Hello, I have a question regarding the LT8638S power-up sequence. Is it supported to drive the EN pin high before VIN is applied? In my setup, VIN and EN are supplied by two independent 5 V power supplies. I observed the following behavior: If VIN is applied first and EN is asserted afterwards, the device starts normally. If EN is driven to 5 V first and VIN is applied afterwards, the VIN power supply immediately enters current limit and its output voltage collapses. The converter is unloaded during this test. Is this operating sequence supported, or does the LT8638S require VIN to be present before EN is asserted ? Thank you.

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LT8638S Power-Up Sequence (EN Before VIN)

Tommybanco on Aug 3, 2026

Category: Hardware

Product Number: LT8638S

Hello,

I have a question regarding the LT8638S power-up sequence.

Is it supported to drive the EN pin high before VIN is applied?

In my setup, VIN and EN are supplied by two independent 5 V power supplies. I observed the following behavior:

If VIN is applied first and EN is asserted afterwards, the device starts normally.
If EN is driven to 5 V first and VIN is applied afterwards, the VIN power supply immediately enters current limit and its output voltage collapses.

The converter is unloaded during this test.

Is this operating sequence supported, or does the LT8638S require VIN to be present before EN is asserted?

Thank you.

DPLUMMER

on Aug 3, 2026 3:45 PM
0
Hello Tommybanco ,

Thank you for your question.

The LT8638S datasheet does not specify a requirement that VIN must be applied before EN/UV is asserted. The EN/UV pin is a logic enable input with a rising threshold of approximately 0.98 V, and the converter becomes active whenever EN/UV is above this threshold and VIN is within the operating range.

That said, operation with EN/UV driven high while VIN is unpowered is not a startup condition that is specifically characterized in the datasheet. In most applications, EN/UV is tied directly to VIN or generated from a VIN-derived resistor divider, which naturally ensures that VIN is present before the device is enabled.

The behavior you observed, where the VIN supply immediately enters current limit when VIN is applied after EN is already at 5 V, is not expected for a typical unloaded converter and may indicate an interaction within the application circuit such as startup inrush current, a back-powering path, or another board-level condition.

Could you provide:

A schematic of the LT8638S circuit

Input and output capacitor values

Whether any external voltage is present on VOUT, BIAS, FB, or other pins during the test

The current-limit setting of the VIN supply

These details will help us determine the source of the VIN current limiting behavior.

Best regards, Douglas Plummer Associate Engineer, Central Applications
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Tommybanco on Aug 4, 2026 3:29 AM
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I am attaching the schematic for reference.

As mentioned previously, the LT8638S is configured as an inverting buck converter.

The board has a solder jumper to select the VIN source. For this test, the jumper is set to the 12 V VIN position; however, I am applying 5 V from the laboratory power supply instead of 12 V. Please note that this VIN rail is used only by this section of the circuit and is not connected to any other circuitry on the board.

The EN pin is not driven directly by the 5 V supply. It is driven by a BJT, which is controlled by a voltage supervisor. The voltage supervisor drives its output high (5 V) once VCC has reached its regulated value and is operating correctly.

Please note that VCC and the 5 V supply applied to the 12 V VIN rail are provided by two separate laboratory power supplies.

With this configuration:

If VIN is applied first and EN is asserted afterwards, the converter starts normally.

If EN is driven high first and VIN is applied afterwards, the VIN power supply immediately enters current limit and its output voltage collapses.

Could you please confirm whether this power-up sequence is supported, or if VIN must always be present before EN is asserted?
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DPLUMMER

on Aug 4, 2026 9:45 AM
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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.

If VIN itself is current limited the recommended fix straight from the datasheet is: program a VIN UVLO by adding a resistor divider from the VIN rail to EN/UV so the LT8638S only enables once VIN is above a safe threshold:

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
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