+ mr_darker
You asked about factors that optimize optocoupler speed, and have had some good explanations by the other members. Here are a few more words starting with the very basic and general and working up to the more detailed and optocoupler specific:
TYPES OF ISOLATING COUPLER
There are a number of different types of isolating couplers (there are also variations within a type. For example some optocouplers can be configured as type 1 or 3, depending on the connections used):
- LED input, photo diode output (medium speed+) (6N135/36)
- LED input, photo transistor output (medium speed) (LTV816/26/46)
- LED input, photo diode plus transitor output (medium speed-) (6N135/36)
- LED input, photo Darlington output (slow and high output saturation voltage) (PS2533)
- LED input, photovoltaic diode output (generates an output voltage/current rather than a change of resistance) (extremely slow) (TLP3905)
- LED input, host powered logic output (10Mbaud) (VO2601/31)
- Communications couplers (RS232, RS485, LVDVS, USB, etc) (10 Mbaud+)
- Capacitively coupled (150 Mbaud) (ICPL2630/31)
- Inductively coupled (150 Mbaud) (ADuM110N)
- Custom
Having investigated all types of isolating couplers, only type (2) above are covered further, as they are the only types that appear to be practicable for the MDB application, as far as I can tell that is.
INPUT LED CURRENT AND CAPACITANCE
To turn an optocoupler input LED on you need current: the more current the faster the optocoupler will turn on.
To turn an optocoupler input LED off you need to remove the current and the faster you remove the current the faster the optocoupler will turn off.
But there is a third factor. To change a current (increase or decrease) you need to change a voltage. The thing is that the LEDs on the input of optocouplers have a relatively high capacitances, so the voltage that controls the current flow, also needs to charge and discharge that capacitance.
OPTOCOUPLER SWEET-SPOT
That takes care of the basic electronics side of things, but there is another factor. An optocoupler's performance, especially speed and Current Transfer Ratio (CTR), vary considerably with input current and, for many optocouplers, the sweet spot is 10mA input current. Any more input current does not increase speed significantly but you are in danger of exceeding the input LED's power rating.
TURNING AN OPTOCOUPLER INPUT LED ON AND OFF FAST
After all that, you have a dilemma. On one hand you need a relatively high current to turn the optocoupler on fast, but on the other hand you need to remove that current as fast as possible to turn the optocoupler off fast, and the more current that is there in the first place the more difficult it is to remove that current.
So how do you turn an optocoupler on and off fast. The first thing to note is that all optocouplers have an inherent speed limitation, and there is nothing that you can do to change that, but what you can do is make sure that you drive the optocoupler input diode to get the highest speed out of an optocoupler that is possible. You do see circuits where the achieved speed is around 100x slower than the inherent optocoupler limiting speed. And in many applications this is acceptable, even desirable.
The speed (delay) of an optocoupler has four distinct regions as shown on most optocoupler datasheets:
- on delay
- on rise time
- off delay
- off fall time
Ultimately, you cannot do much about the turn on and turn off delays. They are more a function of the optocoupler itself, but you can radically change the on rise time and off rise time.
One last bit of optocoupler basics. The input of an optocoupler is simply a LED, although a very specialized one. And like all LEDs it has a maximum reverse voltage before breakdown and damage.
HOST INTERNAL DRIVER CHARACTERISTICS
Armed with all the above data you will be in a position to define the fastest way to drive an optocoupler host side input LED for maximum attainable speed:
- To turn on fast, provide a positive voltage step waveform with 0s rise time, limited to the input LED sweet-spot current. The initial part of the waveform should also allow additional current to charge up the input parasitic capacitance of the input LED.
- To turn off fast, provide a negative step voltage wave form with 0s fall time to a negative voltage 1V less in magnitude than the maximum LED reverse voltage. The negative waveform should be capable of providing infinite current
Note how far removed from the above ideal most optocoupler LED drivers are. One of the worst offending is the commonly used saturation transistor collector with serial current limiting resistor. This arrangement gives a fast turn-on, but a woefully slow turn off, where it just leaves the optocoupler input LED isolated in space with no route to lose its current and charge. Also, at turn off, the transistor needs to climb out of saturation, which is slow. The pull up resistor and pull down saturating transitor collector arrangement has a much faster turn off but significantly slower turn on, although the turn on can be made faster by replacing the pull up resistor with a constant current source..
HOST INTERNAL RECEIVER CHARACTERISTICS
The host-side receiver is much simpler and less critical than the host-side optocoupler LED driver. The output is simply an open collector or drain which, with a pull up resistor can satisfactorily drive logic circuits. The value of pull-up resistor is a compromise between turn off speed (positive going) and CTR. On one hand you want a low value pull up resistor to increase pull up speed, but on the other hand you need a high value to make sure that with the current available from the output transistor that the output transistor will saturate and generate a good 0V logic level.
There are some simple techniques to increase the performance/speed of the host receiver but these are not really worth the bother. It is a good idea, though, to make the input of the the unit receiving the signal a Schmitt trigger input type.
HOST DRIVER LINE SIDE
There is not much you can do here as the characteristics of the line are normally predefined
HOST RECEIVER LINE SIDE
Once again, there is not much that can be done here, except to ensure that the current through the receiver LED is optimum and, worst case, does not exceed the receiver LED maximum current.