BJT & MOSFET transistor function

[JerryLee]

Hi Guys,

I was searching for constant current supply circuit reference design that uses BJT or MOSFET and stumbled upon this design below.



It is used to drive two LED to toggle at different time, meaning one would turn on and the other off and vice versa.

RON & IRON: connected to logic OR gates
VCR & IVCR: connected to a digital-to-analog converter's output

What I'm trying to find out is how it actually works or operate, could someone help enlighten me?

1) Let say RON is feed with logic 1 or 0
- What will happen to the Gate, Drain and Source of MOSFET in term of voltage flow?

2) Let's say IVCR is feed with analog input
- What is the role of the IVCR in this design? To control the brightness of LED?
- If yes how this works in term of current flow?

Thanks 

[T3sl4co1l]

Not sure why PMOS would be involved, the current sinks can be disabled by simply setting them to zero.

Current is defined by the voltage on VCR / VCIR, and R27/R26.  There's a strong tempco due to the transistor Vbe (~0.7V at room temperature, but up to 0.9V or more at low temperature, and 0.5V or less at maximum temperature).

To put it fully,
Ic = (VCR - Vbe) / R27

In the < 0.7V range, Ic falls to ~zero with an exponential cutoff, and has an offset voltage (so it's ~zero by 0.2 or 0.3V, or at least as zero as the transistor will go, which is in the nA range).

With R30/R23 present also, there is additional voltage drop, due to base current.  I would suggest removing these resistors altogether (replace with 0 ohms, or a shorting jumper).  As long as the DAC outputs can supply enough current, it will be fine.  Base current is about 1/100th of Ic (the exact ratio is hFE, which varies as well, but is usually high enough).

Note that the collector won't pull all the way down to the emitter, so all of this is only true as long as:
V(LED1) > Ic * R27 + Vce(sat)
(and so on for the other side), where Vce(sat) is usually small (~0.2V, unless the transistor is being ran at rather high currents).

Tim