ALED[7:0]

EN_ALED

8-Bit IDAC

LP55281

SNVS458B – JUNE 2007 – REVISED JUNE 2007

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ALED Driver

LP55281 has a single ALED driver. It is a constant current sink with an 8-bit control. ALED driver can be used as

a DC current sink or an audio synchronized current sink. Note, that when the audio synchronization function is

enabled, the 8-bit current control register has no effect.

ALED driver is enabled when audio synchronization is enabled (EN_SYNC = 1) or when ALED[7:0] control byte

has other than 00h value.

ADJUSTMENT OF ALED DRIVER

Adjustment of the ALED driver current (Register 0Ch) is described in table below:

ALED[7:0]

Driver Current, mA

(typical)

0000 0000

0

0000 0001

0.06

0000 0010

0.1

...

...

1111 1101

14.8

1111 1110

14.9

1111 1111

15

With other than values on the table, the current value can be calculated to be (15.0 mA / 255) * ALED[7:0],

where ALED[7:0] is value in decimals.

LED Test Interface

All LED pin voltages and boost output voltage in LP55281 can be measured and value can be read through the

the LED outputs or boost output for measurement. The selected output is connected to the internal ADC through

a 55 k

Ω resistor divider. The AD conversion is activated by setting the EN_LTEST bit to '1'. The first conversion

is ready after 128 µs from this. The result can be read from the ADC output register (address 13h). The device

executes the AD conversions automatically once in every 128 µs period, as long as the EN_LTEST bit is '1'.

User can set the preferred DC current level with the LED driver controls. The RGB drivers' PWM must be set to

100%, or otherwise there can appear random variation on results. Note, that the 55 k

Ω resistor divider causes

small additional current through the LED under measurement.

ADC result can be converted into a voltage value (of the selected pin) by multiplying the ADC result (in decimals)

with 27.345 mV (value of LSB). The calculated voltage value is the voltage between the selected pin and ground.

The internal LDO voltage is used as a reference voltage for the conversion. The accuracy of LDO is ± 3%, which

is defining the overall accuracy. The non-linearity and offset figures are both better than 2LSB.

18

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