ADC

20

LP55281

SNVS458D – JUNE 2007 – REVISED OCTOBER 2016

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With values other than those in Table 7, the current value can be calculated to be (15 mA / 255) × ALED[7:0],

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

Figure 15. Principle of LED Connection to ADC

7.3.4 LED Test Interface

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

SPI/I2C compatible interface. MUX_LED[3:0] bits in the LED test register (address 12h) are used to select one of

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 PWM of the RGB drivers must be

set to 100% — otherwise random variation can appear 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.

Table 8. LED Multiplexing (Register 12h)

MUX_LED[3:0]

CONNECTION

0000

R1

0001

G1

0010

B1

0011

R2

0100

G2

0101

B2

0110

R3

0111

G3

1000

B3

1001

R4

1010

G4

1011

B4

1100

ALED

1101

—

1110

1111

Boost output