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Original question:

TPS61291: using with MSP430F going in/out LPM3

TPS61291: Operation with Vin>Vout in boost mode

Anthony Triana1
Prodigy 40 points
Part Number: TPS61291
Other Parts Discussed in Thread: TPS61099

Maximum input voltage is listed as 5V. Recommended Operating Conditions indicate VinMAX=Vout.

What are liabilities of Vin>Vout? e.g. setting output to 3.0V and powering with two series alkaline cells (Vin=3.1V). We notice an increase in output noise in this condition. Output noise is 2X that when Vin<Vout.

  • 0
    TI__Mastermind 21565 points
    Hi Anthony:
    TPS61291 is a boost converter. The normal work mode is step up the Vin to Vout. So it means the Vin should not larger than Vout.
    TPS61291 also has the Bypass mode. It will let Vout equals to the Vin. As 7.4.2 Bypass Mode Operation in datasheet. But this mode couldn't control the value of Vou.

    I'll recommended you to see TPS61099, which has down mode. It could regulate the Vout if the Vin is little higher than Vout.
  • 0 in reply to Minqiu Xie
    Prodigy 40 points

    We understand that TPS61291 is a boost converter.

    Application requires two alkaline cells in series. When batteries are fresh Vin=3.15V. When batteries are low Vin=2.2V or less. Therefore down (buck) mode not suitable.

    Microcontroller requires Vddmin=2.7V, therefore boost converter required.

    3 options:

    1. Boost to 3.3V always- not efficient

    2. Use bypass mode- a little tricky at turn on but could be done. Also Vout will vary as Vin changes.

    3. Use boost mode but allow Vin to slightly exceed Vout (fresh batteries). This does not exceed absolute maximums, but slightly noisier.

    Again, I ask:

    What are liabilities of Vin>Vout?

    Is there a way to predict noise levels?

  • 0 in reply to Anthony Triana1
    TI__Mastermind 21565 points
    Hi Anthony:
    As I mentioned, normal boost mode couldn't regulate Vout well when Vin>Vout.
    So, I'll still recommended you to see TPS61099, which has down mode. It could regulate the Vout if the Vin is little higher than Vout.