• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Subscribers 62 subscribers
  • Views 92 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS54A20: Is RTON the most dominant parameter in determining Vout? Or are RTOP and RBOT the most dominant?

gomi.ryota
Prodigy 140 points
Part Number: TPS54A20

Hello, TI expert.

I want to know whitch parameter is more dominant, RTON or RBOT & RTOP ?

1 Does the output voltage change when the value of the resistor RTON attached to the TON pin changes?
→ When Vout is 1.25, RTON = 21.75kΩ, but if RTON deviates ±1% due to variations in resistance, will Vout also deviate?
    Or does something like the TON pin resistance value discriminator have resolution, and does the output voltage remain the same even with some deviation?
    If so, what percentage of resistance variation is acceptable for the resolution of the resistance value discriminator?

2 I think that the output voltage is also determined by the On time, but looking at the specifications, it seems that it is determined only by the resistor attached to FB.
→ I think that what is determined by RTOP and RBOT attached to FB is not the output voltage value, but the variable range of the output voltage.
　 I think that the voltage that is actually output is determined by the On time (value of RTON).
　 The job of RTOP and RBOT is to divide Vout to make VFB=VREF±10%, right?
　Assuming that R9: 1.4kΩ, R7: 1kΩ, and VREF=0.508V, DCDC will stop when Vout is outside 1.10 to 1.34V.
　In other words, it is RTON that determines the actual output voltage, and I think that R7(RBOT) and R9(RTOP) only determine the error range of the output voltage, am I wrong?

I have attached cutouts and tables of the specifications in the attached EXCEL, so please take a look if you like.

By the way,

3 If the voltage to the EN pin is supplied by an external supply rather than a voltage divider of Vin, do I need a pull-down resistor?

Best regards.


TI_ask-list.xlsx

  • 0
    TI__Expert 6486 points

    Hi Ryota-san,

    RTON has no measurable impact on VOUT. RTON sets the on-time, and regulation of the feedback voltage (VOUT) adjusts the switching frequency to set the output voltage and RTON programmed on-time.

  • 0 in reply to Tomoya Ide
    Prodigy 140 points

    Sorry for not understanding.

    For example, when Vin is 12V and Vout is 1.2V, the voltage is stepped down to 1/10, so I think that On time: Off time = 1:9.

    In other words, I think that determining the On time leads to determining the output voltage, but isn't that the case?

    If determining the output voltage is determining the On time, and it is RBOT and RTOP that determine the output voltage, what is RTON doing?

  • +1 in reply to gomi.ryota
    TI__Expert 6486 points

    It's ok for not understanding. I will explain better.

    First, please understand that RTON only sets the on-time. As you said, the duty cycle/ratio will be maintained (~Vout/Vin). Since this is a type of variable frequency control, what it does is it will have a fixed on-time that you programmed through the RTON resistor and adjusts the off-time to maintain the duty cycle to achieve the output voltage you set through your FB resistor divider, and frequency/period will change accordingly. VOUT voltage is still only set by RTOP and RBOT.

    Also, to answer your third question, yes, you will need a pull-down because this device has an internal pull-up.

    I hope this makes sense!

  • 0 in reply to Tomoya Ide
    Prodigy 140 points

    Hi Ide san,

    PWM制御ではデューティ比によって降圧率が決まり、On時間はTONで決まり、周波数（オン時間＋オフ時間）はSS/FSELで決まり、FBによって出力電圧が決まる。
    入力電圧が変化した時、降圧率が変化する必要があると思うのですが、その際、どのパラメーターが変化するのでしょうか？
    On時間と出力電圧を固定すると、OFF時間を変えるしかないと思うのですが、そうするとOn時間＋オフ時間の逆数である、周波数が変化しなければいけなくなります。
    In PWM control, the step-down rate is determined by the duty ratio, the ON time is determined by TON, the frequency (ON time + OFF time) is determined by SS/FSEL, and the output voltage is determined by FB.
    I think that the step-down ratio should change when the input voltage changes, but which parameter will change?
    If the ON time and output voltage are fixed, the OFF time must be changed, but then the frequency, which is the reciprocal of ON time + OFF time, must be changed.
    best regards.
  • 0 in reply to gomi.ryota
    Prodigy 140 points

    Sorry for the repeated votes. I would like to ask you three additional questions.

    1. Is it correct to understand that the initial on-time is programmed in TON, and then the on-time is fine-tuned by FB?

    2. Is it necessary to change the RTON resistance value when the frequency setting is changed?

    3. When the RTON resistance value changes due to resistance variation, does the On time change as well?

    連投申し訳ございません。
    三点追加で質問させていただきたいです。
    1.　TONでは初期オン時間がプログラミングされていて、その後、FBによってオン時間が微調整されるという認識で合っているか？
    ２.　周波数の設定を変えた時にRTONの抵抗値は変えなくてもよいのか？
    ３.RTONの抵抗値が、抵抗のばらつきにより変わったとき、On時間も同様に変化するのか？

  • +1 in reply to gomi.ryota
    TI__Expert 6486 points

    Hi Ryota-san,

    I understand the confusion now. Yes, RTON sets the initial on-time, but it's the SS/FSEL pin that makes the final on-time adjustment and sets the fixed frequency operation in steady state.