#2 Submitted by Sanjay Chawla on January 15, 2009 - 1:41pm.
Improperly designed LED-multiplexing circuits can create ghost
images. Ghost images result when parasitic current flows through LEDs that are
intended to be in an off state (i.e. no current flowing through them); this
causes very faint illumination or ghosting. These ghost-image currents
typically result from the discharging of stray capacitances associated with the
large, common-LED anode-node tracks and the slightly forward-biased LEDs
themselves.
Faint ghosting images from parasitic currents can occur when the
multiplexing changes phases from MUX0-bar to MUX1-bar and vice versa. The effect
is most pronounced when the LEDs on the multiplexed circuits are different
colors (light wavelengths) and, hence, has significantly different voltage
drops for a given current flow.
Eliminating Ghost-Image Currents
The ghost-image currents can be eliminated by providing a
discharge path for the parasitic node capacitances and providing a time for the
discharge to occur.
Comments
I think you are scanning
I think you are scanning LEDs.
You need to switch off LEDs which are ON before switching on the next bank.
If problem persist, try introducing a small delay during which all LED banks are off at the time of switch over.
Improperly designed
Improperly designed LED-multiplexing circuits can create ghost images. Ghost images result when parasitic current flows through LEDs that are intended to be in an off state (i.e. no current flowing through them); this causes very faint illumination or ghosting. These ghost-image currents typically result from the discharging of stray capacitances associated with the large, common-LED anode-node tracks and the slightly forward-biased LEDs themselves.
Faint ghosting images from parasitic currents can occur when the multiplexing changes phases from MUX0-bar to MUX1-bar and vice versa. The effect is most pronounced when the LEDs on the multiplexed circuits are different colors (light wavelengths) and, hence, has significantly different voltage drops for a given current flow.
Eliminating Ghost-Image Currents
The ghost-image currents can be eliminated by providing a discharge path for the parasitic node capacitances and providing a time for the discharge to occur.
Thanks for your help