CSD16301Q2: How to define a higher bias voltage across the LED

Hello Mohtaba,

Thanks for your interest in TI FETs. I will review the application and TINA-TI simulation and get back to you early next week when I am back in the office.

Best Regards,

John Wallace

TI FET Applications

Hello Mohtaba,

Thanks again for your interest in TI FETs. Your application is operating the FET in the saturation region, where Vds > Vgs - Vth and Ids ~ (Vgs - Vth)². I ran your simulation for DC bias conditions. With the existing bias network and supplies, the Vak = 2.19V, Vgs = 1.59V, Vds = 1.89V and Ids = 12.13mA. Increasing Vak = 2.5V would require changes to the bias network. Increasing the value of R2 = 250Ω results in Vak = 2.49V, Vgs = 1.71V, Vds = 1.54V and Ids = 67.98mA. I do not know how to change the biasing without increasing the power dissipation. I believe the operating point is dictated by the V-I characteristics of the LED. I pulled up the datasheet for the LZ1-00R702 used in the simulation and as Vak is increased, the forward current also increases. Please note, we do not characterize our devices at the conditions in your application. Specifically, we do not know how it will perform with a 30MHz signal riding on top of the DC bias. In general, the CSD16301Q2 is optimized for switched mode power applications up to about 1MHz switching frequency. Please let me know if I can be of any further assistance.

Best Regards,

John Wallace

TI FET Applications

Hi Mojtaba,

Following up to see if this has resolved your issue. Please let me know if I an be of any further assistance.

Best Regards,

John

Hi John,

Thank you for your reply.

Yes, you are right. the operation point is defined by I-V characteristics of the LED.

I want to set the DC voltage across the LED (Vanode - Vcathode) on 2.5 or 2.6V which means the current should be set to about 250mA. Then the input signal comes over this bias voltage like a modulation. Also sometimes I need to increase the Voltage/current across the LED to increase the emission power.

What do you think? is it possible to set the value for this purpose or I have to change the part number?

Thank you

Hi Mojtaba,

You should be able to bias the circuit but there will be normal process variation of the FET parameters including the threshold voltage which results in variation of the bias conditions. If you want to have more precise control then you may want to consider a closed-loop system to regulate Vak of the LED. All of TI's NFET devices will perform similarly in an open loop configuration.

Best Regards,

John