Hello
My customer had still random flicker issue for their board when approx. 0.1% duty.
I had recommended customer to have a resistor to shunt FET source ( So, or higher Rdson of shunt FET), and
it seems effective .
So, customer want to have your advice to proceed innext step design (for pilot run).
Would you please advise about my 4 questions?
Issue on Customer’s board :
Even though L was increased to 330uF ( theoretically, 250uF is enough as per customer)
and MAXOFF were optimized, but at 0.1% duty (shunt FET is 99.9% on) and following,
customer has visible random flicker.
when
VIN is varied
Ambient temperature changed
Verification
So, I had followed using TPS92518EVM , and customer’s issue was verified. ( page 2~ on attached ppt file)
I had verified as customer saying.
Question 1
The flicker exist if duty is slightly changed or VIN were changed or ambient temp changed.
It seems the “ sweet spot “ on approx. every 0.5% duty.
But, do you know the mechanism of this phenomenon?
I ‘ve found another post as following.
Do you agree this is the mechanism?
e2e.ti.com/.../502316 shunt
>I think the main problems are disturbance when switching on the shunt dim transistor and high input voltage together
>with low output voltage when shunt dimming.
>The disturbance will vary a bit depending on when in the regulation cycle the shunt dim transistor is switched on.
>This is probably where the randomness comes from.
>Worst is when the high side transistor is conducting and it is at the end of the on-time.
>At this time the current is already at max through the inductor and there is a short delay between HG pin low and high side
>switch completely off which means it can still conduct when the shunt transistor activates.
>This together with the high current in the inductor results in a short high peak current which can disturb the regulation
>and directly start a new charging cycle.
>
>When input voltage is high and output voltage is low while shunt dimming, the resulting minimum on-time,
>which is limited to a minimum value by the driver, is still too long which results in too high peak current through the inductor.
>I will try to put a resistor in series with the shunt transistor to limit the peak current.
Question 2
Do you have a criteria for Maximum vale (uH) for inductor?
We know eq (41) in data sheet is minimum.
Customer think as large as possible seems good result ( good for random flicker) ,except
size/cost but, but how do you think?
Question3
Do you agree my corrective action to add resistor in shunt FET source for bigger VLED?
The shunt FET which customer is using is very low Rdson 20mohm compared with the FET in
EVM user’s guide:FCD5N60TN 0.86ohm)
Question4
Do you think flywheel diode SL310A is no problem?
goodarksemi.com/.../SL32A-SL310A.pdf
When customer add high temp to flywheel diode, the flicker increased.
Thank you for your cooperation.
Best egards
