TPS61195 Colour Shift using SMBUS

Are you using the TPS61195 to perform analog dimming (i.e., vary the dc current through the LEDs to change the light intensity) instead of PWM dimming (i.e., vary the duty cycle of the maximum LED current through the leds to change the average LED current and therefore the light itensity)?  If so, then analog dimming of the LEDs can lead to a color shift especially at lower LED currents.  The TPS61195 can do PWM dimming with either external an PWM signal applied (Direct PWM Mode) or through SMBus using an internal PWM dimming frequency (No delay PWM).  See Table 1 on page 13 of the datasheet.

Hi

Thanks for the reply. From page 13 I currently have it set up using(below), so not analog dimming, unfortunately.

SEL1    SEL2    MODE               INTERFACE
VDDIO GND     No delay PWM  SMBus

I have tested a single led from the batch I am using on a bench PSU and it does seem to give the expected 5600k.

Were as the panel only gives 5600K or close at the lowest setting on SMBus, with a setting of about 10 as soon as it goes higher it gets into 7500K and stops at 7900k at full power.

Could it be anything to do with the ADJUSTABLE PWM DIMMING FREQUENCY (FDIM) also on page 13, this is currently set to the example standard of 953K causing an internal PWM of 210hZ. Would it help to change this to 100R and therefore have an internal PWM of 2Khz?

(I have tried this changing the FDIM to 100K resistor for 100R and it actually made it slightly worse) 

I have also tired changing the iset resistor to give 30ma instead of the current 20ma (also to give 12ma) both settings seemed to make it slightly worse.

Was wondering what effect the 10uH inductor could make as I notice its an EPCOS part that is specified, however it seems to meet all the required specs

Thanks again for your help.

One benefit of PWM dimming is no color shift because the maximum current through the white LED stays the same, only the average current is different and therefore the intensity.  I am not aware of any color shifting due to PWM dimming frequency.

If I am using SMBus, what logic level should the enable and dpwm lines be at

As if I use a logic 0 for DPWM, the device does not light the LED's, is this correct?

The other question I have is these devices are handsoldered.

Could this have effect there heat sink capabilities and in turn causing them to run differently than expected?

I have the TPS61195 EVm dev board so will try some other forms of driving the LEDS to see what effects this has.

Any ideas fro me to try please let me know.

EN must be logic high for the device to turn on.

DPWM must be logic high, indicating a duty cycle of 100%, or have a PWM signal applied with the duty cycle that relates to the LED current that you desire.

Handsoldering is not recommended and could be causing to the IC to operate unexpectedly.