I make the inductor larger and reduce/eliminate the output capacitor?

Thanks!

  • 0
    TI__Mastermind 18675 points

    Hi Elan,

    I am currently out of office. I will reply to you tomorrow when I am back to office. Thanks for your understanding.

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 80 points

    Thank you!

  • 0 in reply to Elan
    TI__Mastermind 18675 points

    Hi Elan,

    Sorry for the delay.

    Elan said:
    When I plug this into the formula, I get (24 * 12) / ((24 + 12) * 0.5 * 2 * 1200000) = 6.67 uH. What am I doing wrong?

    Please be noticed that the maximum average inductor current I_L(max) = V_OUT * I_LED / efficiency / V_IN + I_LED.

    If we assume that the device can achieve 90% efficiency, then I_L(max) = 12V * 2A / 90% / 24V + 2A = 3.1A

    Use this I_L(max) in the equation (12) and you will get L = (26.4V * 12V) / ((26.4V + 12V) * 0.5 * 2A * 1.2MHz) = 4.4uH.

    Elan said:
    Second question is around the output capacitor. I'm coming from a Shunt FET design where minimizing or eliminating the output capacitance helped achieve higher dimming ratio and dimming-to-dark. With this new chip, is this not the case? Or if I want to achieve the 10000:1 dimming ratio should I make the inductor larger and reduce/eliminate the output capacitor?

    With a normal Buck LED driver combined with shunt FET like TPS92641, reducing the output capacitance can help as this will lead to shorter current pulse that the driver can output. The shorter the pulse is, the higher dimming ratio the device can achieve with PWM dimming.

    For TPS923655, this is a boost / buck-boost LED driver, you need a certain amount of output capacitance so that when the switching FET turns on, there is enough capacitance that source current into the load.

    I can provide you with some advice to achieve 10000:1 dimming ratio. Please provide the spec of your input voltage, LED voltage and LED current. Also, which dimming mode do you prefer?

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 80 points

    Super helpful, Steven, thank you!

    I'm coming from TPS92641 + Shunt FET, so this is great context. Really hoping this part provides similar dimming ratio/quality but with far fewer parts.

    My specs are:

    - V_IN = 48-52
    - V_OUT = 18v or 36v (stack of 1 or 2 LEDs, hoping to make it work for both, 1 LED is the more common case)
    - I_LED = 500mA

    In terms of dimming mode, I was going to try PWM and Hybrid and see which one worked best, but I'm curious to hear what you recommend to maximize the dimming ratio and improve dim-to-dark scenarios.

    Thanks so much for your help!

  • 0 in reply to Elan
    TI__Mastermind 18675 points

    Hi Elan,

    You are welcome.

    For your input & output condition, you should use TPS9222052 / TPS922053, which is Buck LED driver, but not TPS923652.

    You can achieve 10000:1 with PWM dimming mode or flexible dimming mode. I would recommend you use flexible dimming mode to achieve the maximum dimming ratio.

    The maximum dimming ratio for analog dimming mode and hybrid dimming mode is 256:1.

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 80 points

    Ah, thank you, for some reason I hadn't noticed there was a buck-only version of this new series. That's very helpful.

  • 0 in reply to Elan
    TI__Mastermind 18675 points

    Hi Elan,

    Another note for you:

    Although TPS923652 can be used as a buck-boost LED driver, when being used in your application condition, the output voltage (the voltage faced by the CSN and CSP pin, and also sometimes the SW pin) will be 52V+18V = 70V, which is outside the range of Recommended Operating Conditions (Please refer to Section 7.3 from the datasheet) of this IC. This is the reason why you cannot use TPS923652 in your application condition.

    Best Regards, 

    Steven

  • 0 in reply to Steven Li
    Prodigy 80 points

    Ah, that's good to know, I hadn't realized that nuance, thank you!

  • 0 in reply to Elan
    TI__Mastermind 18675 points

    Hi Elan,

    You are welcome. Do you have any other question or support need? If not, I am going to close this thread.

    Best Regards,

    Steven