LM3481: Inductor peak current controller

And also I have pulled down FB pin to ground and pulled up COMP pin via 4.7k ohm from VCC pin as in the EVM schematic. In my application, there is no output voltage to monitor and all it has to do is to control the pulse width based on the current flowing through to 0.05ohm current sense resistor. 

Subash,

Thanks for reaching out with your question.

Are you trying implement a boost controller? It is not clear from the picture as there is no output capacitance and the diode just feeds back into the VIN pin.

Thanks,

Garrett

Hi Garrett,

                         Thanks for your reply. And we are not trying to implement boost converter. We are trying to control the load current by placing inductor in series with mosfet and load. The controller has to control mosfer based on the current flows through current sense resistor. During mosfet off time, whatever enery the inductor stored, it will freewheels thorugh  load  via freewheeling diode.

And also brief us how PWM comparator inside lm3481 works? Is the output voltage necessary to control PWM or can we control peak current alone?

Subash,

I suggest removing the resistor between the VCC pin and COMP. When FB is pulled low COMP should go to the maximum voltage.

The PWM comparator will set turn the switch on when at the beginning of the clock cycle and when the current sense signal reaches the COMP pin voltage the switch turns off. This is the basic operation of peak current mode control.

I suggest probing the current sense signal to ensure the peak current limit is not already tripped at the begging of the clock cycle. This might be why the swtiching frequency is low as well. There is frequency fold back during long periods of over current conditions.

Thanks,

Garrett

Garrett,

Thanks for your response.

When FB pin is pulled low, Vcomp voltage would be 2.7V. So are you meant to say that when ISEN pin reaches to 2.7V, then it will turn off the pulse?. But at ISEN pin maximum voltage would be 200mV as per the datasheet. And also is there limit for current (maximum current) to control or is it purely depends on sense resistor voltage?

Regards,

Subash

Subash,

Current limit will trigger when the ISEN pin is around 160mV. This will terminate the pulse from the gate driver. Based on the threshold and the selection of the current sense resistor the maximum peak current is set.

Thanks,

Garrett

Garrett,

Thanks for the information. I have brought up the ic and I have got 85% duty cycle at 211khz when DR pin is floating but when DR pin is connected to gate resistor(10 ohms) of the MOSFET, the pulse width is very low and frequency is also changed to 27.9khz. And also I have varied gate resistance value like 10R,22R,33R but the result was the same. Both the pictures are attached for your reference. Please clarify.

Garrett,

And also please clarify that can lm3481 be used for the above circuit?

Thanks,

Subash

Garrett,

Please do the needful. I have been waiting for your reply

Thanks.

Garrett,

Now I'm using 10m ohm current sense resistor. So do you want me to reduce further?

Thanks,

Subash

Garrett,

And also I have  checked it  with mosfet drain floating. Same thing happened. Moreover,could please share me document where I can get information about frequency fold back mode in lm3481 .

Thanks,

Subash

Garrett,

            In my application, the load resistance is less than 1ohm and I have to give 20A continuous current to the load from the battery through the series inductor with MOSFET to control the current flow. So current through an inductor can be controlled by any current mode PWM IC. So kindly suggest me the controller which matches my requirement. vin= 3.3 to 5V  vout = 3.3-12V Iout=20A.

Thanks,

Subash

Garrett,

I'm trying to create a constant current converter by maintaining the average current of the inductor. And i tested my circuit , mosfer pulses are not going to zero completely at turn off time. It's shows some offset around 1.5V  during turn off time. How to pull the DR pulse to ground during turnoff time? I have connected 10 ohm gate resistance. Should I reduce it?

Thanks,

Subash