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LM5122-Q1: internal current sense amp , CSP-CSN recommended voltage range

Ryuji
Guru 20090 points
Part Number: LM5122-Q1

Hello,

In my customer, there is the possibility that the CSP and the CSN voltage is under the 3V (for example, 2V) at the cold crank condition.
If there is the heavy load at the cold crank condition and there is input impedance due to the cable between battery to board.Thus the voltage drop is occured.
They already increase input capacitors but they have space limitation in the board.
Thus, they couldn't increase capacitor anymore. 
VIN is biased from the output of LM5122-Q1. Thus, we can keep the voltage of VIN in the recommended operation range.

Then I have the following questions.

(1) 
I think that if wthe CSN-CSP voltage is less than recommended operating conditions 3V, we couldn't guarantee the Vcs-th1 and Vcs-zcd specifications.
Are there any other concerns?

(2)
Could you please let me know the power supply of internal current sense amp?
Is it from  Vcc or VIN?

Best Regards,
Ryuji

 

  • 0
    TI__Guru 68675 points

    (1) the CS pin voltages must be great than 3V, or the sense may become blind.  Solution to this situation is to add offset voltage to pull the sense pin voltage up.  

    cs pullup.docx

    (2) Neither VCC nor VIN.  It is basically a floating circuit, internally the signal is level shifted down to a signal referencing the GND.  

  • 0 in reply to Youhao Xi
    Guru 20090 points
    Hello Youhao san,

    I appreciate your support.
    Regarding (1), could you please let me know the reason that the pull up to the Vout in attached file?
    I thought that it might be enough to just pull up to Vcc.

    Best Regards,
    Ryuji
  • 0 in reply to Ryuji
    TI__Guru 68675 points
    Two reasons: Assuming VOUT is hihger than 3V.
    (1) to take over the VIN supply when input source is below 3V. This will keep the VCC voltage from being collapsed by the dropping input source.
    (2) to pull up CS pin so they do not fall below 3V.