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TPS923652: PWM dimming performance

Hector Chen
Intellectual 410 points
Part Number: TPS923652
Other Parts Discussed in Thread: TPS92641

Tool/software:

Hi Sirs,

I want to confirm the speed at which the LED driver can turn on and off the IRLED current.

Do boost and buck-boost modes have the same performance?

I expect that when PWM pulse = low， the LED current should be zero (like Figure 8-14 and 8-16 in the datasheet).

However, the measured results in EVM look like PWM dimming cannot achieve fast dimming. The LED current is switched off and on slowly (please see Figure 5-7, Figure 5-8 and Figure 5-9 in the EVM User's Guide) 

I also found that the LED ripple current is a little bit high (please see Figure 8-2 in the datasheet). I am afraid that high ripple current will cause image quality issues. 

Can the ripple current be reduced to below 20mA ?

Thanks.

  • 0
    TI__Mastermind 30291 points

    Hi Hector,

    Thanks for reaching out! In datasheet Figure 8-14 and figure 8-16, it's PWM dimming frequency is 1kHz while in User's guide figure 5-7, figure 5-8 and figure 5-9, the PWM dimming frequency is 20kHz. Thus, the response time looks more obvious with 20kHz.

    The fast dimming of PMW dimming means that the inductor current can rise quickly to the target value so that it enables high-freq PWM dim. directly by DC-DC SW converter with highly accurate brightness control and wide dim ratio. Below figure shows the comparison between with inductive fast dimming and without inductive fast dimming.

    In order to get less response time, you can lower the Cout and decrease the inductance.

    In order to get less ripple, it would recommend to increase the Cout or decrease the switching frequency according to below equation. 

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Intellectual 410 points

    Hi Lucia,

    Thanks for your explanation.

    In Figure 5-9 of EVM, the LED current will not reach zero in a low pulse. 

    Do you have a response time during which the LED current can reach its full current and drop to zero?

    Thanks 

    Hector

  • 0 in reply to Hector Chen
    TI__Mastermind 30291 points

    Hi Hector,

    Thank you for your feedback. I suppose you'll need a device with series FET or shunt FET dimming. You can take a look at TPS92641.

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Intellectual 410 points

    Hi Lucia,

    Do you have suggested boost converters?

    Thanks.

    Hector

  • 0 in reply to Hector Chen
    TI__Mastermind 30291 points

    Hi Hector,

    TPS92641 is a good option here if controller is acceptable. BTW, may I know your end equipment and could we discuss by email (lucia-gao@ti.com) we want to learn more about your requirement. Thank you!

    BRs

    Lucia