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tlc5940 with higher current LEDs

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Replies: 21

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Can the tlc5940 be used with much higher current LEDs of about 700mA say with an external FET? If not does TI have another 5940 type device that has a current sink capability of 700mA?

  • This app note is applicable to the TLC5940 also: http://focus.ti.com/lit/an/slva253/slva253.pdf

    We also have high current LED drivers in a boost topology, such as the TPS61500.

    Chris Glaser

    Texas Instruments

    ________________________________

    TPSM82810 - new, 6Vin, 4A, high efficiency power module with clock synchronization Now sampling!

  • In reply to Chris Glaser:

    Thank you Chris. I guess I will try the parelleling technique if nothing else is available. Unfortunately, the tps61500 type devices will not work because I am driving several LEDs and I need multiple channel outputs.

  • In reply to Chris Glaser:

    Ok, I am using the application note suggested above to use the 5940 with 700mA LEDs by ganging up 6 outputs per LED. I am using a 330ohm resistor to set Imax to about 120mA. The power dissipation of the PDIP I am using is rated about 2.45w at 25deg. Celcius. I am not sure on what values to use for Icc and Vout in the PD equation in page 15 of the data sheet. I basically want to make sure that the device will be safe in all operating conditions between 0 ~ 100% duty cycle PWM control. On tests I have done so far, the  5940 devices(daisy chained) get between warm and hot during use. Should I be concerned about damage?

  • In reply to oz46916:

    Icc is the IC's supply current as specified on page 4 of the datasheet.  There are various specs for various operating conditions, so use the spec that matches what you are using in your system.

    Vout is the voltage at the OUT pin (LED cathode).  This voltage multiplied by the current being sunk (700mA) is how much power is lost across the internal drive transistors of the TLC5940.  You can measure this voltage on your board or calculate it based on the forward voltage of your LED at 700mA.  To reduce the power lost in the TLC5940 (and reduce its temperature), you can reduce the voltage on this pin by using a lower voltage for the LED anode.  We have many buck converters or LDOs that will do this for you.  See figure 5 for the minimum Vout headroom required.  For 120mA, it is about 1.2V.

    Yes, as long as the maximum junction temperature rating is not exceeded (maximum power dissipation spec at 25 C), the IC will operate fine.  But it will be hot!  Note that the maximum power dissipation rating assumes a JEDEC board.  Junction temperature is a function of the actual theta JA on your board, power dissipation, and ambient temperature.

    Chris Glaser

    Texas Instruments

    ________________________________

    TPSM82810 - new, 6Vin, 4A, high efficiency power module with clock synchronization Now sampling!

  • In reply to Chris Glaser:

    I have the same question as the original post in this thread. In my application, I need PWM control of 64 different LED strips, which require 12V and draw ~400mA. If at all possible, I would prefer to use only 4 TLC5940, with 1 output channel per LED strip, rather than having to use 6 outputs in parallel as noted in slva 253 - LED Driver--Paralleled Outputs Provide High Current Outputs. Is there some way to use an FET as a current switch?

  • In reply to Michael Grabscheid:

    Michael,

    You might have a look at the TLC5960 (8-channel) driver.  It is designed for use with external FETs.  It was intended for backlighting applications, but can be used in other high current / multi-channel apps as well.  Unlike most of the other TLC series LED drivers, this one does not have a SPI or I2C port.  String dimming is done using the PWM input pins.

    Hope this helps.

    John

  • In reply to John Perry:

    Thank you John for the quick response.

    First off, let me say that I completed an undergrad Electrical Engineering (EE) degree over 30 years ago, then went into IT, so my EE skills are quite rusty.

    I am trying to work with someone else's existing design, where they are driving 64 individual RGB common cathode LED's, using an Arduino board (ATMEGA microprocessor) to multiplex power (via simple transistor) to the R or G or B anodes and to control 4 TLC5940s to PWM the current sink on the LED cathodes.

    If at all possible, I would like to make use of their design including use of the TLC5940's. Is there anything you could suggest for a circuit that would take the PWM'd constant current sink of the 5940 outputs and use that to switch ~500mA of current sink for my LED strips. The strips themselves have integrated resistors, so controlling their current is not an issue.

    Michael

  • In reply to Michael Grabscheid:

    Michael,

    Looks like there a lot of folks using this processor for LED projects.  I saw a number of posts similar to yours on the Arduino forum (http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1238693450/30).

    The TLC5940 was designed primarily for LED display (think jumbotron) applications.  When used for this purpose, the LED current regulation accuracy is important and is therefore sensed/controlled inside the IC.  This is how we are able to achieve good current matching between the channels on the driver.

    Having said all of this, the TLC5940 and other TLC59xx drivers get used in a lot of creative, non-signage applications, hence the app notes for paralleling channels or using external FETs for higher voltage strings.

     If it is not possible to significantly redesign the circuit, then you could do something like this.  The LED current would then be given by below and is no longer internally regulated by the TLC5940.

    ILED = (VLED - n x Vf) / (R + Rdson)

    where:

    • VLED = supply voltage for LED strings
    • Vf = forward voltage of the LEDs (note these vary by color and within a particular color)
    • n = number of LEDs in series

    Since the LED forward voltages vary slightly and change with temperature, you may notice some brightness differences between strings especially at deeper dimming levels.

    Hope this helps.

    John

  • In reply to John Perry:

    Thank you John. This is quite helpful.

    I have some logic level p-channel mosfet transistors I can use to test this. The led strips I am using have integral current regulating resistors, so I don't think I need resistor R in your diagram. I realized that Rdson will cause a bit of a voltage/current drop.

    If Vled is 12V, do you have recommendations for the target current for the TLC5940, and for Rpu? I am guessing that will be determined by the characteristics of the mosfet, which I do not have handy at present. If I post the mosfet characteristics, can you help with sizing the target Out current and Rpu?

    I really appreciate you taking the time to help with this.

    Michael

  • In reply to Michael Grabscheid:

    Michael,

    It is probably going to take some iterations to get the performance you want.  Before you start, be sure to check the ratings of the logic PFET to make sure it can handle the current you are going to put through it.

    You can start with 1k for Rpu.  The LED turn off will be slow, but it won't waste a lot of power.  You can decrease it incrementally until you get the results you want or become limited by what the TLC5940 can sink and turn the FET on.  For LED turn on, you'll need to play with the current set resistor on the TLC5940 to make sure you have enough current to pull the gate low with the Rpu you have chosen.

    John

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