LP8551TLXSLASHNOPB datasheet(13/47 Pages) TI1

13 / 47 page

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]

16-bit

Sloper

13-bit

0/1

LED Drivers

1-4

PLL clock 5...40 MHz

PWM_RESOLUTION

[1:0]

HYSTERESIS

[1:0]

...

LP8551

www.ti.com

SNVS673E – APRIL 2010 – REVISED SEPTEMBER 2014

8.3.2.4 PWM Calculation Data Flow

Figure 9 shows the PWM calculation data flow. In PWM direct control mode most of the blocks are bypassed,

and this flow chart does not apply.

Figure 9. 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.

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 Comparator

The PWM counter clocks the PWM comparator based on the duty cycle value. 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.

8.3.2.10 Current Setting

Maximum current of the LED outputs is controlled with CURRENT[7:0] EEPROM register bits linearly from 0 to

30 mA. If <EN_I_RES> = 1 the maximum LED output current can be scaled also with external resistor, RISET.

RISET controls the LED current as follows:

(1)

Default value for CURRENT[7:0] = 7Fh (127d). Therefore, the output current can be calculated as follows:

(2)

For example, if a 16-k

Ω RISET resistor is used, then the LED maximum current is 23 mA.

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