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LP5907: the impact on phase margin of LP5907 by remote output capacitor

michael YANG
Prodigy 195 points
Part Number: LP5907

Hi Team ,

Regarding the page 12  of the datasheet , LP5907 support remote output capacitor ,could you please help on  follows questions?

Great thanks for your support in advance. 

 

Q2 . about the LP5907 , i find there is a good transient performance with 4.7uf X7R  output capacitor (it is about 50 degree), but in a real project , the phase margin decrease caused by the additional output capacitor in real project( it is about 30 degree) . for example , there are 0.1uf, 10uf ,4.7uf and 2.2uf in parallel for a real project  to meet the requirement of  RF , output noise ...  

I know the ESR will be decreased caused by parallel conenction , so the position of ESR will be changed .  the cross frequncy of LP5907 is 1.6-2MHZ , and the position of ESR zero is smaller than 10*2MHZ.  .

is there any other reasons (other change of pole or zero  by ESR )  to analysis this phenomenon ?  and how to resolve the phase margin decrease caused by load capacitor ?

 

Q3 . we know the ESR zero is sensitive for LDO base on P-MOSFET , how to define the real ESR ? i mean excepting  the ESR value shown in the datasheet of LDO ,which trace in LDO or DC-DC  should be consideried for ESR  ?

 

 

Best Regards

Michael.Yang

  • 0
    TI__Mastermind 23361 points
    Hi Michael,

    Please allow some time for us to look into this.

    Regards,
    Jason
  • 0 in reply to Jason Song
    Prodigy 195 points
    Hi Team ,
    is there some professional engineer help me on this question?
    which compasation mode is selected in LP5907??

    Great thanks !
  • 0
    TI__Genius 14245 points

    Hi Michael,

    Unfortunately we cannot discuss the method of compensating any of our products so the majority of your first two questions cannot be addressed for competitive/intellectual property reasons. 

    Regarding the change in the phase margin mentioned in your second question it seems like you may be exceeding the maximum capacitance listed in the datasheet which could significantly change the phase margin. See the screen shot below from the datasheet. 

    Regarding your third question on what ESR should be considered for stability requirement of 5mOhm to 500 mOhm, the major contributors will be

    1. ESR of the output capacitor(s)
    2. resistance of the trace between the Vout pin and the output capacitor
    3. resistance of the trace between the GND pin and the output capacitor
    4. there is a very small amount of resistance that is added by the solder joint between the board and the device as well as between the board and the output capacitor. These solder joint resistances are negligible compared to the first three. 

  • 0 in reply to Kyle Van Renterghem
    Prodigy 195 points
    Hi Bernard,
    Got it , i will delete the questions related with the compasation mode of LP5907.
    Great thanks for your reply!