LM76202-Q1: Actual required R_ILIM resistor value does not match the datasheet

Hello Jakub,

Find the recommended operating conditions for RILIM. It should be less than 120 Kohms.

Use below design calculator to find out RILIM values and other design parameters.

https://dr-download.ti.com/design-tools-simulation/calculation-tool/MD-9nGUExAPle/1.1/LM76202-Q1_LM74202-Q1_Design_Calculator.xlsm

Thanks 

Amrit 

Hello Amrit,

Thank you for your prompt answer and the design document.

To the table you sent over: My resistor R_ILIM is within the specification. It is 88 kΩ, so between 5.36 kΩ and 120 kΩ. You refer to is that the calculation according to equation 5 would give a value over the higher limit. It might mean I'd need to increase the limiting current in the design, which I could do if the need arises, but as I said, My R_ILIM is not out of specification at the moment.

The Excel worksheet you sent is neat and would have helped with the design, but it is not for the circuit I use. In the excel sheet, MODE is connected to GND. In my case, MODE is left floating. For the schematics in the excel sheet, I think equation 4 applies, which is exactly what cell D32 is showing.

I am out of ideas as to why the current-limiting resistor is not what the datasheet says. I am wondering if it has anything to do with lack of resistor between IMON output and ground in my circuit. Although, the datasheet explicitly says I can leave it floating. The other potential source of conflict could be that I have a 100uF load capacitor. However it is important to note that there is no load applied to the output during the capacitor-charging phase, so the capacitor inrush and load currents are not summed in my situation and my tests verify that.

I wonder if you could have another look at my problem. Like I said, I am happy to send over my test report or do more tests at your instruction.

Kind regards. Jakub

Hello Jakub,

Test result will help. You are getting different ICB value than set value?

What's the load you are using?

Thanks

Amrit 

Hi Amrit. That is exactly the case. I_(CB) in my design is between 110 mA and 115 mA. This was verified through different test as shown in the attached report. The the resistor R_ILIM is 83 kΩ. According to Eq 5, the I_(CB) should be ~270 mA with this resistor. I am trying find an explanation for this discrepancy.

PROJ-LED Current-Limiting Fuse Problem-170625-093327.pdf

Hello Kakub,

With 85K RILIM, ICB= 0.171 mA. It's not 270mA. I am not sure if I am missing something.

Are you testing with actual LED load or E-load?

Thanks

Amrit 

Hi Amrit. You're absolutely right. I made a mistake in the report. It is ~170 mA, not ~270 mA (new report attached). This brings it much closer to the ~110 mA, but still is ~150 % of what I measure.

Having said that, I look at the datasheet and see the following:

The circuit-breaker current with R_ILIM = 120 kΩ is specified to be typically 0.073 mA, but could be as little as 0.045 mA. It means the typical current is ~160 % of the minimum current. This would mean that my circuit performance is still within the margin of the acceptable error. I think I could mark it as resolved therefore.

I don't know what you mean by "E-load". In the different dynamic and static tests I measured specifically the LED current, but I also measured the total power consumption of the circuit. They were found to be within <1% of each other.

Question: All four of my chips gave very similar results (within ±2% of the mean). Is it expected that chips from the same production batch will have a very similar performance, but different batches could lead to the large spread specified by the datasheet (±50%)?

PROJ-LED Current-Limiting Fuse Problem-190625-081330.pdf

Dear Amrit.

I found another problem with the part and its datasheet.

• Equation 5 (page 19) is: I_(CB) = 12 / R_(ILIM) + 0.03 A. Let's do this calculation for R_(ILIM) = 120 kΩ and we get I_(CB) = (12 / 120) A + 0.03 A = 0.13 A.
• Scroll up to page 5 and at the bottom of the table. It says for R_(ILIM) = 120 kΩ, the typical value of I_(CB) = 0.073 A.

Let's do the same calculation for R_(ILIM) = 5.36 kΩ:

•  I_(CB) = (12 / 5.36) A + 0.03 A = 2.27 A.
• In table on page 5 says for R_(ILIM) = 5.36 kΩ, the typical value of I_(CB) = 2.21 A.

Is there a mistake in equation 5 and instead of a plus (+) there should be a minus (−)?

In both instances, the above calculations result in 0.06 A larger values than shown in table on page 5. If there was a minus (−) in the equation, the results of the equation and the table would match.

Also my circuit would behave exactly according to the equation: I measured ~0.113 A, the corrected equation 5 with a minus sign predicts ~0.115 A.

Could you please look into this with extra care? I suspect the datasheet is wrong and should be corrected.