Have a 10 x 10 LED array sourced with CD74HC154 Decoder/Demultiplexer to PNP Transistors (rows) and sinked with STP16CPS05 Sink Driver (columns). This is driven with an Atmega328P. Rows and columns are multiplexed. It works great.
Problem: I want to adjust LED brightness by increasing sink current through STP16CPS05 via the external resistor R-EXT pin 23. Using data sheet Table 11, when I change R-EXt from 1K to 215 ohm there is no change in brightness. This is still true when I light just one row and turn of multiplexing. If I disconnect an LED from an STP16CPS05 output and go straight to ground it gets much brighter.
See attached schematic. Note: I am using an inverter gate from Latch LE/DM1 pin 4 to Output Enable pin 21. This was to eliminate random flickers when multiplexing. Only turns on outputs when Latched. Also works great.
Source: Row Current is 200mA with transistor Base resistor at 2.5K. Dropped this down to 215 Ohm to make sure I was not restricting current here. Transistor is Fairchild SS8550.
Question: Why doesn't R-EXT value change LED brightness?
I took the measurements as shown in the image below. While multiplexing is paused, measuring the single LED in a column that is on Anode = 4.84V Cathode =0.80V. Transistor Emitter = 5.06V Collector = 4.88V. At the pin for this column on STP16CPS05 = 0.69V
Looking at the Datasheet graph of FWD Current vs FWD Voltage for a blue LED, if my FWD Voltage is 4.0 Volts then the current must be 69mA? I added the dashed lines to the graph.