LP8553

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SNVS701B – SEPTEMBER 2011 – REVISED JANUARY 2014

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.

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.

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:

•

Default value for CURRENT[7:0] = 7Fh (127d).

(1)

Therefore the output current can be calculated as follows:

Note: formula is only approximation for the actual current.

•

E.g. If 16 k

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

(2)

PWM Frequency Setting

PWM frequency is selected with PWM_FREQ[4:0] EEPROM register. If PLL clock frequency multiplication is

used, it will affect the output PWM frequency as well. <PWM_RESOLUTION[1:0]> EEPROM bits will select the

PLL output frequency and hence the PWM frequency and resolution. Below are listed PWM frequencies with

<EN_VSYNC]> = 0. PWM resolution setting affects the PLL clock frequency (5 MHz…40 MHz). Highlighted

frequencies with boldface can be selected also with external resistor RFSET. To activate RFSET frequency selection

the <EN_F_RES> EEPROM bit must be 1.

PWM_RES[1:0]

00

01

10

11

PWM_FREQ[4:0]

5 MHz

10 MHz

20 MHz

40 MHz

Resolution (bits)

11111

19232

-

-

-

8

11110

16828

-

-

-

8

11101

14424

-

-

-

8

11100

12020

-

-

-

8

11011

9616

19232

-

-

9

11010

7963

15927

-

-

9

11001

6386

12771

-

-

9

11000

4808

9616

19232

-

10

10111

4658

9316

18631

-

10

10110

4508

9015

18030

-

10

10101

4357

8715

17429

-

10

10100

4207

8414

16828

-

10

10011

4057

8114

16227

-

10

10010

3907

7813

15626

-

10

10001

3756

7513

15025

-

10

10000

3606

7212

14424

-

10

01111

3456

6912

13823

-

10

01110

3306

6611

13222

-

10

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