LP8551_14 Datasheet, PDF (15/47 Pages) Texas Instruments – LP8551 High-Efficiency LED Backlight Driver for Notebooks

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LP8551
SNVS673E – APRIL 2010 – REVISED SEPTEMBER 2014
R_FSET resistance values with corresponding PWM frequencies:
PWM_RES[1:0]
RFSET (kΩ)
10...15
26...29
36...41
50...60
85...100
135...150
200...300
450...
5 MHz
CLOCK
19232
16828
14424
12020
9616
7963
6386
4808
00
RESOLUTION
8
8
8
8
9
9
9
10
10 MHz
CLOCK
19232
15927
12771
9616
8715
7813
6311
4808
01
RESOLUTION
9
9
9
10
10
10
10
11
20 MHz
CLOCK
19232
16227
14424
12020
9616
7813
6010
4808
10
RESOLUTION
10
10
10
10
11
11
11
12
40 MHz
CLOCK
19232
17429
15626
12020
9616
8414
6811
4808
11
RESOLUTION
11
11
11
11
12
12
12
13
8.3.2.12 Phase Shift PWM (PSPWM) Scheme
The PSPWM scheme allows delaying the time when each LED output is active. When the LED output are not
activated simultaneously, the peak load current from the boost output is greatly decreased. This reduces the
ripple seen on the boost output and allows smaller output capacitors. Reduced ripple also reduces the output
ceramic capacitor audible ringing. PSPWM scheme also increases the load frequency seen on boost output by
x4 and therefore transfers the possible audible noise to so high frequency that human ear cannot hear it.
Description of the PSPWM mode is seen on the following diagram. PSPWM mode is enabled by setting
<EN_PSPWM> EEPROM bit to 1. Shift time is the delay between outputs and it is defined as 1 / (f_PWM x 4). If the
<EN_PSPWM> bit is 0, then the delay is 0 and all outputs are active simultaneously.
Shift time
Cycle time
t_SHIFT = 1/(F_PWM x 4) 1/(F_PWM)
OUT1
OUT2
OUT3
OUT4
Figure 10. PSPWM Mode
8.3.2.13 Slope
Slope time can be programmed with EEPROM bits <SLOPE[3:0]> from 0 to 500 ms. The same slope time is
used for sloping up and down. Advanced slope makes brightness changes smooth for eye.
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