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TPS61196: TPS61196 soft start time

Part Number: TPS61196

Hi

VIN=12V,  LED= 10S x 6Ch,  ILED/CH=160mA. VOUT=32.5V=3.2x10 + 0.5V

In this case how to calculate soft start time ?

Fig13 is showing 15ms and 200ms time. what is this condition ?

best regards K.Kanao

  • Hi Katsuhiro,
    The 15ms and 200ms is just draft data for your reference. These two time interval is dependent on the capacitor connected to REF pin, which is C7 in "7.2 Fuctional Block Diagram".
    The soft-start combines two stage. The first one is charging C7 with 200uA from 0V to 2V. This is relatively fast stage. The second stage C7 is charged with 10uA from 2V to (Vout+VIFB(200mV))*R3/(R3+R4). VIFB is the minimum volatge of 6 chanels. R3 and R4 is determined by OVP voltage.
    If anything unclear, please let me know.

    Best Regards
    Caryss
  • Hi Caryss

    Thank you for your reply.

    I calculated the soft start time to find 15ms and 200ms condition.
    * (Vout+VIFB(200mV))*R3/(R3+R4) seems wrong , I use (Vout+VIFB(200mV))*R4/(R3+R4).
    *C7 =1.5uF
    First Soft time = 1000*1.5uF*(2-0)V/200uA=15mS

    From 8.2.1.1 Design Requirements
    VOUT=3.2*20= 64V, R3=255K, R4=10K ohm

    (Vout+VIFB(200mV))*R4/(R3+R4)= 2.42V
    So 2nd solt start =63.4mS = 1000*1.5uF*(2.42-2)V/10uA

    2nd soft start of data sheet is 200mS, otherwise my caluclation is 63.4mS. Not match.
    Would you please let me knot what vaule of Vout, R3 and R4 for 200mS ?

    best regards K.Kanao
  • Hi Katsuhiro,
    The 200ms in datasheet is a draft value, the real soft start time is dependent on specific design. The theoretical calculation is as what I replied above.
    thanks.

    Best Regards
    Caryss
  • Hi

    *Whch is correct  ?

     (Vout+VIFB(200mV))*R3/(R3+R4)  or  (Vout+VIFB(200mV))*R4/(R3+R4).

    *Where comes from VIFB(200mV) value  ?

     

    *How to caluclate VOUT ?

         VOUT= VF*N+VFB = VF*N+0.5

         N is number of LEDs per strings, VFB is diode foward voltage

    best regards K.Kanao

  • Hi Katsuhiro,

    (Vout+VIFB(200mV))*R4/(R3+R4) is correct.
    The VIFB=200mV comes from that the charge current stops when VIFB reaches 200mV.
    You are right about Vour calculation, if you want to be more accurate, you could add the head room voltage ( around 400mV according to your application.)

    Best Regards
    Caryss