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LP5520

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SNVS440B – MAY 2007 – REVISED MARCH 2016

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Copyright © 2007–2016, Texas Instruments Incorporated

Current setting for each LED comes from EEPROM in the automatic mode. The same current values must be

programmed as were used in the calibration. Current control range is from 0 to 60 mA with 8-bit resolution and

the step size is 235 µA.

Common brightness control for all LEDs can be done using the pwm_brightness (05H) register. The

pwm_brightness register makes 8 level logarithmic brightness control with 3 bits. An automatic fade function

allows smooth turnon, turnoff, and brightness changes of the LEDs. White balance is maintained during fading.

A brightness correction value can be given for each LED. The PWM value obtained from the EEPROM memory

is multiplied by this correction value. This feature can be used for example for LED aging compensation or for

color adjustment by user. These values are kept in R_correction (0AH), G_correction (0BH) and B_correction

(0CH) registers. The correction multiplier can be between 0 and 2.

Due to LED self-heating, the temperature sensor and the LED temperatures will differ. The difference depends

on the thermal structure of the display module and the distance between the sensor and the LEDs. This

temperature difference can be compensated by storing the temperature difference value at highest power (100%

red LED PWM) in the EEPROM memory. The system then corrects the measured temperature based on the

actual PWM value used. The correction assumes that the red LED PWM value is representing the whole RGB

LED power consumption.

Sequential (non-overlapping) drive is possible using external PWM control inputs to trigger a new sequence in

each LED output. 60 mA maximum current setting makes possible 20 mA maximum averaged current for each

output in the non-overlapping mode.

7.4.3 Stand-Alone Mode

In stand-alone mode the operation is controlled through a single PWM brightness input, BRC. After power-up or

reset the LP5520 is ready for stand-alone operation without any setup through the serial interface. The stand-

alone mode is entered with a rising edge in the BRC input. The boost converter operates in adaptive mode. The

LED current settings are loaded from EEPROM. The LED brightness is controlled with a PWM signal in the BRC

input. The BRC PWM frequency must be from 2 to 10 kHz. The PWM signal in the BRC input is not used as

such for the LED outputs, but it is converted to 3-bit value and a logarithmic brightness control is based on this 3-

bit value, as shown in Table 9. There is hysteresis in the conversion to avoid blinking when the BRC duty cycle is

close to a threshold. When the PWM pulses end in the BRC input and the input stays low, the circuit goes to the

standby mode.

Figure 21 shows the waveforms in BRC input and ROUT output in the stand-alone mode. The circuit is in

standby mode until the first rising edge in BRC input is detected. The circuit starts up, and the outputs activate

after 30 ms from the first rising edge in BRC. The BRC frequency is assumed to 2 kHz in this example giving 0.5

ms BRC period. When the duty cycle changes in BRC, it takes two BRC periods before the change is reflected in

the output. When BRC goes permanently low, the circuit enters standby mode after 15 ms from the last BRC

pulse.

All controls through the serial interface can be used in the stand-alone mode. In Automatic and Manual mode the

control bit <brc_off> must be written high and BRC input kept low to prevent the LP5520 device from entering

stand-alone mode.

Table 9. Stand-Alone Mode Brightness Control

BRC DUTY CYCLE THRESHOLD VALUES (%)

INTENSITY

(% of maximum)

RECOMMENDED BRC PWM CONTROL VALUES

INCREASING

DECREASING

INCREASING

DECREASING

0

off

0

1

15

0.8

10

10

20

28

1.6

28

22

35

42

3.1

40

32

48

52

6.3

53

47

58

62

12.5

63

58

68

75

25

75

70

82

90

50

88

85

97

100

99