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# LM3429-Q1: Is current limit

Part Number: LM3429-Q1
Other Parts Discussed in Thread: LM3429

Hi,

I'm designing a LED driver with the LM3429 as a boost:

Vin=12V.

Vo = 15 to 35V (typical: 29,5V).

Io = 0.4 - 1A. (analog dim with i2c digital potentiometer)

I don't quite understant when Is works:

Rlim = 0,08 ohm, so it limites at 3.06A.

FET current It max = ( Iled Dmax / (1-Dmax) ) + 10% = 2,86 A.

There would be a difference with and without (Is connected to PGND) Rlim in normal operation? Or during startup?

What about EMI? Is it better to limit the current or then it would lose efficiency so would be a worse case?

Is there a rule for max current limiting?

• Hello Joan,

When calculating Rlim you need use the minimum Vlim threshold of 0.215V. You also have to include the inductor ripple current and efficiency.

I do not understand "There would be a difference with and without (Is connected to PGND) Rlim in normal operation? Or during startup?" During start-up, if the bandwidth is high and there is a large output capacitor it can hit the peak current limit, though that shouldn't be an issue.

Limiting the current will limit the output power, efficiency doesn't have anything to do with that. If you set Rlim to 0.2 ohm you just limit how much power you will be able to get out of it with a fixed 12V input.

If you need to have 35V at 1A the Rlim will not change the EMI. Rlim is for peak current protection and the control loop. Peak current protection should not be functioning in the normal operating range.

Maximum current limit should be to protect the devices such as the MOSFET. It should be set high enough that it doesn't trip Ilim during normal operation. This requires knowing the inductor ripple current, efficiency, using 0.215V or Ilim and adding margin (this can be much more than 10% depending on your design.

Regards,