AS3687 datasheet(35/55 Pages) AMSCO

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AS3687/87XM

Datasheet

www.austriamicrosystems.com/AS3687

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7.4.2 Function Testing for LEDs connected to the Step Up DCDC Converter

For LEDs connected to the DCDC converter (usually current sinks CURR1,CURR2 and CURR6) use the

following procedure:

Table

12 – Function Testing for LEDs connected to the DCDC converter

Step

Action

Example Code

Switch on the current sink for the LED string to be

tested (CURR1,2 or 6)

e.g. Test LEDs on CURR1:

Reg 01h <- 01h (curr1_mode=on)

Reg 09h <- 3ch (curr1 = 9mA)

Select the feedback path for the LED string to be

tested (e.g. step_up_fb = 01 for LED string on

CURR1)

Reg 21h <- 02h (feedback=curr1)

Set the current for step_up_vtuning exactly above the

maximum forward voltage of the tested LED string +

0.6V (for the current sink) + 0.25V; add 6% margin

(accuracy of step_up_vtuning); this sets the maximum

output voltage limit for the DCDC converter

e.g. 4 LEDs with UfMAX = 4.1V gives 17.25V

+6% = 18.29V; if R3=1M

Ω and R4 = open, then

select step_up_vtuning = 18 (Reg 21h <- 92h;

results in 19.25V overvoltage protection voltage –

see table in DCDC section)

Set stepup_prot = 1

Reg 22h <- 04h

Switch on the DCDC converter

Reg 00h <- 08h

Wait 80ms (DCDC_FB settling time)

Measure the voltage on DCDC_FB (ADC)

Reg 26h <- 96h (adc_select=DCDC_FB, start

ADC; Fetch the ADC result from Reg 27h and

28h)

If the voltage on DCDC_FB is above 1.0V, the tested

LED string is broken – then skip the following steps

(Code >199h)

Switch off the overvoltage protection (stepup_prot = 0) Reg 22h <- 00h

10.

Reduce step_up_vtuning step by step until the

measured voltage on DCDC_FB (ADC) is above 1.0V.

After changing step_up_vtuning always wait 80ms,

before AD-conversion

e.g.: Reg 21h <- 62h (step_up_vtuning=12): ADC

result=1,602V

11. Measure voltage on DCDC_FB

e.g. DCDC_FB=1.602V

12. Switch off the DCDC converter

Reg 00h <- 00h

13.

The voltage on the LED string can be calculated now

as follows (R4 = open):

VLEDSTRING = V(DCDC_FB) + I(step_up_vtuning) *

R3 – 0.5V (current sinks feedback voltage: VFB2).

V(DCDC_FB) = ADC Measurement from point 11

I(step_up_vtuing) = last setting used for point 10

e.g.: VLED = (1.602V + 12V – 0.5V) / 4 = 3.276V

14.

Compare the calculated value against the

specification limits of the tested LEDs

With the above described procedures electrically open and shorted LEDs can be automatically detected.

7.5

Analog-To-Digital Converter

The AS3687/87XM has a built-in 10-bit successive approximation analog-to-digital converter (ADC). It is

internally supplied by V2_5, which is also the full-scale input range (0V defines the ADC zero-code). For input

signal exceeding V2_5 (typ. 2.5V) a resistor divider with a gain of 0.4 (Ratioprescaler) is used to scale the input of

the ADC converter. Consequently the resolution is:

Table

13 – ADC Input Ranges, Compliances and Resolution

Channels (Pins)

Input Range

VLSB

Note

DCDC_FB

0V-2.5V

2.44mV

VLSB=2.5/1024

ADCTEMP_CODE

-30°C to 125°C

1 / ADCTC

junction temperature

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