• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 38 subscribers
  • Views 207 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TIDA-01415: Design Inquiry

Renan Santos Adriano
Mastermind 22260 points
Part Number: TIDA-01415
Other Parts Discussed in Thread: LM3409, , UCC27519, UCC27511

Tool/software:

Hello, I had some questions regarding the TIDA-01415 design that uses the LM3409 as a LED driver. I’m attaching screenshots to help clarify the questions:

  1. What exactly are the 10k resistors R7, R6; R20, R21; R44, R45; R56, and R57 for? I see they are in series with a 470-ohm resistor (which I also don't know the purpose of), connected from an MCU pin to the EN pin of the LED driver or the gate of the parallel MOSFET. What are they used for?

  2. Regarding the 100k resistor connected between EN and VIN_LED on the LED driver, why is it there? The LM3409 datasheet states that EN should either be connected directly to VIN_LED or used for PWM dimming by applying a PWM signal.

  3. Why is the UCC27511DBVR used, which has two outputs (OUT_H and OUT_L), instead of another similar part from the same family, such as the UCC27519, which has only one OUT? In this context, how are R3 and R4 calculated? I understand these are the gate resistors and that their value depends on the MOSFET, but I don’t know how to calculate them using a MOSFET's datasheet (or at least get an approximation).

  4. The temperature differences observed in experimental tests I suppose that are due to the MOSFETs conducting more or less current; Is this related to variations in the MOSFETs Vgs(th)? Are the R3/R4 resistors included to compensate for these variations? Or has this not been considered in the design, and something additional needs to be done to address these variations?

Regards,

Renan

  • 0
    TI__Intellectual 1410 points

    Hi Renan,

    1. There are a few different functions for the 10kOhm resistors; R6, R21, R45 and R57 are pull-down resistors, they ensure that the PWM logic is low when there is no signal coming from the MCU. R7, R20, R44 and R56 can work as pull-down resistors, but they also are part of the resistor divider for the enable pin.
    2. The divider at the enable pin changes when the device starts switching. While it can be directly connected to the input, the divider means that the switching will not start until the input is well above the typical starting threshold.
    3. UCC27511 has asymmetrical drive, where the sink current is actually faster than the source. Having separate pins also allows the designer to independently adjust the speeds of the turn-on and -off edges to balance the efficiency and EMI performance. Calculating the gate resistor is not exact. it is typically recommended to have 1 - 2 Ohms to account for noise in the system. Higher resistances will slow down transition time of the FETs which will lead to reduced switch node ringing from parasitic inductance but will also increase switching losses in the FET. For most cases a gate resistor is between 1 - 20 Ohms. in practice i have found drivers struggle when the resistance is too high.
    4. The difference in temperature rise is due to the color of the LEDs being driven. The efficiency data in section 3.3.3.1 shows the change in forward drop due to the different diodes used to create the different colors.