TPS92518: TPS92518 PWM DIMMING

Hello Tarun,

1) Is this the 700 mA design increased to 1.2A? Was the design verified to be able to operated to 1.2A? Do you have the protection diodes, 8.3.3.1 in the datasheet, to protect the IC from ringing? What is failing when the TPS92518 gets damaged? Just the IC, the MOSFETs, anything else?

2) Minimum frequency will have to do with visual effects and possible the boot capacitor discharge, you can go quite low. Maximum will be limited by inductor slew and linearity. The inductor current can only ramp up and fall so fast and those rates depend on the applied Von and Voff, for example if Vin is 48V and Vled is 40V the applied on voltage is 8V, the off voltage is 40V+Vfd (freewheel diode). Of course these vary with current, temperature as well as other circuit variables.

Best Regards,

Hi Irwin,

Thanks for the prompt reply.

Design is same. Actually I made design for up to 2.5 A max current as per TI's reference guide. Till now i am using 700mA Max current only. Right now I want to use 1.2 A  Current. 

I haven't used protection diodes as it is told for Shunt Dimming method. Do i need to use it for PWM Dimming and Analog Dimming also?

When IC gets damaged, only TPS92518 IC gets damaged. only that channel is not working and IC got heated. switch node pin gets shorted to GND.

Currently  I am using Vin = 24V, VLED = 20V, Rsense = 0.1 Ohm, INDUCTOR = 100uH, 5 A rating, 

for 1.2 A i am setting Toff = 54, DAC thresold = 120, and PWM freq 20 KHz.

What should i need if I want to drive VLED = 24V same as the input Voltage? Do i need to increase Vin ?

Regards,

Tarun Patel 

Hello Tarun,

I would try the diodes, not sure why it's failing however if the IC is the only failed part it could be because of PWM dimming and damage caused by the output ringing.  If you read the section describing the addition of the diode:  "During shunt dimming, ringing may occur at the channel output due to PCB and device parasitic capacitances and inductances. This should be checked as part of the design process. If the ringing approaches the absolute maximum of any pin, a clamping diode must be added to the design. Connect the diode anode to the output at VLEDx and the cathode to the input voltage. This protection must also be used if the LED load is ever to be connected or removed while the output is enabled."

Another question is how much capacitance do you have across the LEDs?  Are you doing anything to the output besides just turning it on and off?  What does the output voltage look like when PWM dimming?  What does the inductor current look like when PWM dimming?  When does it fail?

It would be easiest to increase the input voltage.  A different topology can be use however a higher input voltage is probably easiest, or reconfiguring the LEDs to a lower stack voltage.

Best Regards,

Hi Irvin Sir,

Ok, I will connect the diodes as shown in Figure 12.  ( I have ordered it)  Additional Circuitry Required for Shunt FET Dimming for output ringing.
I need to connect Dshunt(1N4148W-7-F) diode as shown in figure to both the channels. right?

I am using 100 uH, 5A Inductor and Clamping didode instead of Schottkey diode(D3,D4 as per Datasheet schematic). Is it ok to use this ?

Regards,
Tarun

Hello Tarun,

Yes, two diodes from each LED+ (anode) to Vin (cathode).  They have to be able to handle the peak current (maximum operating current). Again, there are a lot of possible reasons.

D3 and D4 are the freewheel diodes, what part number are you using for those?  They need to be able to handle the freewheel current.

The 1N4148 can have a fairly high peak current rating.  Since this is being used just to clamp parasitic ringing the power dissipation will be very low but the peak current can be as high as the output current.

How quickly does the IC fail in your application?

Best Regards,

Hi Sir,

I replaced clamping diode with Schottkey Diode as per reference design (D3 and D4). Previously I have used SMAJ188CA diode.

After using Schottkey diode, i think, problem is solved. I am testing now for different LEDs. But As of now, i got the Solution.

Do i still need to use Dshunt(1N4148W-7-F) diode?

Thanks for the prompt replies.

regards,

Tarun Patel