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Original question:

TPS922052: Question about high speed ILED transient waveform.

TPS922052: How to calculate the delay time from PWM signal input to LED current turning on/off.

Kazuya Nakai59
Guru 21595 points
Part Number: TPS922052
Other Parts Discussed in Thread: TPS92641, TPS922054

Tool/software:

Hello guys,

One of my customers is considering using TPS922052 to their new products.

At this moment, they have the following questions.
Could you please give me your reply?
Q1.
It is important for them that the delay time from PWM signal input to LED turning on/off completely.
The complete LED turning on means that LED current reaches target current. And complete LED turning off means that LED current becomes zero.
Could you tell me how to calculate the delay time under Vin=24V, LED total Vf=20V and If=2A conditions?
I think the delay time would be changed by inductor value.
Could you please give me how to calculate the inductor value under same conditions?
Q2.
What is TPS922052 maximum average drive current? Is it more than 2A?
If PWM signal on duty is limited less than 70%, 50% or 30%, can the maximum average drive current be increased?

Q3.
Can shunt FET LED on/off method like TPS92641 be used for TPS922052?

Q4.
Do you have any LED driver which can turns LED on/off with us level delay time under conditions above except TPS922052 family and TPS92641?

Your reply would be much appreciated.

Best regards,
Kazuya.

  • 0
    TI__Mastermind 23190 points

    Hi Kazuya,

    Please refer to my answers below.

    A1.

    The inductor current is filtered by the output capacitors to form the LED current. Therefore, the actual LED current rise / fall time will be affected a lot by the output capacitors which vary from application to application. Therefore, I will only talk about inductor current rise / fall time here to avoid the influence of output capacitors.

    • For the inductor current rise time (let's suppose we are not at a first-time startup where soft starts will happen), it can be estimated by T_rise(max) = I_L_max * L / (V_IN - V_LED), where I_L_max is the max inductor current.
    • For the inductor current fall time, it can be estimated by T_fall(max) = I_L_max * L / V_LED. 

    A2.

    Kazuya Nakai59 said:
    What is TPS922052 maximum average drive current? Is it more than 2A?

    TPS922052 can output more than 2A current if you keep the max inductor current (also the max switching FET current) within the switching FET cycle-by-cycle current limit I_LIM (Please refer to Section 7.5 Electrical Characteristics from the datasheet) and keep the device away from thermal shutdown.

    Kazuya Nakai59 said:
    If PWM signal on duty is limited less than 70%, 50% or 30%, can the maximum average drive current be increased?

    If the thermal performance (but not the witching FET cycle-by-cycle current limit) limits the output current capability of the device, having smaller EN/PWM duty cycle can decrease the thermal stress on the IC and you can then output higher instantaneous output current. However, the average drive current may not be increased as you lower the EN/PWM duty cycle (and therefore the output current duty cycle).

    A3.

    Yes. TPS922052 can work with shunt FET.

    A4.

    May I understand what is your customer's target output current rise / fall time? Do they have any requirements on the output current ripple?

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Guru 21595 points

    Hi Steven,

    Thank you very much for your reply.

    About A4.
    It is important for the customer that short delay time from PWM signal input to LED current reaching to the target value.
    Therefore they need fast LED current rising/falling time.

    Could I ask you an additional question?

    About A1.
    Is there any formula for calculating rising/falling time in case that the output capacitor is attached to output?

    Thank you very much again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 23190 points

    Hi Kazuya,

    Kazuya Nakai59 said:
    It is important for the customer that short delay time from PWM signal input to LED current reaching to the target value.
    Therefore they need fast LED current rising/falling time.

    Is there any specific (quantitative value) requirement for the LED current rise / fall time?

    Kazuya Nakai59 said:
    Is there any formula for calculating rising/falling time in case that the output capacitor is attached to output?

    In this case, the LED current rise / fall time can be estimated by solving some differential equations. It would be easier to estimate the LED current rise / fall time by using the simulation software. (The SIMPLIS model for TPS92205x is available upon request. Please send me an e-mail if the customer needs this model.)

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Guru 21595 points

    Hi Steven,

    Thank you very much for your reply and I'm sorry to be late my response.

    Do you have any model for "PSPICE for TI"? Is TPS922052 simulation model SIMPIS model only?

    Also could I ask you about rising current waveform in TPS922052 datasheet?

    Q1.
    Figure 9-17 in TPS922052~55 datasheet shows LED rising current waveform after the current out starting by PWM pin rising from L to H.
    This waveform is taken using TPS922054.
    What is the capacitor value of Cout in Figure 9-14?

    Q2.
    Do you have any waveform like Figure 9-14 for TPS922052 case?

    Thank you again and best  regards,
    Kazuya.
     

  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 23190 points

    Hi Kazuya,

    No, we do not have Pspice model for TPS922052. Only SIMPLIS model is available.

    A1. Regarding the Cout, you can find the value in Section 9.2.2.2.3 Output Capacitor Selection from the datasheet.

    A2. I think you are referring to Figure 9-17. Sorry. Currently I do not have such waveform for TPS922052. But it should be similar. 

    Best Regards,

    Steven