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.
How do you make such an assumption for the beta, since I don't see it in the datasheet?
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.
The 560 Ohms, what is the '4V' that you have divided by Ib?
5 volts minus VBE(sat)
of at least 0.75V and the drop in the driver IC (not researched).
What is the 4K7ohms resistor for and how did we come this value?
It keeps the transistor switched off when not driven. Value is arbitrary, same as in Mike's tutorial.
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.
at 150 mA
What do you mean by the "sink" driver?
Your driver IC which pulls the resistor down
to switch on the transistor.
Why do you believe it will be hard to multiplex through the PCA9635?TL;DR
- because I haven't studied the datasheet to understand just what it does. Is is supposed to do the multiplexing for
you, or do you have to keep feeding it stuff from the MCU? I see it does PWM at a high frequency, but didn't see reference to multiplexing.
I need to keep everything on the I2C bus and that too in FM+.
Umm - why
"FM+" - didn't catch that.
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.
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.