LP8550TLX-ASLASHNOPB datasheet(14/49 Pages) TI2

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PWM input

signal

PWM

detector

5 MHz

clock

13-bit

PWM

comparator

PWM

Counter

Brightness

control

BRT_MODE

[1:0]

Temperature

sensor

12-bit

Brightness

8-bit

Resolution

selector

13-bit

PWM_FREQ[4:0]

8...13-bit

SLOPE[3:0]

Dither

16-bit

DITHER[1:0]

8...16-bit

Sloper

13-bit

0/1

LED Drive

1-6

PLL clock 5...40 MHz

PWM_RESOLUTION

[1:0]

HYSTERESIS

[1:0]

...

PWM &

Current

Control

16-bit

I_SLOPE[1:0]

MODE_25/50%_SEL

EN_PWM_&_I_CTRL

LP8550

SNVS657E – SEPTEMBER 2010 – REVISED SEPTEMBER 2014

www.ti.com

Figure 13. PWM Calculation Data Flow

8.3.2.5 PWM Detector

The PWM detector block measures the duty cycle of the input PWM signal. Resolution depends on the input

signal frequency. Hysteresis selection sets the minimum allowable change to the input. If smaller change is

detected, it is ignored. With hysteresis the constant changing between two brightness values is avoided if there is

small jitter in the input signal.

8.3.2.6 Brightness Control

Brightness control block gets 13-bit value from the PWM detector, 12-bit value from the temperature sensor and

also 8-bit value from the brightness register. <BRT_MODE[1:0]> selects whether to use PWM input duty cycle

value or the brightness register value as described earlier. Based on the temperature sensor value the duty cycle

is reduced if the temperature has reached the temperature limit set to the <TEMP_LIM[1:0]> EEPROM bits.

8.3.2.7 Resolution Selector

Resolution selector takes the necessary MSB bits from the input data to match the output resolution. For

example if 11-bit resolution is used for output, then 11 MSB bits are selected from the input. Dither bits are not

taken into account for the output resolution. This is to make sure that in steady state condition, there is no

dithering used for the output.

8.3.2.8 Sloper

Sloper makes the smooth transition from one brightness value to another. Slope time can be adjusted from 0 to

500 ms with <SLOPE[3:0]> EEPROM bits. The sloper output is 16-bit value.

8.3.2.9 PWM & Current Control

Automatic PWM & current control improves the optical efficiency of the LEDs by using PWM control with small

brightness values and current control with bigger values. <EN_PWM_&_I_CTRL > EEPROM bit selects whether

the PWM & current control is used instead of PWM control or not. PWM to current dimming switch point can be

set to 25% or 50% of the brightness range with <MODE_25/50_SEL> EEPROM bit. Current slope can be

adjusted by using the <I_SLOPE[1:0]> EEPROM bits.

8.3.2.10 Dither

With dithering the output resolution can be “artificially” increased during sloping from one brightness value to

another. This way the brightness change steps are not visible to eye. Dithering can be from 0 to 3 bits, and is

selected with <DITHER[1:0]> EEPROM bits.

8.3.2.11 PWM Comparator

The PWM counter clocks the PWM comparator based on the duty cycle value received from Dither block. Output

of the PWM comparator controls directly the LED drivers. If PSPWM mode is used, then the signal to each LED

output is delayed certain amount.

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