Slow turn-off time on PNP when driving 7 segment led

Question

I know that there has already been question about optimizing PNP turn-off time, but my circuit is a bit different from that question so the solution presented there does not seem to work for me. Here is my circuit

I use MAX7219 to drive this circuit. The MAX7219 digit output is inverted by CD4069 so it can drive the common anode 7 segment that I have. D1 represents segment A, D2 represents segment B, this configuration continues up until segment G.

The problem that I have now is because MAX7219 scanning time is at about 800Hz for 8 Digits display, the PNP 2N3906 does not turn off fast enough, so there is ghosting on the display. Here I have tried measuring the 2n3906 collector voltage on CH-1 (yellow), and base input of the 2n3906 on CH-2 (green).

From those measurements i think that my turn off delay time of my pnp is way to slow, but i dont know how can i possibly improve it.

Here is the 7 segment that I use:

EDIT:

Here is the output waveform of MAX7219 on Digit 1 and Digit 2. From what I see in the datasheet the interdigit blanking should be at least 1/32 of a cycle, but here I see almost no interdigit blanking. Any idea why?

EDIT2:

Here is the waveform of the current measurement on a segment as suggested by Spehro, the current looks fine to me. So it should be eliminate pnp turn off time as the cause of the ghosting problem right?

Answer 1

You would be better to measure the current through the 330 ohm resistors than the voltage- the current translates into light emitted whereas the voltage doesn't mean much once the LED current drops to near zero.

As well as moving the resistors as others have suggested, I suggest trying to digitally reduce the brightness from full to maybe 80-90% which will increase the interdigit blanking time by a factor of 4 to 6.

You could also replace the BJTs with MOSFETs 2N7000/BSS84 for a large increase in switching speed.

Answer 2

Turns out after some reading on the documentation and application notes of MAX7219, I found out that my Rset for the MAX7219 was too low, because I am driving the 7segment led using external driver. I have tried changing it to 60kOhm (the recommended value was 47kOhm, but because I have 60k lying around I just tried that one) and the ghosting on other 7 segment is completely gone. I have not been able to measure the signal output on the pnp again, but I will give some updates later.