Issue on Customers board Even though L was
Issue on Customer’s board : Even though L was increased to 330 u. F ( theoretically, 250 u. F 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 TPS 92518 EVM , and customer’s issue was verified. ( page 2~ on this ppt file) I had verified as customer saying.
24 V 1) 2) 3) L=330 u. F (increased from 150 u. F) C# set to 1000 p. F Optimize MAXOFF Not resolved - When VIN was changed from 24 V to 25 V Or from 24 V to 22 V, the flicker still exists. - When flywheel diode(D 4) were high temp (add heat to D 4), the flicker exists. - Shunt FET frequency decreased to 800 Hz, but if duty was changed, flicker still exists TPN 1200 APL(TOSHIBA) Rdson=20 mohm Flywheel diode SL 310 A 330 u 220 p/100 C# = 1000 p 3) Add resistor ~0. 5 ohm on Shunt FET source. Not perfect ( if VIN were changed, SW waveform move around, but the flicker seems significantly improves. 10 K MCU 1 JHz ~800 Hz SL 310 A PWM pull up to 3. 3 V TPN 1200 APL(TOSHIBA) Rdson=20 mohm Vf=12 V Drain resistor =0. 25 ohm
L=330 u. H shunt freq=800 Hz VIN=26 V, Peakth=150(MAX 1 A), TOFF=72, MAX OFF=82(optimized) duty 99. 6%(Iled=21 m. A(av. ) on fluke 179 tester) Fig 1 L=330 u. H shunt freq=800 Hz VIN=26 V, Peakth=150(MAX 1 A), TOFF=72, MAX OFF=82(optimized) duty 99. 6%(Iled=21 m. A(av. ) on fluke 179 tester) Add resistor on shunt FET source SW Iinductor Small pick due to resistor added VLED Flicker Around 6~7 Hz at VIN=26 V (if chage to VIN=24 V, no problem) No Flicker At VIN=26 V Fig 2
Same condition as Fig 2, but VIN is back to VIN=24 V Fig 3 The waveform at SW pin still has disturbance ( jitter ) but not visible as Flicker At VIN=24 V
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? https: //e 2 e. ti. com/support/power-management/f/196/t/502316? tisearch=e 2 e-sitesearch&keymatch=flicker 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 (u. H) 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? Question 3 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 20 mohm compared with the FET in EVM user’s guide: FCD 5 N 60 TN 0. 86 ohm) Question 4 Do you think flywheel diode SL 310 A is no problem? https: //goodarksemi. com/docs/datasheets/schottky_rectifiers/SL 32 A-SL 310 A. pdf When customer add high temp to flywheel diode, the flicker increased.
- Slides: 6