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Krunal SHah
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2020/05/06 05:39:33 (permalink)
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MCP1725_ADJ_LoadingIssue

Hi
We are designing a system where we want to drive the LED using PWM. Now we have the 5V voltage which is fed to the system as the system supply. There are some MOSFET that is driving the LEDs and Gate of the  Mosfet is driven by the Microcontroller. 
Now to power up the microcontroller, we have used MCP1725-ADJ which converts the 5V to 3.3V. And PWM driving frequency is 200kHz with the varying duty cycle from 10% (on Time)  to 100% (On time)  based on the selection. Now between 25% (On Time) to 70% (On Time), 3.3V becomes 3.52 or even 3.6V. We suspect that is due to the high-Frequency switching happening in the supply voltage. 
Can you please guide us on how we can control the same.
Attached is the snap of the DSO for the zoomed out and zoomed in, showing the PWM Output from the Micro Controller, waveforms captured when C2= 10uF, 25V, and others are as per the snap.
 
Filename
1) 1.BMP (25% On Time)
2) 2.BMP (25% On Time, Zoomed In Image) 
3) 3.BMP (70% On Time)
  
We change the C2 to 20uF and We are no longer seeing this issue, but as per datasheet maximum value of the C2 should be 4.7uF. Can you please suggest if we can use 20UF for the C2 or not for such frequency.
 
 
Thanks
 

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#1

8 Replies Related Threads

    Krunal SHah
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    Re: MCP1725_ADJ_LoadingIssue 2020/05/11 01:04:10 (permalink)
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    Hi
     
    Can anyone answers this please, It a little bit urgent.
     
    Thanks
    Krunal Shah
    #2
    ric
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    Re: MCP1725_ADJ_LoadingIssue 2020/05/12 22:04:59 (permalink)
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    I doubt you're going to get many answers without showing a complete schematic of your circuit.
     

    I also post at: PicForum
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    NEW USERS: Posting images, links and code - workaround for restrictions.
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    #3
    Krunal SHah
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    Re: MCP1725_ADJ_LoadingIssue 2020/05/12 22:31:17 (permalink)
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    Hi
     
    Here is the snap of the schematic where we have used the MCP1725-ADJ IC. Sorry, We can not share the complete schematic. The Reset Pin is going to the Reset Pin of the HUB IC USB5744-I/2G from Microchip. We can answer the question you have to support us on this query.
     
    Support forums are meant to help and every time if someone has to share the complete schematic of their product which is not good. 
     
    I hope to receive some answers based on the details we have posted here.
    THanks
     
     
    post edited by Krunal SHah - 2020/05/12 22:32:35

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    Krunal SHah
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    Re: MCP1725_ADJ_LoadingIssue 2020/05/14 04:59:07 (permalink)
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    Hi
     
    Can anyone help with this?
     
    Thanks
    #5
    mbrowning
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    Re: MCP1725_ADJ_LoadingIssue 2020/05/14 09:53:26 (permalink)
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    drive the LED using PWM.
    What kind of LED and how much current?
    some MOSFET driving the LEDs
    What kind of FET, and now it's not LED but LEDs?
    Gate of the  Mosfet
    once again, what kind of FET?
    waveforms captured when C2= 10uF, 25V, and others are as per the snap.
    What is C2, and what do you mean by "others"?
    datasheet maximum value of the C2 should be 4.7uF
    I can't see a max value of 4.7uF on any cap in the datasheet. What "C2" are you talking about? C2 in Figure 4-1 can be max 22uF. Input capacitor has no maximum.
    Can you please suggest if we can use 20UF for the C2 or not for such frequency.
    If you want an official Microchip answer rather than fellow users you need to submit a support ticket at microchip.com/support.
     
    The pictures seem to show the output of the PWM, but what do the output of the LDO and the 5V from USB look like?
    Krunal SHahevery time if someone has to share the complete schematic of their product which is not good.
    This is absurd. The only way someone can diagnose such an issue is to see all relevant schematic and layout aspects, and if you can't find the issue you may not be qualified to determine what's relevant.
     
    Now that you've shown a tidbit of schematic, at least one issue is clear. A 6.3V ceramic capacitor is not appropriate for a 5V rail. Ceramics should be derated minimum 50% so for a 5V rail, 10V caps minimum. For higher voltage rails like 15V, I will go down to 25V caps if necessary, but not less. 4.7uF 0603 35V caps are readily available, and a 4.7uF 0603 6.3V cap is really just 1.5uF due to capacitance-voltage derating. A 35V version will be about 2.5uF.
    #6
    Krunal SHah
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    Re: MCP1725_ADJ_LoadingIssue 2020/05/14 11:29:01 (permalink)
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    Please be noted: where we are referring to the [link=mailto:C@]C[/link]2, is the Input Capacitor to the IC. C2 is the Input capacitor reference in or schematic. As per the Typical application Circuit in the Datasheet, its reference is C1.
     
    - LED is Normal Lighting LED, LED are drawing a maximum of 1.5A of the current
     
    - We are using STS5P3LLH6 to drive the LEDs
     
    - C2 is the Input Capacitor as per the attached snap of the Schematic, which we have changed to 10Uf (But the Schematic is showing 4.7uF) So consider that is 10uF, 25V. Like the Waveforms, snaps are named accordingly when they have been captured. In all waveform Snaps C2 is 10uF, 25V only.
     
    - See Page # 17 of the datasheet, A minimum of 1.0 µF to 4.7 µF is recommended for most applications. And in Figure 4-1, I am asking about value for the C1, not C2. 

     
    - Attached is the Snip of the Mosfet Connection. Will Capture the Supply 5V Waveforms along with 3V3 voltage and send you the same soon.

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    #7
    Bob White
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    Re: MCP1725_ADJ_LoadingIssue 2020/05/14 11:38:00 (permalink)
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    What else, especially capacitors, is attached to the 3.3 V rail?
     
    My thoughts:  During the time the LEDs are on there is current filter inductor L3.  When you turn off the LEDs that current still has to flow somewhere.  L3 is discharging into whatever capacitance you have on the 3.3 V rail and driving up the voltage (sort of like a switching boost converter).
     
    Again, without a full schematic we are stumbling in the dark.  This forum is a simply other users of Microchip products and not Microchip people.  We are willing to help but not if you can't provide all the information needed.  And by that I mean *all* the information, not just what you think might be relevant.
    post edited by Bob White - 2020/05/14 13:41:00
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    mbrowning
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    Re: MCP1725_ADJ_LoadingIssue 2020/05/15 04:44:37 (permalink)
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    Krunal SHah
    Page # 17 of the datasheet, A minimum of 1.0 µF to 4.7 µF is recommended for most applications.
    how are you getting a max value from that? That’s a range of minimums.
    #9
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