I wish to use the PCA9635 LED driver connected to and ARM based microporocessor
the PCA9635 has 16 pins lines available, which can operate as 25mA sink or 10mA source, thus naturally I decided to control them in common anode.
Quote from: mohitgarg on Jan 24, 2014, 12:45 pmI wish to use the PCA9635 LED driver connected to and ARM based microporocessorNot an Arduino?
Let's assume a beta of 30 at 200 mA, (minimum specified 100 at 150 mA, 20 at 500 mA) so you need about 7 mA to drive it. 4V divided by 7 mA - 560 ohms, keep the 4k7 to the 5V line.
The LED resistors? Well, you are going to lose half a volt in the PNP transistors (according to the datasheet) and no doubt at least another half in the "sink" driver, so that leaves about 4 volts, if the LEDs do drop 3V, that's just a volt in the resistor so 25 mA - 39 ohm.
Good pick. The mode that is - not so sure about how easy it will be to multiplex through the PCA9635.
Personally, I'd use a MAX7219 - does all the multiplexing for you, bright enough, 15 brightness levels for the whole lot, and 3-wire interface (part of which can be shared).
Quote from: Paul__B on Jan 24, 2014, 01:41 pmLet's assume a beta of 30 at 200 mA, (minimum specified 100 at 150 mA, 20 at 500 mA) so you need about 7 mA to drive it. 4V divided by 7 mA - 560 ohms, keep the 4k7 to the 5V line.How do you make such an assumption for the beta, since I don't see it in the datasheet?
The 560 Ohms, what is the '4V' that you have divided by Ib?
What is the 4K7ohms resistor for and how did we come this value?
Where in the datasheet did you find that the transistor will eat up half a volt? I can't find it since I am very new to this.
What do you mean by the "sink" driver?
Why do you believe it will be hard to multiplex through the PCA9635?
I need to keep everything on the I2C bus and that too in FM+.
Also, cost is a big big factor and I want to keep it as low as possible, whilst balancing with PCB size and complexity constraints.
Page 2, "ON CHARACTERISTICS" - DC Current Gain/ hFE. And note I am being conservative here - that is specified as VCE = 2.0 V, not saturation.
5 volts minus VBE(sat) of at least 0.75V and the drop in the driver IC (not researched).
VCE(sat) at 150 mA
Umm - why? "FM+" - didn't catch that.
Not having to multiplex the display - or provide driver transistors and resistors - is a big "plus" and I'll bet the MAX7219 is cheaper than the PCA9635.
Okay, so I can't omit resistors or use PWM for individual LEDs since I am running the LEDs common anode, correct?
Also why are 7 NPN Darlington pair ICs so much cheaper than 8PNP Darlington pair ICs?