The LP5866 device uses time-multiplexing matrix
scheme to support up to 108 LED dots with one chip. The device integrates 18 current
sinks with 6 scan lines to drive 18 × 6 = 108 LED dots or 6 × 6 = 36 RGB pixels. In
matrix control scheme, the device scans from Line 0 to Line 5 sequentially as shown
in Figure 7-1. Current gain and PWM duty registers are programmable for each
LED dot to support individual analog and PWM dimming.
There are 6 high-side p-channel MOSFETs (PMOS)
integrated in LP5866 device. Users can flexibly set the active scan numbers from 1
to 6 by configuring the 'Max_Line_Num' in Dev_initial register. The
time-multiplexing matrix timing sequence follows the Figure 7-2.
One cycle time of the line switching can be calculated as below:
Equation 1. tline_switch =
tPWM + tSW_BLK + 2 × tphase_shift
- tPWM is the current
sink active time, which equals to 8 us (PWM frequency set at 125 kHz) or 16 us
(PWM frequency set at 62.5 kHz) by configuring 'PWM_Fre' in Dev_initial
register.
- tSW_BLK is the switch
blank time, which equals to 1 us or 0.5 us by configuring 'SW_BLK' in
Dev_config1 register.
- tphase_shift is the
PWM phase shift time, which equal to 0 or 125 ns by configuring
'PWM_Phase_Shift' in Dev_config1 register.
Total display time for one complete
sub-period is tsub_period and it can be calculated by the following
equation:
Equation 2. tsub_period =
tline_switch × Scan_line#
- Scan_line# is the scan line
number determined by 'Max_Line_Num' in Dev_initial register.
The time-multiplexing matrix scheme
time diagram is shown in Figure 7-3. The tCS_ON_Shift is the current sink turning on shift by configuring
'CS_ON_Shift' bit in Dev_config1 register.
The LP5866 device implements
deghosting and low brightness compensation to remove the side effects of matrix
topology:
- Deghosting: Both upside deghosting and
downside deghosting are implemented to eliminate the LED unexpected weak
turn-on.
- Upside_deghosting:
discharge each scan line during its off state. By configuring the
'Up_Deghost' in Dev_config3 register, the LP5866 discharges and
clamps the scan line switch to a certain voltage.
- Downside_deghosting:
pre-charge each current sink voltage during its OFF state. The
deghosting capability can be adjusted through the 'Down_Deghost' in
Dev_config3 register.
- Low Brightness Compensation: three groups
compensation are implemented to overcome the color-shift and non-uniformity
in low brightness conditions. The compensation capability can be through
'Comp_Group1', 'Comp_Group2', and 'Comp_Group3' in Dev_config2 register.
- Compensation_group 1:
CS0, CS3, CS6, CS9, CS12, CS15.
- Compensation_group 2:
CS1, CS4, CS7, CS10, CS13, CS16.
- Compensation_group 3:
CS2, CS5, CS8, CS11, CS14, CS17.