AS3687 datasheet(34/55 Pages) AMSCO

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AS3687/87XM

Datasheet

www.austriamicrosystems.com/AS3687

1v3-4

33 - 54

7.4

LED Test

Figure

25 – LED Function Testing

4.7µF

CPOUT

2.2µF

ADC

Baseband

Processor

Interface

Detect Open LEDs

Detect Shorted LEDs

From DCDC

Step Up

Converter

From Charge

Pump

DCDC_FB

I(step_up_vtuning)

The AS3687/87XM supports the verification of the functionality of the connected LEDs (open and shorted LEDs

can be detected). This feature is especially useful in production test to verify the correct assembly of the LEDs,

all its connectors and cables. It can also be used in the field to verify if any of the LEDs is damaged. A damaged

LED can then be disabled (to avoid unnecessary currents).

The current sources, charge pump, dcdc converter and the internal ADC are used to verify the forward voltage of

the LEDs. If this forward voltage is within the specified limits of the LEDs, the external circuitry is assumed to

operate.

7.4.1 Function Testing for single LEDs connected to the Charge Pump

For any current source connected to the charge pump (CURR30-33) where only one LED is connected between

the charge pump and the current sink (see Figure 1) use:

Table

11 – Function Testing for LEDs connected to the Charge Pump

Step

Action

Example Code

Switch on the charge pump and set it into manual 1:2

mode (to avoid automatic mode switching during

measurements)

Reg 23h <- 14h (cp_mode = 1:2, manual)

Reg 00h <- 04h (cp_on = 1)

Switch on the current sink for the LED to be tested

e.g. for register CURR31set to 9mA use

Reg 10h <- 0Fh (curr31_other = 9mA)

Reg 03h <- 0ch (curr31_mode = curr31_other)

Measure with the ADC the voltage on CP_OUT

Reg 26h <- 95h (adc_select=CP_OUT,start ADC)

Fetch the ADC result from Reg 27h and 28h

Measure with the ADC the voltage on the switched on

current sink

Reg 26h <- 8bh (adc_select=CURR31,start ADC)

Fetch the ADC result from Reg 27h and 28h

Switch off the current sink for the LED to be tested

Reg 03h <- 00h (curr31_mode = off)

Compare the difference between the ADC

measurements (which is the actual voltage across the

tested LED) against the specification limits of the

tested LED

Calculation performed in baseband uProcessor

Do the same procedure for the next LED starting from

point 2

Jump to 2. If not all the LEDs have been tested

Switch off the charge pump

set chargepump automatic mode

Reg 00h <- 00h (cp_on = 0)

Reg 23h <- 00h

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