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LM3423-Q1: Current control problems in ILED

Part Number: LM3423-Q1

Hi TI,

I made the following circuit configuration.

The current problem is that the current of ILED is set at 290mA but it is output at 520mA.

<setting value>

- ILED = VSNS/RSNS = 1.24V/RSNS*RHSP/RCSH
- LED Current : 290mA
- RCSH = 12.4K ohm, RHSP = 1K ohm
- ILED = (1.24/RSNS) x (1K/12.4k) = (1.24/RSNS) x 0.081
- RSNS = (1.24/ILED) x 0.081 = (1.24/0.29) x 0.081 = 0.346 ohm
- Applied resistor power rating : PR =0.29*1.24 = 0.3596
- 0.3596 * 2 = 0.692W [Double the margin]
- SMD 3216 Resistor : 1/4W[0.25W] *3 =0.75W
- 0.346ohm * 3 = 1.038ohm

- ILIM = 245mV/RLIM
- ILIM = about 3.2A
- RLIM = 245mV/3.2A
= 0.075 ohm
= 0.075 * 2 = 0.150 ohm

- OVP
ROV1 = 15.4K, ROV2 = 511K
VTURN-OFF = 1.24V * {(15.4K+511K)/15.4K}
= 1.24V * (34.18/15.4K) = about 42.39V
- Switching Frequency
RT = 120K, CT = 1nF
Fsw = 25/(120,000*0.000000001)
= 25/0.00012 = about 208 kHz

  • Hello BJ,

    Do you have any waveforms you could share? Possibly Q2 drain and the HSP-HSN voltage to verify it is correct? I would like to see the current sense voltage particularly because I don't know how it will behave with ferrite beads. There could be an oscillation with the filter cap.

    Have you tried shorting L7 and replacing L6 with a 10ohm resistor to see if the beads are causing it? Is this on a prototype PCB or are you using a modified EVM?

    Thanks,

    Clint