LT8638S Power-Up Sequence (EN Before VIN)

Hi Tommybanco ,

Asserting EN before VIN seems to be an allowable condition for the LT8638S. The datasheet imposes no required VIN-before-EN sequencing, and no pin rating is violated by holding EN high with VIN absent. When VIN is applied, the part performs its normal startup, INTVCC comes up, the VIN UVLO releases, and soft-start ramps the output in a controlled manner... regardless of whether EN was already high.

The supply collapse you're seeing is not evidence that this sequence is unsupported. It's an interaction with your current-limited bench supply. With EN already high, the converter turns on the moment VIN crosses its ~2.6 V minimum. i.e., while VIN is still ramping up and well below your 5 V target, and while your lab supply is still in its own current-limit/ramp phase. At that point the LT8638S looks like a constant-power (negative-resistance) load, which is described in the Enable Pin section of the datasheet (p. 21). In an inverting configuration charging a large negative-rail capacitor bank, the inrush is enough to pull your ramping current-limited supply low and latch it there. In your working case, VIN is fully established and out of current limit before EN asserts, so soft-start brings it up without tripping the supply.

To confirm this: please repeat the EN-first sequence with the VIN supply's current limit raised (or from a stiffer source). You should see it start normally, which demonstrates the sequence is supported and the collapse was purely a source-inrush interaction.

V_IN(EN) = (R3/R4 + 1) × 0.98 V

Although this is essentially a different flavor of your working configuration where VIN is applied and above a safe level before EN is asserted which is completely valid as you've already seen.

Best,
Doug

Hi Tommybanco ,

Asserting EN before VIN seems to be an allowable condition for the LT8638S. The datasheet imposes no required VIN-before-EN sequencing, and no pin rating is violated by holding EN high with VIN absent. When VIN is applied, the part performs its normal startup, INTVCC comes up, the VIN UVLO releases, and soft-start ramps the output in a controlled manner... regardless of whether EN was already high.

The supply collapse you're seeing is not evidence that this sequence is unsupported. It's an interaction with your current-limited bench supply. With EN already high, the converter turns on the moment VIN crosses its ~2.6 V minimum. i.e., while VIN is still ramping up and well below your 5 V target, and while your lab supply is still in its own current-limit/ramp phase. At that point the LT8638S looks like a constant-power (negative-resistance) load, which is described in the Enable Pin section of the datasheet (p. 21). In an inverting configuration charging a large negative-rail capacitor bank, the inrush is enough to pull your ramping current-limited supply low and latch it there. In your working case, VIN is fully established and out of current limit before EN asserts, so soft-start brings it up without tripping the supply.

To confirm this: please repeat the EN-first sequence with the VIN supply's current limit raised (or from a stiffer source). You should see it start normally, which demonstrates the sequence is supported and the collapse was purely a source-inrush interaction.

V_IN(EN) = (R3/R4 + 1) × 0.98 V

Although this is essentially a different flavor of your working configuration where VIN is applied and above a safe level before EN is asserted which is completely valid as you've already seen.

Best,
Doug