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Original question:

TIDA-01371: some questions about equation(1) and TIDA-01592

TIDA-01371: questions on floating LDO working

BX27
Genius 3900 points
Part Number: TIDA-01371

Hi there

I am reading the user guide of TIDA01371. Some detail question about how the circuits working.

1. for the yellow highlight text, I think it should be R10 and R47 across B and E junction of Q4. Why just use 0.7V/47ohm = 15mA? 

2. fo the red highlight text, could you help explain how the driver circuit (Q4 ? Q3? D2? )for Q1/Q2/Q14 working? do not uderstande it very clear. 

3. So the floating LDO is used to stable the output voltage as well as eliminating the use of 9000uF capacitor as mentioned in previous section of ther user guide.and three parallel MOSFET is providing large current path. seems large capacitors may be unnecessary ( C4/C5/C6) ? if they are a must, why use 3*330uF?

Thanks very much!

  • +1
    Genius 3900 points

    could anyone help? thanks

  • +1
    TI__Genius 12951 points

    Hi, I have forwarded this to one of our engineers who should respond to you in a day or two. Thank you for your patience.

  • +1
    TI__Intellectual 1320 points

    Hi, Dear

    for 1: 0.7V/47ohm=15mA. this 15mA current is threshold current. if the LDO(U2) output current increasing large than 15mA, then parallel MOSFET's will start action and scaling current increasing. current less than 15mA, parallel MOSFET should switch off since there has very weak driver current.

    for 2:Q4 was used for driver parallel MOSFET. D2 was used for limite the drive current with Q3, if current throught D2 increasing, then the voltage of D2 will increasing. the voltage of D2 will cause Q3 acting and shunt drive current. in this way keep system response quickly but without or less over-response.

    for 3: since shear wave need transient high-power the 330u capacitor is needed. 3*330u capacitor will get better filter performance than a single 1000uF capacitor.

    feel free to let us know if you have further question. thanks.

    regards,

    Bill

  • +1 in reply to Bill Xu
    Genius 3900 points

    Hi Bill

    For question 1, R10 and R47 are across the Q4 B and E, maybe the current should be i = 0.7/ (R10+R47) = 0.7 / (47+ 4700) ? I think we can not omit the voltage on R47. how do you think?

    thanks.

  • +1 in reply to BX27
    TI__Intellectual 1320 points

    Hi, Dear

    strictly, the current of Ib(Q4)=(V(R10)-Vbe)/R47. in actually, Vbe could not equal 0.7V. it should be between 0.6~0.7V. the voltage of R10 was used for drive the Q4. and then drive the parallel MOSFET.  feel free to let us know if you need further supporting. thanks.

    regards,

    Bill  

  • +1 in reply to Bill Xu
    Genius 3900 points

    hi Bill

    When use SMPS (TIDA01352)+ Floating LDO (TIDA-01371), if Enable control is needed, is there requirement about power on / off sequence? thanks.

  • +1 in reply to BX27
    TI__Intellectual 1320 points

    it should be no need sequence for floating LDO circuit. but I'm not sure that your application need sequence or not. thanks.

    regards,

    Bill