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LED Drivers/LCD Bias
LED Drivers/LCD Bias Forum
interfacing the TLC5947
I'm new on this forum, so i hope i posted this topic in the right forum section.
I'm using a TLC5947, and i want to drive 1W LEDs. For this is I want to use a buck converter from Zetex; the ZXLD1350.
The outputs of the TLC5947 are constant current and the input of the ZXLD1350 wants to be hooked to an open collector output. The TLC5947 is an open collector, so that should work.
However, my analog skills are not top notch, and the internal schematic of both the TLC5947 and the ZXLD1350 are unclear to me.
Am i right that i CAN hookup the ZXLD1350 straight to the output of the TLC5947?
Or..should I put a pullup resistor to the output of the TLC5947 and the connect the output (via a resistor divider) into the base of an external NPN transistor? The collector of this external transistor can then be hooked to the input of the Zetex.
I'm very confused in this.
Of course I want to minimize the amount of components on my board, but I am really not sure if i can directly drive the Zetex chip with the TLC5947.
Hope somebody can clear this up for me..
Thanx in advance!
I understand that if i do so, i will have inverted PWM, but doesnt matter, my software can correct that.
Can somebody help me out with this?
Rene, for driving 1W LEDs you need 350 mA drive capability. TLC5947 hs only 30 mA. I'm not sure which application you design, but I'd suggest:
- try to cascade 6 TLC59213
- or ZXLD1350 for each LED + MCU (like Piccolo) with as many PWMs as possible
The thing i wanted to do was indeed hook up a ZXLD1350 per output of a TLC5947. With this i could use the 24channels of PWM control with a current boost of 350mA per channel.
The thing that confused me was the fact that the output circuitry of the TLC5947 does not become clear from the datasheet.
The only thing the datasheet says, is that it has an open collector output, with a fixed output current. In other words, its a current source.
My main question was, what happens if you hook this up to a ZXLD1350?
Is the TLC gonna force the set current..or not? The datasheet of the TLC does NOT say that the output current is the MAX outputcurrent. I just wanted to be sure.
Also the inputcircuitry of the ZXLD (the adj input) confused me, and did not become clear to me from the datasheet.
Anyway, my PCB's have allready arrived, and they are stuffed now and it seems that everything goes okey with hooking up the ZXLD1350 straight to the output of the TLC chip.
thanx for reacting.
I still would like to know HOW the output circuit of the TLC5847 functions.
Rene, ZXLD1350 does DC-V into DC-I transformation. It's controlled by voltages, not by currents. If you need PWM control of LED drivers, you should us an MCU with PWM outputs (like Piccolo).
From which country are you? From your name I guess - from Germany. You can ask then for local support from our Arrow (former Spoerle office), we have both Zetex and TI in linecard. Please, check here
I see what you mean, and thats why i did my first post.
The zetex IC has an adj-input, which should be hooked up to a NPN open collector output.
This is the recomended way of interfacing the zetex IC with logic or u-controllers.
I wanted to know if it was possible to use the open collector output of the TLC IC, just to be able to have an easy I/O expansion for my u-controller.
The reason is that i need 144 individual PWM channels.
The u-controller runs a program and also the LEDs, so PWM in the ucontroller becomes a bit a no-go.
Anyway, the zetex IC has an input circuit i dont really understand, and the TLC datasheet doesnt tell me if it is a real current source, or a current limited output.
(the zetex adj pin has 1.25V (with 200K input resistance) present, created from the internal circuitry, switching that to zero turns off the internal mosfet, you can control it with PWM)
Do you understand my confusion?
(FPGA design becomes a bit more logical here i think...)
I don't fully understand what you are trying to accomplish by connecting the Zetex IC to the TLC5947. A quick explanation of the TLC5947 is that it sinks a constant amount of current regardless of the voltage on the OUT pins. If you can generate a supply voltage to connect to the anode of your LED, and then connect the LED cathode to the TLC5947, the TLC5947 will sink a contant amount of current. Your microprocessor can control the turn on and turn off the LED. Also note that the TLC5947 outputs can be connected in parallel to sink a total of 24*0.030A = 0.72A. If you need less functionality than the TLC5947, you might consider the TLC5927. It has 16 outputs and each output can sink 45mA. If the output of the TLC5927 is turned off, it is open collector. When the processor tells a TLC5927 output to turn on, the output sinks the programmed constant current. The voltage at OUT gets pulled down to whatever voltage is necessary to drive the current through the LED. If you connect a resistor to the OUT pins, the TLC5927 will still try to sink a constant current. If the programmed sink current times the resistor value is greater than the pullup voltage on the resistor, the OUT voltage goes to 0V.
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