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TPS62860: Frequency issues

Minghan Dong
Prodigy 40 points
Part Number: TPS62860
Other Parts Discussed in Thread: TPS62130

Hi expert,

I found frequency of TPS628600 TPS628610 have issues as shown below.

(1) TPS628600YCH datasheet shows the frequency is 1,5MHz. But in my applications, I found the frequency is about 4MHz. This makes the impact on effiency. Why?

(2) TPS628610YCH use lower frequency at low duty operations (such as 5.5V~0.4V). Leads to higher efficiency. Why?

Thanks,

Minghan

  • 0
    TI__Mastermind 18280 points

    Hello Minghan,

    Thank you for using E2E,

    Minghan Dong said:
    (1) TPS628600YCH datasheet shows the frequency is 1,5MHz. But in my applications, I found the frequency is about 4MHz. This makes the impact on effiency. Why?

    Can you please check the device marking, to confirm if you are using TPS628600 or TPS628610?

    Minghan Dong said:
    (2) TPS628610YCH use lower frequency at low duty operations (such as 5.5V~0.4V). Leads to higher efficiency. Why?

    Its a DCS control device and the switching changes with duty cycle and load current. Understanding frequency variation in the DCS-Control(TM) topology

    Thank you & regards,

    Moheddin.

  • 0 in reply to Moheddin Pasha Shaik
    Prodigy 40 points

    Hi Moheddin,

    For question(1), I will double check the IC under evaluation.

    For question(2), I have understood that the frequency will decrease under higher duty.(Vin=5V Vout=4V etc..) But why TPS628610YCH use lower frequency at lower duty operations (such as 5.5V~0.4V)?

    Thanks for your kindly reply!

    Minghan

  • +1 in reply to Minghan Dong
    TI__Mastermind 18280 points

    Hello Minghan,

    Minghan Dong said:
    For question(2), I have understood that the frequency will decrease under higher duty.(Vin=5V Vout=4V etc..) But why TPS628610YCH use lower frequency at lower duty operations (such as 5.5V~0.4V)?

    At very low duty cycle, this is due to the min on time set internally in the device as explained in the app note.

    "Low duty cycles occur with lower output voltages, such as 1 V and 1.8 V. The relatively high 12-V input voltage requires duty cycles of sometimes less than 10%. With respect to the desired 400-ns period, this requires on-times near and even below 40 ns. Such small on-times are challenging for any converter to achieve, or are actually impossible due to absolute minimum on-times. The TPS62130 data sheet notes a typical 80-ns absolute minimum on-time that occurs in these cases. This is the primary source of frequency variation at low duty cycles. Fixed propagation delays added to small on-times are another source of variation, as explained before. Figure 4 shows measured data for a 1.8-V output voltage

    The 2.5-MHz curves in Figure 4b clearly show a minimum on-time in the 80-ns range. This sets an upper boundary on the achievable switching frequency. The 1.25-MHz curves show good frequency variation similar to Figures 2a and 3a. Due to smaller on-times with this 1.8-V output, fixed propagation delays cause a sharper downward frequency shift versus higher output voltages, which result in a lower frequency."

    Thank you & regards,

    Moheddin.

     

  • 0 in reply to Moheddin Pasha Shaik
    Prodigy 40 points

    Thanks Moheddin. Thanks for your kindly reply!

  • 0 in reply to Moheddin Pasha Shaik
    Prodigy 40 points

    The picture above shows the Vout=0.4V Vin=2.5V 4MHz(set in TPS628610) 2.917MHz(Real freq under test).

    This is a important application in my system. The on-time shows as 80ns. But the min-on-time only 40ns under our evaluation. It is seems that min-on-time has not been triggered. Why TPS628610YCH use lower frequency at lower duty operations (such as 2.5V~0.4V)?

    Thanks for your kindly reply!

    Minghan

  • +1 in reply to Minghan Dong
    TI__Mastermind 18280 points

    Hello Minghan,

    Minghan Dong said:
    The on-time shows as 80ns

    This value is form the different converter (TPS62130) which is taken as an example in the app note - Understanding frequency variation in the DCS-Control(TM) topology

    Minghan Dong said:
    Why TPS628610YCH use lower frequency at lower duty operations (such as 2.5V~0.4V)?

    At very low duty cycle the hysteretic converter 

    We have an adaptive on-timer that limits the switching frequency to relative fixed value. So for certain duty cycles (e.g. 30%-70%), the frequency will be limited by the on timer and will be quasi-constant. outside that dutycycle range, you will have the characteristic of the hysteretic converter. I hope this helps.

    Thank you & regards,

    Moheddin.