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TPS62903: 12.7Vin to 0.6V output application

Toshiro Imi
Expert 6260 points
Part Number: TPS62903

Hello,

Our customer has looked for 12.7V input to 0.6V/3A output DCDC converter in strict power on/off sequence and we proposed TPS62903 is the best.

While we reviewed the datasheet, we have a question how TPS62903 can drive 0.6V/3A output from 12.7Vinput voltage. Because the datasheet specify 50ns minimum on time and selectable 1MHz or 2.5MHz switching frequency. Even if we can select 1MHz switching frequency, the on time in this case is 47ns which is less than minimum on time 50ns.

Could you let us know how TPS62903 can drive 0.6V/3A output from 12.7V input voltage.

Best regards,

  • 0
    TI__Genius 13070 points

    Hi Toshiro,

    Thanks for reaching out. My colleague will give you response soon. Thanks!

    BRs

    Lucia

  • 0
    TI__Expert 3285 points

    Hi Toshiro,

    Thank you for your question.

    TPS62903 has an input range of 3V-17V and an output range of 0.6V-5.5V, so it will be a great fit for your customer. It has two output voltage options that are set by a voltage divider and will be able to drive the required voltage at 3A as required.

    1. VFB external divider: 0.6 V to 5.5 V

    2. VSET internal divider: 16 options between 0.4 V and 5.5 V

     For the Ton time, we can look at equation 3 in the datasheet:

    Ton = (Vout/Vin) * (1/Fsw) 

    As the switching Frequency decreases, the Ton will increase. This is all part of the Power Save Mode feature which is entered at the boundary to discontinuous conduction mode. The reason for the 50 ns minimum pulse is to limit switching losses when driving smaller voltages. The operation frequency is automatically reduced from its nominal value of 1Mhz linearly and this keeps the efficiency high. Therefore, in the customer's case, the frequency will reduce and this will increase the Ton above the min on time.

    Regards,

    Akshat

  • 0 in reply to Akshat Murali
    TI__Expert 6260 points

    Hi Akshat,

    Thank you for your explanation. I understood that the switching frequency will be down linearly lower than 1MHz or 2.5MHz to make the efficiency highest.

    If my understanding is correct,

    - Could you tell me how we can expect or calculate the switching frequency in this case.

    - Would you also let me know if this feature comes from100% duty cycle operation.

    Best regards,

    T.Imi

  • 0 in reply to Toshiro Imi
    TI__Expert 3285 points

    Hi Toshiro,

    The switching frequency will change automatically to maintain a steady Vout. I will check to see if we have a specific formula to calculate the switching frequency in this case.

    Regarding the 100% Duty Cycle operation, I don't think I understand your question correctly. In 100% Duty Cycle Operation, once the minimum off-time reaches 80ns, the switching frequency will reduce automatically until it reaches the 100% mode (Always high). In this mode the high side switch is always on and the low side switch is always off. Therefore, there will be no on time or off time as the switch is always on. 

    VIN(min) =VOUT + IOUT(Rds + RL) where RL is the DC resistance of the inductor and Rds is the on resistance of the high side FET. These resistances will be very small so VIN will almost exactly equal VOUT in this operation mode. Does this answer the second part of your question?

    Regards,

    Akshat

  • 0 in reply to Akshat Murali
    TI__Expert 6260 points

    Thank you very much for your explanation.

    In order to understand how TPS62903 works, I have ran some simulation and attached the result. The switching frequency is 1.2MHz, ton=50ns at Forced PWM 2.5MHz setting, 540kHz, ton=100ns at AEE 2.5MHz setting. It looks the Forced PWM switching frequency looks calculated based on ton=50ns of minimum on time. So I think AEE mode would be the best for both efficiency and on time perspective.

    please review the file and let me know if something incorrect.

    Best regards,

    T.Imi

    TPS62903 pspice test.pdf

  • 0 in reply to Toshiro Imi
    TI__Expert 3285 points

    Hi Toshiro,

    Thank you for sharing the presentation. It is very helpful to see visually!

    You're simulations are absolutely correct. In forced PWM mode, if the Fsw is 1MHz then the Ton time will be 47ns which is not possible. That's is why the simulation shows the Fsw increasing to 1.17MHz. When we use small output voltages, the Power save mode should be enabled and the AEE function will automatically change the Fsw based on Vin and Vout. The best way to calculate the Fsw when using power save mode is to use the PSPICE simulation.

    The benefits of this device is that it has DCS control which is a seamless transition into power save mode. This gives it a high efficiency as power loss is reduced and therefore, it is very good to have power save mode enabled when using small loads.

    Regards,

    Akshat 

  • 0 in reply to Akshat Murali
    TI__Expert 6260 points

    Hi Akshat,

    Thank you very much. This is closed.

    Best regards,