LM51772: current limiter mode

Hi Stoil,

The engineer is out of office today. Please expect a delayed reply.

Best Regards,
Feng

Hi Stoil,

Thanks for using the E2E forum.

To activate the current limiter via I2C, follow the following configurations.

1. Connect a capacitor or a type 2 R-C network on ILIMCOMP/ISET pin, if the current limit is set by the internal DAC.

2. The current monitor block limiter operation is activated via MON_LIMITER_EN bit. Set it to 0x1 to enable it. (See table 8-10 in datasheet)

3. The value for current limit for the DAC is set by the ILIM_THRESHOLD register. See table 8-4 in datasheet.

(www.ti.com/.../lm51772.pdf

Yes, you can set the output voltage to 50V and the output voltage will regulate according to the current limit threshold. You will see the output voltage drop when the load current is higher than the current limit. 

Best Regards,

Hassan 

Hi, Hassan. I try but don't work. What about pin CDC? Is it important for current regulator or is just for current monitoring ?

You wrote - "Connect a capacitor or a type 2 R-C network on ILIMCOMP/ISET pin, if the current limit is set by the internal DAC". What mean last one - if the current limit is set by internal DAC? . I need to do it. How to do it ?

Hi Stoil,

The CDC pin is use for Cable drop compensation or current monitor, but it is not important for current regulation. Connect it to GND, if it is not in use. 

Current limit set by the internal DAC means you are using the I2C to use internal Digital to Analog converter to set the current limit. 

In order to set current limit via DAC (I2C), you need to follow the steps mentioned in my aforementioned reply.

Can you please share your schematic?

Best Regards,

Hassan  

1. How can share pdf ? 

PS: I find some error in my PCB , the pin ISNSP was not connected. Now is connected but have other problem. When is set IMON_LIMITER_EN, output voltage is zero. When is not active this bit the voltage regulator work well.

I can't share file . I don't know how to do it.

Seems like current limiter is always in over current mode. But I try with short-cut between ISNSP and ISNSN and the result is the same.

Hi Stoil,

You just need to pick and drop the file in reply section. It will upload automatically.  

Best Regards,

Hassan 

Note: C4 on BIAS pin is 0 ohm resistor 

Hi Hassan, the problem was SEL_ISET_PIN - this bit was not disabled. So now current limiter work. One more Q:

When i increase output voltage register , the output voltage stops increase when is close to input voltage (power) and again start increase when value of register correspond  around  4v  more. For example: Input Voltage is 12V. When increase output voltage register to correspond value 12v and keep increase it the output voltage stops increase. And when voltage register correspond to around 17v then output voltage keep again increase. But then according register must be 17v or more but output voltage is 12 or more. 

PS: Current is in times lower than limit.

Hi Stoll,

If I understand correctly, the issue is that the output voltage is not increasing more than input voltage. So your converter is only working in buck topology.

Am I correct?

Best Regards,

Hassan 

NO. work in both modes. but close to power voltage stops working and keep as output voltage the input one. When increase voltage register to 17v then starts again to increase voltage.