7.3.2.1 White Balance Control

The LP5520 is designed to provide spectrally rich white light using a three-color RGB LED. White light is obtained when the red, green, and blue LED intensities are in proper balance. The LED intensities change independently with temperature. For maintaining the purity of the white color and the targeted total intensity, precise temperature dependent intensity control for each LED is required. The color coordinates in this document refer to the CIE 1931 color graph (x,y system).

Figure 10. CIE 1931 Color Graph

Figure 11 shows a typical RGB LED intensity behavior on a 12-bit scale (0 to 4095) at constant 20-mA LED currents. Figure 12 shows the typical color coordinate change for an uncompensated RGB LED. Figure 13 shows the corresponding PWM values for achieving constant intensity white light across the temperature range. The PWM values have been saturated at 104°C to avoid overheating the LED and to better utilize the PWM range. The white balance is not maintained above 104°C in this case.

Figure 11. LED Intensity vs Temperature

Figure 12. Typical Color Coordinates vs Temperature for Uncompensated RGB LED

The compensation values for the measured temperatures can be easily calculated when the intensity vs temperature information is available. For the best accuracy the iterative calibration approach must be used.

Figure 13. Compensation PWM Values

The compensation values must be converted to 16°C intervals when they are programmed to the calibration EEPROM. The evaluation software has import function, which can be used to convert the measured compensation data to the 16°C interval format. The measured data can have any temperature points, and the software fits a curve through the measured points and calculate new PWM values in fixed temperatures using the curves.

Typical color coordinate and intensity stability over temperature are shown in Figure 14 and Figure 15.

Figure 14. Compensated Color Coordinates vs Temperature

Figure 15. Compensated Blue LED Intensity vs Temperature

7.3.2.2 LED Brightness Control

The LED brightness is defined by two factors, the current through the LED and the PWM duty cycle. The constant current outputs ROUT, GOUT, and BOUT can be independently set to sink between 0 and 60 mA. The 8-bit current control has 255 levels, and the step size is 235 µA. In manual mode the current is defined with the current control (R/G/B) registers (01H, 02H, and 03H). In automatic mode the current settings are loaded from the EEPROM.

The PWM control has 12-bit resolution, which means 4095 steps. The minimum pulse width is 200 ns, and the frequency can be set to either 1.2 kHz or 19.2 kHz. The duty cycle range is from 0 to 100% (0 to 4095). The output PWM value is obtained by multiplication of three factors. The first factor is the temperature-based value from the EEPROM. The second factor is the correction register setting, which is independent for each color. The third factor is the brightness register setting, which is common to all colors.

The temperature-based PWM values are stored in the EEPROM at 16°C intervals starting from –40°C and ending to 120°C. PWM values for the temperatures between the stored points are interpolated.

LED brightness has 3-bit logarithmic control. The control bits are in the pwm_brightness (04H) register. The 3-bit value defines a multiplier for the 12-bit PWM value obtained from the memory according to Table 1.

The brightness correction can be used for aging compensation or other fine-tuning. There is an 8-bit correction register for each output. The PWM value obtained from the memory is multiplied by the correction value. The default correction value is 1. Correction range is from 0 to 2 and the LSB is 0.78% (1/128).

Shown complete only for red channel

Figure 16. LED Control Principle

7.3.2.3 LED PWM Control

The PWM frequency can be selected of two alternatives, slow and fast, with the control bit <pwm_fast>. The slow frequency is 1.2 kHz. In the fast mode the PWM frequency is multiplied by 16, and the frequency is 19.2 kHz. Fast mode is the default mode after reset. The single pulse in normal PWM is split in 16 narrow pulses in fast PWM. Higher frequency helps eliminate possible noise from the ceramic capacitors and it also reduces the ripple in the boost voltage. Minimum pulse length is 200 ns in both modes.

The PWM pulses of each output do not start simultaneously in order to avoid high current spike. Red starts in the beginning of the PWM cycle, Green is symmetric with the cycle center and Blue ends in the end of the cycle. For PWM values less than 33% for each output, the output currents are completely non-overlapping. With higher PWM values the overlapping increases.

Figure 17. Pulse Positions in the PWM Cycle

7.3.2.4 Sequential Mode

Completely non-overlapping timing can be obtained by using the sequential mode as shown in Figure 18. The timing is defined with external PWM control inputs. The minimum trigger pulse width in the PWM inputs is 1 µs. There is no limitation on the maximum width of the pulse as long as it is shorter than the whole sequence.

Figure 18. Non-Overlapping External Synchronized Sequential Mode

In sequential mode the PWM cycle is synchronized to trigger pulses and the amount of PWM pulses per trigger can be defined to 2, 3 or 4 using the <seq_mode0> and <seq_mode1> control bits. This makes possible to use sequence lengths of about 5 ms, 7.5 ms or 10 ms. Fast PWM can be used in sequential mode, but the frame timing is as with normal PWM.

The PWM timing and synchronization timing originate from different clock sources. Some margin must be allowed for clock tolerances. This margin shows as a dead time in the waveform graph. Some dead time must be allowed so that no PWM pulse is clipped. Clipping would distort the intensity balance between the LEDs. The dead time causes some intensity reduction, but assures the current balance.

PWM mode is defined by <seq_mode1> and <seq_mode2> control bits of rgb_control (00H) register:

7.3.2.5 Current Control of the LEDs

The LP5520 has a separate 8-bit current control for each LED output. In manual mode the current for red LED is controlled with the current_control_r (01H) register, the green LED is controlled with the current_control_g (02H), and the blue LED with current_control_b (03H). Output current can be calculated with formula: current (mA) = code × 0.235; for example, a 20-mA current is obtained with code 85 (55H).

In automatic and stand-alone modes the LED current values programmed in EEPROM are used, and the current control registers have no effect. There are two ways to change the default current if needed. The defaults can be changed permanently by programming new values to the EEPROM. The other option is to make a temporary change by writing new current values in SRAM.

7.3.2.6 Output Enables

ROUT, GOUT, and BOUT output activity is controlled with 3 enable bits of the rgb_control (00H) register:

PWM control inputs PWMR, PWMG and PWMB can be used as external output enables in normal and automatic mode. In the sequential mode these inputs are the trigger inputs for respective outputs.

7.3.2.7 Fade In and Fade Out

The LP5520 has an automatic fade in and out for the LED outputs. Fading makes the transitions smooth in on and off switching or when brightness is changed. It is not applied for the changes caused by the compensation algorithm. The fade can be turned on and off using the <en_fade> bit in the rgb_control (00H) register. The fade time is constant 520 ms, and it does not depend on how big the brightness change is. The white balance is maintained during fading. Fading is off in the stand-alone mode.

Fading only works in automatic mode. The LED current registers must be written to 0 for proper fade operation. When the LEDs are turned on with fading, it is best to set the brightness first and then enable the outputs and automatic mode. The LEDs can be turned off then by turning off the automatic mode (write rgb_auto to 0).

7.3.2.8 Temperature and Light Measurement

The LP5520 has a 12-bit analog-to-digital converter (ADC) for the measurements. The ADC has two inputs. S1_IN input is intended for the LM20 temperature sensor and S2_IN input for light measurement or any DC voltage measurement. The conversion results are filtered with average filter for 134 ms. The <adc_ch> bit in the Control register selects, which conversion result can be read out from the registers ADC_hi_byte and ADC_low_byte. The ADC_hi_byte must be read first. The <comp_ch> bit selects, which input is used for compensation. The ADC uses the LDO voltage 2.8 V as the reference voltage. The input signal range is 0 V to 2.8 V, and the inputs are buffered on the chip.

If S2_IN is used for light measurement using TDK optical sensor BCS2015G1 as shown in the Functional Block Diagram, the measurement range is from 10 to 20 000 lux when using a 100-kΩ resistor.

Figure 19. ADC Operation Block Diagram

Table 6. Boost DC-DC Converter Control

7.3.3.1 Boost Control

User can set the boost converter to standby mode by writing the register bit <en_boost> low. When <en_boost> is written high, the converter starts for 50 ms in low current PWM mode and then goes to normal PWM mode.

User can control the boost output voltage by boost output boost_output (05H) register.

If register value is lower than 5, then value of 5 is used internally. If register value is higher than 20, then value of 20 is used internally.

7.3.3.2 Adaptive Output Voltage Control

When automatic boost voltage control is selected using the <vout_auto> bit in the Control (06H) register, the user-defined boost output voltage is ignored. The boost output voltage is adjusted for sufficient operating headroom by monitoring all enabled LED driver outputs. The boosted voltage is adjusted so that the lowest driver voltage is from 0.85 V to 1.35 V when the LED output currents are below 30 mA and from 1 V to 1.5V when any LED current is above 30 mA. The output voltage range is from 5 V to 20 V in adaptive mode.

The adaptive voltage control helps saving energy by always setting the boost voltage to minimum sufficient value. It eliminates the need for extra voltage margins due to LED forward voltage variation or temperature variation. With very small brightness settings, when the PWM pulses in LED outputs are very narrow, the adaptive voltage setting gives higher than necessary boost voltage. This does not harm the overall efficiency, because this happens only at low power levels.

After reset the adaptive control is on by default. In stand-alone mode the adaptive output voltage is always used.

Table 9. Stand-Alone Mode Brightness Control

7.5.1.1 I²C Compatible Interface

7.5.1.1.2 I²C Data Validity

The data on SDA line must be stable during the HIGH period of the clock signal (SCL). In other words, state of the data line can only be changed when CLK is LOW.

Figure 24. I²C Signals: Data Validity

7.5.1.1.3 I²C Start and Stop Conditions

START and STOP bits classify the beginning and the end of the I²C session. START condition is defined as SDA signal transitioning from HIGH to LOW while SCL line is HIGH. STOP condition is defined as the SDA transitioning from LOW to HIGH while SCL is HIGH. The I²C master always generates START and STOP bits. The I²C bus is considered to be busy after START condition and free after STOP condition. During data transmission, I²C master can generate repeated START conditions. First START and repeated START conditions are equivalent, function-wise.

Figure 25. I²C Start and Stop Conditions

7.5.1.1.4 Transferring Data

Every byte put on the SDA line must be eight bits long, with the most significant bit (MSB) being transferred first. Each byte of data has to be followed by an acknowledge bit. The acknowledge related clock pulse is generated by the master. The transmitter releases the SDA line (HIGH) during the acknowledge clock pulse. The receiver must pull down the SDA line during the 9^th clock pulse, signifying an acknowledge. A receiver which has been addressed must generate an acknowledge after each byte has been received.

After the START condition, the I²C master sends a chip address. This address is seven bits long followed by an eighth bit which is a data direction bit (R/W). The LP5520 address is 20h when SI=0 and 21h when SI=1. For the eighth bit, a 0 indicates a WRITE and a 1 indicates a READ. The second byte selects the register to which the data is written. The third byte contains data to write to the selected register.

Figure 26. I²C Chip Address

w = write (SDA = 0)
r = read (SDA = 1)
ack = acknowledge (SDA pulled down by either master or slave)
rs = repeated start
id = 7-bit chip address, 20h when SI=0 and 21h when SI=1 for LP5520.

Figure 27. I²C Write Cycle

When a READ function is to be accomplished, a WRITE function must precede the READ function, as shown in the I²C Read Cycle waveform.

Figure 28. I²C Read Cycle

7.5.1.2 SPI Interface

The LP5520 is compatible with SPI serial-bus specification, and it operates as a slave. The transmission consists of 16-bit write and read cycles. One cycle consists of 7 address bits, 1 read/write (RW) bit, and 8 data bits. RW-bit high state defines a write cycle and low defines a read cycle. SO output is normally in high-impedance state, and it is active only when data is sent out during a read cycle. The address and data are transmitted MSB first. The slave select signal (SS) must be low during the cycle transmission. SS resets the interface when high, and it must be taken high between successive cycles. Data is clocked in on the rising edge of the SCK clock signal, while data is clocked out on the falling edge of SCK.

Figure 29. SPI Write Cycle

Figure 30. SPI Read Cycle

Table 10. EEPROM Contents

Table 11. EEPROM Memory Map

Table 12. EEPROM Pages

7.6.1.1 Register Bit Conventions

Each register is shown with a key indicating the accessibility of the each individual bit, and the initial condition:

rgb_control (00H) – RGB LEDs Control Register

current_control_R (01H) – Red LED Current Control Register

current_control_G (02H) – Green LED Current Control Register

current_control_B (03H) – Blue LED Current Control Register

pwm_brightness (04H) – Brightness Control Register

boost_output (05H) – Boost Output Voltage Control Register

control (06H) – Control Register

ADC_hi_byte (08H) – Analog Digital Converter Output, bits 8-11

ADC_low_byte (09H) – Analog Digital Converter Output, bits 0-7

r_correction (0AH) – Additional Brightness Correction Value Register for Red LED

g_correction (0BH) – Additional Brightness Correction Value Register for Green LED

b_correction (0CH) – Additional Brightness Correction Value Register for Blue LED

EEPROM_control (0DH) – EEPROM Control Register