I am currently designing on a switch matrix board for custom measurement equipment.
To route the signals I plan on using optical relays, the G3VM-61DR1 to be precise.
So I need to control >250! LEDs (1,5-1,8V 6-8mA) somehow.
- Only <25 will be turned on at the same time
- Supply voltage will be 3.3V or lower, temperature should be around 40°C
- We just need to turn them on or off, no dimming or PWM
- I don't want to use an LED driver because of the noise it might/will introduce
- I can't use a classic matrix arrangement since it is unknown which switches will be on at the same time
So the only option is to have a discrete output for each relay/LED, your IO Expanders look promising for this, eg. the PCAL6524.
Because of the high channel count I want to reduce the components per channel as much as possible, so I have some questions:
- Can I rely on the 7,5mA current setting of the Agile IO devices and omit the LED series resistor?
- If this is ok, what are possible side effects?
- Since the devices start configured as Input some experience high currents when they are used to control LEDs(eg. PCA9535A Datasheet page 14/15). The classic fix with a pullup for each channel is not cool for so many channels. Does this also affect the Agile IO devices?
- Can this be circumvented by supplying the LEDs with 3.3V and the expanders with 1.65V?
Thank you in advance
Otto
