Dimming function of ADD5203

Can you reply this question?

Our customer is still waiting the answer to use or not !!

Thanks

Hi Kaos,

Sorry for the delay.  For some reason, your initial post didn't show up on Engineer Zone until today. 

Q1:  No.  It can do both reduction and growth.  The reduction shown in Figure 18 was just for example.  We don't play any tricks on the customer! 

Q2:  The total power dissipation (Pdiss) of the part is: 

     Pdiss = Vin*Iq + N * VFBx * ILED

          where Iq is the current consumption of the part (4.2mA typ)

          N = number of active sinks (between 1 and 8)

          VFBx is the average voltage on the active FBx pins.  For ILED=20mA, this will be something >0.65V (it depends on the voltage mismatch between the LED strings)

          ILED is the LED current

For example, Pdiss might be:

     Pdiss = 12V*4.2mA + 8 * 1.0V * 30 mA = 290 mW

This Pdiss is then used to calculate the die temperature, using the theta JA given in Table 6.  The exposed pad of the ADD5203 helps it to easily dissipate this power without heating the part beyond the thermal limits.  So in general, the internal heating of the IC is not an issue. 

Best Regards,

- Jon

Hi Jon,

Thank you for your kindly answer.

Here is one additional question,

Can ADD5203 drive max 2A/1 LED by ganged FB1+FB2+.......FB8?

Thanks Kaos

Hi Kaos,

No, the maximum current through 1 LED is 8x30mA=240mA.  For 2A of LED current, you could use the ADP8140.  Or a standalone boost converter (like the ADP1621) with some external modifications.  I am writing an app note on how to do that, but I can send you an advance copy if you are interested. 

Best Regards,

- Jon