Other Parts Discussed in Thread: TPS2662
Hi All,
We are counter a problem to calibrate the LM5069-1 and hope you can help understand the problem.
I’m using the excel calculator from TI website and trying to calibrate the IC overcurrent and power dissipation protection.
The design is attached (ignore the values there, it’s just for bring-up) , basically we have 6 - 48V rails with the LM5069-1 on them and DC-DC from 48V to 12V or 5V after the LM5069.
System requirement –
- 48V power in
- Max load –
- Overcurrent protection at 15W/20W Load
- Power dissipation protection – our transistor for this design is up to 1.3W (we will replace for larger in the updated design)
First problem is the big differences between the calculations and the actual measurement for the overcurrent protection.
For 100mR resistor the calculator point that the typical detect current limit is 0.6A with 0.2A pk-pk but the results are very different.
Also with lower value resistor 75mR, typical should be 0.7A with 0.2A pk-pk but the actual measurement are way off.
|
Measured on board |
Excel calculator |
||||||
|
Rsense [ohm] |
Isense [A] Avg |
Current PK-PK [A] |
V load [V] |
I load [A] |
W load [W] |
Isense [A] Typ |
Current PK-PK [A] |
|
0.1 |
0.424 |
0.3 |
4.88 |
3.3 |
16.1 |
0.6 |
0.2 |
|
0.075 |
0.562 |
0.4 |
4.76 |
4.21 |
20.0 |
0.7 |
0.2 |
Second problem is with the power dissipation resistor. There isn’t limit on this resistor according to datasheet but low value resistors aren’t allowed the gate to open up.
For example, according to calculations, a 100mR resistor allowed minimum dissipation of 0.7W which mean 8.75kR but we calibrate it to 1.3W which mean 16.25kR.
And for 75mR resistor allowed minimum dissipation of 0.9W which mean 8.45kR but we calibrate it to 1.3W which mean 12.2kR.
For both situations I tried a lower resistor down to 10kR and a bit larger resistor 20kR but the system doesn’t work with both, only with larger resistor which mean no protection.
It’s look like this IC cant work well with low power system?
Why on the website the minimum detection current is 0.01A? in what conditions?
please your help
thank you,
Aviv.
