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TPS56339: TPS56339 as adjustable 9v or 12v

Hossam Moghrabi
Intellectual 360 points
Part Number: TPS56339
Other Parts Discussed in Thread: LM317, TPS54308, TPS62933F, TPS56637, TPS560430

Tool/software:

Hello,

I plan to use TPS56339 as either 9v or 12v buck regulator from 15v supply. output is less than 300 mA which is limited by eFuse and actual current is less than this.

I have the following circuit:

I have the following choices:

1- EN pin is float so that the device is enabled by default. i want this to be robust and no glitches. I do not require any over or under voltage protection, just enable by default. is this correct? or I must use resistor divider? i can use 430k upper resistor and 154k lower resistor, which gives me about 3.96v at EN pin when VIN = 15v. what is the best and more reliable option?

2- Output caps are 2x22u 1206 X5R + 1x10u 0805 X5R + 1x100nF 0402. L=6.8uH, which after derating C*L are within range of datasheet...

total capacitance @12v after derating = 13.4u

total capacitance @9v after derating = 17.5u

is this ok?

3- input cap is the same + other bigger polymer caps at the source and near it.

4- feedback resistors were chosen as follows:

for 12v: 155k+510R with 11k.

for 9v: it needs 113.67k with 11k... to get 113.67k i kept the 155.51k resistor and added 430k in parallel with it.

the switch SW2 switches 430k resistor in and out of the circuit which achieves 9v or 12v.

please kindly check this and advise if options are correct. I used similar technique for LM317 and it worked perfectly, but now changed the values to suit this buck.

5- using this circuit, can i achieve <10mV p-p ripple assuming output current is expected to be less than 300mA?

thanks,

  • 0
    TI__Mastermind 27306 points

    Hi Hossam

    1. Yes EN can be left floating to enable the IC, which reduce the BOM and reduce solution size. The EN UVLO resistors can be configured to set specific value for Viin_UVLO. Both way should be ok and reliable.

    2. It should be ok as referred to the Recommended component values in datasheet.

    3.Place C41 close to the IC Vin and GND pins.

    4. It's ok, please make sure FB resistors are close to FB pin and GND pin(RFBB). and the vo sample trace should route away from noisy nodes like SW/BOOT.

    5. when light load, the device will enter into light load. For this Vin/Vo/500kHz and 0.3A operate condition, it should be under DCM mode. Do you consider other device like TPS62933F/ TPS54308, which is FCCM mode and should be able to achieve smaller Vo_ripple under light load. <10mV is really small, might need to add larger Cout.

    Thanks and best regards.

  • 0 in reply to Gui He
    Intellectual 360 points

    Hello Gui He,

    as for output ripple, what is the expected using this current L and C? 

    if i want 5mV or so, what should I add to the output filter? I can add other 10u ceramic caps in parallel but since it is very light load i don't know the expected ripple figure.

    I saw TPS62933F which seems promising but tps56339 is a lot cheaper (both from lcsc) and i like using it if it can deliver such low ripple figure. if not, then i will have to use TPS62933F.

    I also check for FPWM and found these optoins:

    TPS560430XF: fixed 1.1MHz.

    TPS560430YF: fixed 2.1MHz.

    the problem with them is that they ask for very high inductor value for 12v like 33uH and 18uH, so I wonder if I can use a smaller value like 4.7uH/10uH and put multiple ceramics in parallel as output capacitor to achieve the same result.

    another option is TPS56637 which is dcap 3, i can use 4.7uH inductor here with multiple caps in parallel. this will take more space for sure. 

    please  check these options and give me your recommendation.

  • 0 in reply to Hossam Moghrabi
    Intellectual 360 points

    Hello

    I have completely modified the design using TPS560430YF, please check the images here: https://imgur.com/a/tV1q3US

    I optimized the layout as much as possible in the current board, no other space is available. input caps are not perfect but very good. 2x100nF 0402 caps are very close to Vin pin as well as other 1x10u 0805 + 2x22u 1206 caps.

    the selected feedback resistors as follows:

    12v
    R_top = 243k
    R_bot = 22k


    9v
    R_add = 649k -> this is switched using the switch to either be out of the circuit or in parallel with top resistor 243k to form about 176.5k top resistor for 9v.

    please check the calculations as well as layout and provide your feedback

    thanks for your kind support.

  • 0 in reply to Hossam Moghrabi
    Intellectual 360 points



    Hello sir,

    kindly check this modified circuit for a new buck and provide your feedback. if it goes well, then i will make a new prototype with it.

    thank you

  • 0 in reply to Hossam Moghrabi
    TI__Mastermind 27306 points

    Hi Hossam

    sorry for late reply. Please submit new thread about tps560430.

    Thanks and best regards.