LM3549: Timing Questions

Ryan.Bishop

Part Number: LM3549

Hello!

I am working with a customer who will be using this for a projection application like the chip is designed for. A few questions we have that aren't addressed in the datasheet:

What is the minimum on time for any particular channel?
When a channel is enabled, what is the typical setting time for the current to reach it's programmed value?
Related to #2, is the setting time related to the previous channels current? (Meaning does it take longer when switching from a dim channel to a bright channel vs. when switching from a bright channel to a bright channel?)

Thank you! Let me know if you need any other info.

Regards,
Ryan B.

over 6 years ago

0 Kenneth Du over 6 years ago

TI__Genius 14295 points

Hi, Ryan,

1. Please refer to the PWM inout parameter for reference, and it has typical value of 1us.

2. Please refer to the rising time in the following figure for the rise time:

When it changes from one channel to another, the current sink are independent but the Vout may need to adjust for different Vf. I suggest customer to test themselves.

0 Ryan.Bishop over 6 years ago in reply to Kenneth Du

TI__Expert 3900 points

Hi Kenneth,

Thank you for the quick answer to my questions. Sorry the formatting in my post got messed up, I have fixed it in case anything was unclear.

For #1, I believe that 1us minimum on time is for the PWM pin. I'm curious about the minimum on time for the R_EN, G_EN, and B_EN pins. Is there a spec for this?

Thank you,
Ryan B.

0 Charles-Chen over 6 years ago in reply to Ryan.Bishop

TI__Genius 11836 points

Hi, Ryan,

What kind of application did cusotmer plan to use this part? And how about the accuracy requirement?

0 Ryan.Bishop over 6 years ago in reply to Charles-Chen

TI__Expert 3900 points

Hi Charles,

This is a mini-projection application.

They would like to have the R,G,B channels enabled for as short as 100us. I realize the datasheet specifies the current rise time as 50us, but ignoring that, will the logic on the EN pins have any issue with 100us on-time?

They will be adjusting the brightness using i2c.

Thank you,
Ryan B.

0 Charles-Chen over 6 years ago in reply to Ryan.Bishop

TI__Genius 11836 points

Hi, Ryan,

What's your PWM frequency? Since the minimum on time of PWM is 1 us, it can meet 100 us on time PWM accuracy. Do you want to shut down the PWM in one cycle by using x_EN? The on time of x_EN will be long if your PWM of one channel lasts many cycles. However, one point should be noted that in order to ensure that control takes effect for the next color time from PWM change to next enable needs to be greater than timeout time(300 us typical). Please refer to datasheet page 11 PWM TIMMING for details.

0 Ryan.Bishop over 6 years ago in reply to Charles-Chen

TI__Expert 3900 points

Hi Charles,

In this application, we won't be adjusting the PWM duty cycle and will only be dimming via the I2C master fader register.

So for example, if the brightness is set to 100% via the register and we are not changing it, could we cycle through the colors in intervals of 100us? Could it be even lower than that?