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I am using a TPS61235p to boost a 3.7V 18650 battery to 5.1v with a pure-resitive load of 2Ω. I expect it to draw 2.5A, but upon pulling the enable pin high the output current is limited to about ~1.75A due to the soft-start functionality described on page 10 of the datasheet.

I have confirmed this by increasing the load resistance higher than the Fig 8 Minimum Load Resistance at Startup, and then bypassing my increased resistance, confirming I now have the proper output. The booster never comes out of soft-start unless the load resistance is increased first, then reduced after the booster started switching.

The enable pin is controlled by an Arduino, so now I'm wondering how do I decrease the load resistance once I've enabled the booster.

My first thought was to put a resistor in-line like I am now, and then use a N-channel MOSFET to bypass the resistor after the booster has started. But is there a more robust / effecient way to do this?

Forgive my cramped circuit, I'm still learning.

  • SYS is VCC, battery voltage unless its being charged
  • VOUT goes straight to the load
  • EN and INACT go to the Arduino

Boost circuit

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  • \$\begingroup\$ Soft start is basically always desirable. Why exactly is it a problem? \$\endgroup\$
    – Matt Young
    Commented Mar 1, 2017 at 2:15
  • \$\begingroup\$ When using such a low resistance load, the boost converter never comes out of soft-start. Its described on the top of page 11, and they recommend applying a small load or no load at all until the booster starts up. I just don't know the best way to do that yet. \$\endgroup\$
    – Jeff
    Commented Mar 1, 2017 at 2:22
  • \$\begingroup\$ The problem with a PFET is that Vout is 0V to start and it only stays off as long as Vout < Vgsth! Also want to make sure the PFET can handle the power dissipation during soft-start. What did you end up doing? \$\endgroup\$ Commented Jun 2, 2017 at 23:40

1 Answer 1

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Costs a GPIO pin, but simple.

schematic

simulate this circuit – Schematic created using CircuitLab

Make sure the MOSFET you choose is rated for the 2.5A continuous,has a logic level gate and a low Rds on. Pull the gate low after Vout stabilizes. If that part had a power good output, you could even tie the power good signal to the gate.

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