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SN6505B: Ultra low-power isolated DC-DC converter

Luigi Vigone
Prodigy 30 points
Part Number: SN6505B
Other Parts Discussed in Thread: TPS7A05, SN6505A

Hi,

We have to power supply a 4 channels isolated data acquisition system and we are looking for an ultra low-power isolated DC-DC converter with the following specs:

VIN: from a 3.7V lithium battery

VOUT: 2.7/2.8VDC

IOUT: 100/200uA

EFFICIENCY: >80%

Attached a starting idea, but efficiency is very low (55%).

Isolated DC-DC.TSC

Regards,

Luigi

  • 0
    TI__Mastermind 18955 points

    Hello Luigi,

    Thank you for reaching out. The device does not have low power modes and will not able to meet 80% at this current. At 200uA of current the efficiency will be less than 10% (pink dot). Please see the below datasheet Figure 6-14. Efficiency vs Load Current.

    Let me know if you have further questions.

    Best,
    Andrew

  • 0 in reply to Andrew Jackiw
    Prodigy 30 points

    Hi Andrew,
    thank you for your answer, but the circuit I attached is working in a different way:
    - SN6505B is enabled only for 10ms every 20s to charge the 2 x 470uF tank capacitor up to 4.7V: in this condition SN6505B is working at nearly 40mA and efficiency is 80% in this condition (with the transformer I used Figure 6.20 has to be used.
    - when the voltage on the tank capacitors drops to 3V, SN6505B is enabled again to recharge the tank capacitors.

    TPS78228 LDO is used to supply 2.8V @100-150uA to the AFE, but the total efficiency drops to 55%.

    I'm looking for a better solution than my idea.

    BR,
    Luigi

  • 0 in reply to Luigi Vigone
    TI__Mastermind 18955 points

    Hi Luigi, 

    Let me review with the team and get back to you. 

    Best,
    Andrew

  • 0 in reply to Andrew Jackiw
    Prodigy 30 points

    Hi Andrew,

    thank you!

    Another option I'm evaluating it's to use a size A lithium battery to supply each channel followed by a LDO regulator (like TPS7A05: any hint from you?): with this solution I will not need an isolated SN6505A, because the power supply will be indipendent.

    BR,

    Luigi

  • 0 in reply to Luigi Vigone
    TI__Mastermind 18955 points

    Hi Luigi, 

    We are still evaluating this solution. The LDO regulator sounds like a good option since it will not have as many losses as a transformer-based solution. 

    Best,
    Andrew

  • 0 in reply to Andrew Jackiw
    TI__Mastermind 18955 points

    Hi Luigi,

    Thank you for your patience. 

    To summarize the issue. I understand that you are enabling the SN6505 for short periods of time to charge a capacitor back. While enabled the efficiency is 80%, however after the LDO stage efficiency drops to 55%. The best solution to continue using the isolated power topology would be to change the SN6505 output voltage to something smaller like 3V, the closer to 2.8V the better since this will minimize losses.

    Luigi Vigone said:
    A lithium battery to supply each channel followed by a LDO regulator (like TPS7A05: any hint from you?): with this solution I will not need an isolated SN6505A, because the power supply will be indipendent.

    This would be a viable option. For LDO recommendations please post on the power management forum: (+) Power management forum - Power management - TI E2E support forums.

    Best,
    Andrew