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Current limit Problem ?

YS SHIN
Prodigy 10 points

Dear. Valuable Supplier,

My name is YS. Shin at R&D department.
Recently, I am making a design for DC_DC BUCK CONVERT using TPS40170Q PWM IC of TI Company.
And I hope to hear your valuable advice.

Our required specification is like the below.
 - Input  voltage : 18V ~ 32V
 - Output voltage :  13.5V
 - Output current :  11A ( Rated )
 - Output current limit  :  about 13A

At the present, it is being designed with TPS40170Q + IPP80N06S4-07 + 10uH Coil.

Here are my questions.

Q1. How can I increase the limit of current point?

    The current limit is being operated with 9.5A when the R3(ILIM) is 24.9k.http://e2e.ti.com/cfs-file.ashx/__key/communityserver-discussions-components-</p> <p>&nbsp;&nbsp;&nbsp; If the R3(ILIM) resistance is changed,<br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Using higher value : the current limit point didn&#39;t increase.<br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Using lower value : the current limit point was decreased.<br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; R3&nbsp; :&nbsp; 5K&nbsp;&nbsp; (&nbsp; current limit point = about 5A )</p> <p>Q2. Should I use lower value part for Rds(ON) of FET than IPP80N06S4-07?<br />&nbsp;&nbsp;&nbsp; Isn&#39;t there different way to increase the current limit point except the Rds(ON) resistance fact?</p> <p>Q3. Can be the current limit point changed according to the variable resistance value of Rrt(R3)?<br />&nbsp;&nbsp;&nbsp; The current limit point didn&#39;t changed from 9.5A even though the frequency was varied with decreased resistance value.<br />&nbsp;&nbsp;&nbsp; <br />Q4. Please inform us the important elements for FET selecting which can satisfy the present design specification.<br />&nbsp;&nbsp;&nbsp; There are many facts.( Rds(ON), Qg, Ciss, Coss, Tron, Troff ... )<br />&nbsp;&nbsp;&nbsp; Could you let me know the rank by importance among these facts?<br />&nbsp;&nbsp;&nbsp; And let me know the value range which should be controlled.<br />&nbsp;&nbsp;&nbsp; <br />Q5. Confirm the value of Pin5.&nbsp; SS ? <br />&nbsp;&nbsp;&nbsp; The Switcerpro simulation shows it as 47N.<br />&nbsp;&nbsp;&nbsp; However, when using 45N, oscillation doesn&#39;t started immediately under Enable port High condition.<br />&nbsp;&nbsp;&nbsp; Meanwhile, when I used 450N, oscillation can be started effectively more than 45N.</p> <p>&nbsp;&nbsp;&nbsp; Is there any special problem when I change it from 45N to 450N?<br />&nbsp;&nbsp;&nbsp; Can you recommend a capacitor value? <br />&nbsp;&nbsp;&nbsp;</p> <p>[View:http://e2e.ti.com/cfs-file.ashx/__key/communityserver-discussions-components-files/196/0525.TI_5F00_REQUEST.pdf

 

Best Regards,
Shin.                                 

  • 0
    TI__Mastermind 35675 points

    Hi Shin, please see my comments below.

    Q1. How can I increase the limit of current point?

        The current limit is being operated with 9.5A when the R3(ILIM) is 24.9k.

        If the R3(ILIM) resistance is changed,
          Using higher value : the current limit point didn't increase.
          Using lower value : the current limit point was decreased.
                              R3  :  5K   (  current limit point = about 5A )

    You may be falsely tripping the current limit. You are correct, increasing RILIM should increase it. A source of the false tripping may be because you are using leaded package power FETs. These will have higher lead inductance. Can you show some screenshots of the current tripping including the SW pin voltage? Please show some zoomed to see individual pulses at SW and also zoomed out to see the restart cycle.

    In addition to this, using an RC snubber to from SW to GND may be needed to remove noise and prevent false tripping of the over current limit.

    Q2. Should I use lower value part for Rds(ON) of FET than IPP80N06S4-07?
        Isn't there different way to increase the current limit point except the Rds(ON) resistance fact?

    Q3. Can be the current limit point changed according to the variable resistance value of Rrt(R3)?
        The current limit point didn't changed from 9.5A even though the frequency was varied with decreased resistance value.

    Increasing R3 is the way to increase your current limit point.
        
    Q4. Please inform us the important elements for FET selecting which can satisfy the present design specification.
        There are many facts.( Rds(ON), Qg, Ciss, Coss, Tron, Troff ... )
        Could you let me know the rank by importance among these facts?
        And let me know the value range which should be controlled.

    The specifications on your FET are look about right for this application. The important ones are Rds(ON) and Qg. As I mentioned before the issue may be because you are using a leaded package FET increasing noise causing false tripping.
        
    Q5. Confirm the value of Pin5.  SS ? 
        The Switcerpro simulation shows it as 47N.
        However, when using 45N, oscillation doesn't started immediately under Enable port High condition.
        Meanwhile, when I used 450N, oscillation can be started effectively more than 45N.

        Is there any special problem when I change it from 45N to 450N?
        Can you recommend a capacitor value? 

    Can you describe or show me the oscillation you are referring to? There is no danger in increasing to 450nF, the higher capacitance will slow down the rise time of the output voltage during startup. It will also increase the restart time during a fault shown in Figure 22 on the datasheet.

    Regards,
    Anthony