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LM3492HC: Current limit ICL= ICL(max) – 4290 × IILIM

Part Number: LM3492HC

Hi all

Would you mind if we ask LM3492HC?
There is the description "Current limit  ICL= ICL(max) – 4290 × IILIM" on the datasheet P13.

<Customer's condtion>
Vin : 12V
Vout : 31.2V/each out 230mA(Iout1=Iout2=230mA)
L=15uH
Fsw=414kHz

<Question>
Our customer would like to configfure Iimitation=0.67A(=ICL).
So, according to equation,
0.67= 3.9 – 4290 × IILIM(5.5(VCC)-0.7/Rlim)
Rlim=6.4kohm

In case of Rlim=6.4kohm,
-Vout=29V, IOUTs did't output 230mA.
-Switching frequency was more than 1MHz.

So, if Rlim was 10kohm(1.84A), operation was normal condtion as follows;
-Vout=31V, IOUTs output 250mA.
-Switching frequency was 414kHz.

Therefore, the customer would like to know how much RLIM is correct for Customer's condtion and formula for it.

We assume that ICL=0.67 is difficult to configure it, because,,,
Vin=12V, Vout=31.2V/ Iout=230mA×2
-31.2V×0.46A×100/80(assumption of the worst efficiency)=17.942W
-17.942/Vin=12V=1.495A
So, in order to configure Vout=31.2V Iout=230mA×2, we assume that ICL requires more than 1.5A limitation.
Is it correct?

Kind regards,

Hirotaka Matsumoto

  • Hello Hirotaka,

    Where did ICL = 0.67 come from? If Vin is 12V and Vout is 31.2V at 460 mA the input current needs to be more than 1.2A due to efficiency and DHC. The 10 Kohm is fine to use, it's just the current limit of the boost regulator. It will also depend on how low the 12V can go.

    Also, I would be using Vcc max for the calculation.  5.5V is typical, 6.3V is maximum, after subtracting 0.7V it become 4.8V and 5.6V which would drop IC to 1.5A from 1.84A.

    Best Regards,