AS3676 datasheet(65/92 Pages) AMSCO

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AS3676

Datasheet - D e ta i l e d D e s c r i p t i o n

8.6.2

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 pro-

cedure:

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

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 charge pump automatic mode

Reg 00h

 00h (cp_on = 0)

Reg 23h

 00h

Table 103. 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_current = 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 (step_up_fb=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 R2=1M

 and R3 = open, then select

step_up_vtuning = 18 (Reg 21h

 92h; results in

19.25V over voltage protection voltage – Table 9 on

page 14)

4Set step_up_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 over voltage protection

(step_up_prot=0)

Reg 22h

 00h

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

Measure voltage on DCDC_FB

e.g. DCDC_FB=1.602V

Switch off the DCDC converter

Reg 00h

 00h

The voltage on the LED string can be calculated

now as follows (R4 = open):

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

R2 – 0.5V (current sinks feedback voltage: VFB2).

V(DCDC_FB) = ADC Measurement from point 11

I(step_up_vtuning) = last setting used for point 10

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

Compare the calculated value against the

specification limits of the tested LEDs

Table 102. Function Testing for LEDs connected to the Charge Pump

Step

Action

Example Code

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