LP5520 datasheet(16/46 Pages) TI1

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2:1

MUX

AVERAGE

S1_RESULT

S2_RESULT

1:2

MUX

S1_IN

S2_IN

12 BIT ADC

LP5520

SNVS440B – MAY 2007 – REVISED MARCH 2016

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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-k

Ω resistor.

Table 5. ADC Configuration

adc_ch(bit5)

S1 input can be read

S2 input can be read

comp_sel(bit4)

S1 input is used for compensation

S2 input is used for compensation

Figure 19. ADC Operation Block Diagram

7.3.3 Magnetic High-Voltage Boost DC-DC Converter

The LP5520 boost DC-DC converter generates a 5-V to 20-V supply voltage for the LEDs from single Li-Ion

battery (2.9 V to 4.5V). The output voltage is controlled with four bits in 18 steps. In adaptive mode the output

voltage is automatically adjusted so that the LED drivers have enough voltage for proper operation. The

converter is a magnetic switching PWM mode DC-DC converter with a current limit. Switching frequency is 1

MHz. Boost converter options are controlled with few bits of Control (06H) register.

Table 6. Boost DC-DC Converter Control

<en_autoload> (bit 3)

Internal boost converter loader off

Internal boost converter loader on

<vout_auto> (bit 2)

Manual boost output adjustment

Adaptive boost output adjustment

<en_boost> (bit 1)

Boost converter standby mode

Boost converter active mode

<nstby> (bit 0)

LP5520 standby mode

LP5520 active mode

The LP5520 boost converter uses pulse-skipping elimination to stabilize the noise spectrum. Even with light load

or no load a minimum length current pulse is fed to the inductor. An active load is used to remove the excess

charge from the output capacitor at very light loads. Active load can be disabled with the <en_autoload> bit.

Disabling active load increases slightly the efficiency at light loads, but the downside is that pulse skipping

occurs. The boost converter must be stopped when there is no load to minimize the current consumption.

The topology of the magnetic boost converter is called current programmed mode (CPM) control, where the

inductor current is measured and controlled with the feedback. The user can program the output voltage of the

boost converter. The output voltage control changes the resistor divider in the feedback loop.

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