LM3407 PWM Dimming Control

Sourav Sarkar

Other Parts Discussed in Thread: LM3407, CC2538

Hi All,

I am planning to use LM3407 LED Driver. I have gone through the data sheet. I have one doubt. As per data sheet PLM (Pulse Level Modulation) is used to generate on/off signal to Power MOSFET for current regulation independent of inductor and input voltage. My question is how PWM dimming signal control the avg output current? If LM3407 is already controlling the on/off signal to Power MOSFET how external PWM Dimming signal control the duty cycle. Is it in addition to PLM control?

Thank you

Sourav Sarkar

over 11 years ago

0 Victor Chen_HK over 11 years ago

TI__Expert 5130 points

Hi Sourav,

Thank you for your question.

Normally, the DIM pin will be tied to Vcc so that enables the switch control block. The switch control block will be disabled as long as the DIM pin is pulled low, thus no current outputs to the LED.

While during the DIM pin is being pulled high, the PLM method will provide precise constant current control.

That's how the PMW dimming works.

You may refer to the figures in data sheet pg.6 and 7 for better understanding.

Thanks.

Victor

0 Sourav Sarkar over 11 years ago in reply to Victor Chen_HK

Prodigy 50 points

Hi Victor,

Thank you for the reply. I want to use the the PWM Dimming signal to dynamically control the intensity of LED light to save power. I want to know how this external dimming control signal works along with internal PLM mechanism to control the average output LED current. So that I can decide what frequency and duty cycle should be used for dimming signal. Also what is the source/sink current of dimming signal? I can not find this information in data sheet. I want to drive multiple(max 5) LM3407 in parallel. What is your opinion?

Thanks

Sourav

0 Victor Chen_HK over 11 years ago in reply to Sourav Sarkar

TI__Expert 5130 points

Hi Sourav,

I have attached two files for you. One is a reference design of LM3407 which has a detail explanation about how to set the frequency and duty cycle of the PWM dimming signal(pg.6).

The other is a design calculator. You can set your operation conditions and it will generate suitable components values automatically.

Hope this can help.

Thanks.

2046.LM3407_Component_Calculator.xls

1638.snva308a[1].pdf

0 Sourav Sarkar over 11 years ago in reply to Victor Chen_HK

Prodigy 50 points

Hi Victor,

Thank you for the document. It was really helpful. One last question. What is source/sink current of Dimming signal? Is it LVCMOS compatible?

0 Victor Chen_HK over 11 years ago in reply to Sourav Sarkar

TI__Expert 5130 points

Hi Sourav,

Looking at the block diagram on pg.8 in datasheet, there is a 400k pull-down resistor. A few micro-amp of input current is enough for the dimming signal. And of course, you can drive it easily with LVCMOS or other microcontrollers output.

Thanks.

Victor

0 Sourav Sarkar over 11 years ago in reply to Victor Chen_HK

Prodigy 50 points

Hi Victor,

Thank you very much for reply. I am going to drive 5 such LED driver by a buffer with 10mA source current @ 500Hz. What is your opinion? What is the input capacitance of the buffer shown on page 8? From that value and slew rate of dimming signal we can decide whether it is possible or not. Incidentally I am planning to use PWM output of TI's CC2538.

0 Victor Chen_HK over 11 years ago in reply to Sourav Sarkar

TI__Expert 5130 points

Hello Sourav,

The input capacitance of the buffer is less than 3pF. It should be OK to drive 5 LM3407 with 10mA source current.

Regards,

Victor

0 Sourav Sarkar over 11 years ago in reply to Victor Chen_HK

Prodigy 50 points

Thank you very Victor. It was real help for me.

Sourav

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LM3407 PWM Dimming Control