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

register

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 Drivers

1-6

PLL clock 5...40 MHz

PWM_RESOLUTION

[1:0]

HYSTERESIS

[1:0]

...

LP8545

www.ti.com

SNVS635D – APRIL 2010 – REVISED DECEMBER 2013

Brightness Control Methods

LP8545 controls the brightness of the backlight with PWM. PWM control is received either from PWM input pin or

BRT_MODE[1]

BRT_MODE[0]

PWM source

0

0

PWM input pin duty cycle control. Default.

0

1

PWM input pin duty cycle control.

1

0

Brightness register

1

1

PWM direct control (PWM in = PWM out)

PWM Input Duty Cycle

With PWM input pin duty cycle control the output PWM is controlled by PWM input duty cycle. PWM detector

block measures the duty cycle in the PWM pin and uses this 13-bit value to generate the output PWM. Output

PWM can have different frequency than input in this mode and also phase shift PWM mode can be used. Slope

and dither are effective in this mode. PWM input resolution is defined by the input PWM clock frequency.

Brightness Register Control

With brightness register control the output PWM is controlled with 8-bit resolution <BRT7:0> register bits. Phase

shift scheme can be used with this and the output PWM frequency can be freely selected. Slope and dither are

effective in this mode.

PWM Direct Control

With PWM direct control the output PWM will directly follow the input PWM. Due to the internal logic structure the

input is anyway clocked with the 5 MHz clock or the PLL clock. PSPWM mode is not possible in this mode. Slope

and dither are not effective in this mode.

PWM Calculation Data Flow

Below is flow chart of the PWM calculation data flow. In PWM direct control mode most of the blocks are

bypassed and this flow chart does not apply.

Figure 13. PWM Calculation Data Flow

PWM Detector

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.

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.

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