The LP5868T device
uses a time-multiplexing matrix scheme to support up to 144 LED dots with one chip. The device
integrates 18
current sinks with 8 scan lines to drive 18
× 8 = 144
LED dots or 6 ×
8 = 48
RGB pixels. In matrix control scheme, the device scans from Line 0 to Line 7
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 8
high-side p-channel MOSFETs (PMOS) integrated in LP5868T device. Users can flexibly
set the active scan numbers from 6 to 8 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 8μs (PWM frequency set at 125kHz) or 16μs (PWM frequency set at
62.5kHz) by configuring 'PWM_Fre' in Dev_initial register.
- tSW_BLK is the switch blank time,
which equals to 1μs or 0.5μs by configuring 'SW_BLK' in Dev_config1
register.
- tphase_shift is the PWM phase shift
time, which equal to 0 or 125ns by configuring 'PWM_Phase_Shift' in Dev_config1
register.
Total display time for one complete sub-period is
tsub_period and 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 LP5868T device implements
de-ghosting and low brightness compensation to remove the side effects of matrix
topology:
- De-ghosting: Both
upside de-ghosting and downside de-ghosting are implemented to eliminate the
LED's unexpected weak turn-on.
- Upside_de-ghosting: discharge each scan line
during blank state. By configuring the 'Up_Deghost' in Dev_config3
register, the LP5868T discharges and
clamps the scan line switch to a certain voltage.
- Downside_deghosting: pre-charge each current sink
voltage during blank state. The de-ghosting 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.