LP5520TL NSC [National Semiconductor], LP5520TL Datasheet - Page 14

• Manufacturer Part Number: LP5520TL
• Description: RGB Backlight LED Driver
• Manufacturer: NSC [National Semiconductor]

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LED BRIGHTNESS CONTROL
The LED brightness is defined by two factors, the current
through the LED and the PWM duty cycle. The constant cur-
rent outputs ROUT, GOUT and BOUT can be independently
set to sink between 0 and 60 mA. The 8-bit current control has
255 levels and the step size is 235 µA. In manual mode the
current is defined with the current control (R/G/B) registers
(01H, 02H and 03H). In automatic mode the current settings
come from the EEPROM.
The PWM control has 12-bit resolution, which means 4095
steps. The minimum pulse width is 200 ns and the frequency
can be set to either 1.2 kHz or 19.2 kHz. The duty cycle range
is from 0 to 100% (0 to 4095). The output PWM value is ob-
tained by multiplication of three factors. The first factor is the
temperature-based value from the EEPROM. The second
factor is the correction register setting, which is independent
for each color. The third factor is the brightness register set-
ting, which is common to all colors.
The temperature based PWM values are stored in the EEP-
ROM at 16°C intervals starting from -40°C and ending to 120°
C. PWM values for the temperatures between the stored
points are interpolated.
LED brightness has 3-bit logarithmic control. The control bits
are in the pwm_brightness (04H) register. The 3-bit value de-
LED PWM CONTROL
The PWM frequency can be selected of two alternatives, slow
and fast, with the control bit <pwm_fast>. The slow frequency
is 1.2 kHz. In the fast mode the PWM frequency is multiplied
by 16 and the frequency is 19.2 kHz. Fast mode is the default
mode after reset. The single pulse in normal PWM is split in
16 narrow pulses in fast PWM. Higher frequency helps elim-
inate possible noise from the ceramic capacitors and it also
reduces the ripple in the boost voltage. Minimum pulse length
is 200 ns in both modes.
LED control principle, shown complete only for red channel
14
fines a multiplier for the 12-bit PWM value obtained from the
memory according to the following table.
PWM Brightness register control
The brightness correction can be used for aging compensa-
tion or other fine-tuning. There is an 8-bit correction register
for each output. The PWM value obtained from the memory
is multiplied by the correction value. The default correction
value is 1. Correction range is from 0 to 2 and the lsb is 0.78%
(1/128).
The PWM pulses of each output do not start simultaneously
in order to avoid high current spike. Red starts in the begin-
ning of the PWM cycle, Green is symmetric with the cycle
center and Blue ends in the end of the cycle. For PWM values
less than 33% for each output, the output currents are com-
pletely non-overlapping. With higher PWM values the over-
lapping increases.
Control byte
<bri[2:0]>
0
1
2
3
4
5
6
7
Multiplier
0.008
0.016
0.031
0.063
0.125
0.250
0.500
1.000
Intensity %
100.0
12.5
25.0
50.0
0.8
1.6
3.1
6.3
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