IO Expansion for LEDs

[0tto]

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:

1. Can I rely on the 7,5mA current setting of the Agile IO devices and omit the LED series resistor?
2. If this is ok, what are possible side effects?
3. 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?
4. Can this be circumvented by supplying the LEDs with 3.3V and the expanders with 1.65V?

Thank you in advance

Otto

[guoweisun · Accepted solution]

PCAL6524 is well suited for driving a large number of optical relays and LEDs. However, the Agile I/O drive strength is not intended to replace the required series current-limiting resistors. To mitigate the risk of unintended current during power-up when the I/Os are still in their input state, options such as using a 3.3 V LED supply with a lower VDD(P), or implementing a global LED power switch, may be considered. Nevertheless, individual current-limiting resistors should always be retained for each LED channel.