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LM3492-Q1: Switching frequency selection

Rajesh
Expert 1010 points
Part Number: LM3492-Q1
Other Parts Discussed in Thread: LM3492

Hi All,

I am using LM3492-Q1 in my design as a LED backlight driver. As per my application the backlight requirement as follows:

LED string current: 280mA

Total no of strings:1

Output voltage required: 10V

LED driver input voltage: 6V to 16V(car battery)

Anyone suggest that how to configure the LM3492 for the above application(switching freq, inductor & capacitor calculations)

Regards,

Rajesh

  • 0
    TI__Mastermind 25500 points

    Hello Rajesh,

    The equations are in the datasheet and your frequency selection is based on your needs. Often for automotive 350kHz to 390kHz is chosen to help make EMI filtering easier, but others can be chosen. Basically if you go lower you improve efficiency at the cost of increased circuit size and vise-versa.

    In your case you will need to use buck-boost (SEPIC with this device) but the equations are the same if you substitute Vout with Vin+Vout in the datasheet equations.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Expert 1010 points

    Hi,

    Thanks for sharing the info. Based on my understanding,  I have calculated the circuit values as follows, please correct me, if any mismatch or any value to be changed :

    Input voltage: 9 to 16V(12V nominal taken for calculation)

    LED total current: 280mA

    Planned to operate both the LED out channels in parallel to meet the load requirement

    Output voltage: 10.14V

    VFB nominal assumed: 1.5V(considered for feedback value calculation)

    RFB1: 57.6K ohms

    RFB2: 10K ohms

    CFB: 10pf

    CCDHC: 0.47uf

    FSW selected: ~ 380KHz

    RRT chosed: 357K ohms

    Formulas used: 

    IN = (VOUT(nom) × ILED ) / VIN

    Input current calculated: 0.62A(vout nom=10.14+12)

    ton = (1 – VIN/VOUT) / fSW 

    ton:1.2us

    L1 = (VIN × ton) / 2IIN

    L1:11.6uH

    used value: 15uH, current rating:2.3A with 50mohms DCR

    output capacitor: 2*150uf-aluminium, 10uf-ceramic,100nf

    input capacitor: 330uf-aluminium,10uf-ceramic,100nf

    Regards,

    Rajesh

  • 0 in reply to Rajesh
    TI__Mastermind 25500 points

    Hello Rajesh,

    I'm out of the office today, but I will look over this and comment tomorrow. Thanks.

    Regards,

    Clint

  • 0 in reply to Rajesh
    TI__Mastermind 25500 points

    Hello Rajesh,

    The equations that require using VIN+VOUT depend on whether it is the switch voltage you are looking at. In your case most of that is good with two exceptions:

    IIN = (VOUT * ILED) / VIN ------ VOUT is true VOUT in this equation

    ton = (1 - VIN / (VIN+VOUT)) / Fsw --------- This one uses switch voltage so you sub VOUT with VIN+VOUT

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Expert 1010 points

    Hi,

    Thanks for sharing the feedback. I have updated my calculations as per the above formula:

    IIN:0.284A

    Ton:1.2us 

    L : 25uH calculated value, 22uH used value(MAG-IND,22uH,20%,2A,SMD).

    Regards,

    Rajesh