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LM5140-Q1: Under Voltage Lockout (UVLO)

Part Number: LM5140-Q1
Other Parts Discussed in Thread: TPS3710-Q1

I'm using LM5140QRWGTQ1 with pretty much the example schematics. VOUT1 outputs 3.3V, VOUT2 outputs 5.0V. EN1 is connected to VCC.

I'm using LM5140QRWGTQ1 with pretty much the example schematics. VOUT1 outputs 3.3V and VOUT2 outputs 5.0V (FB1+FB2 is connected to VDDA). EN1 is connected to VIN and VCCX is connected to VOUT2.

When VIN is raised to ~3.5 V the circuit begins trying to output VOUT1, but the voltage is not stable until VIN is raised to ~4.5 V. This all matches what the datasheet says. VCC(uvlo) is expected to be TYP 3.4 V and Vih of EN1 is MIN 2.4 V.

I’d like the circuit not to start outputting voltage until VIN is > 6 V. Is there a recommended way of doing this with LM5140-Q1?

I could put a simple resistor voltage divider between VIN and EN1 to have it enable the circuit when VIN is > 6 V, but there’s only a MIN value specified on Vih of EN1. My guess is that this is the voltage needed for you to guarantee that the IC will be enabled? Could you also specify at which voltage the IC might get enabled? The value must be valid over the entire temperature range of the IC. The Vil threshold of EN1 is specified to be MAX 0.4 V. I’m hoping that this is partially due to hysteresis and that the IC not might be getting enabled when EN1 is >0.4 V.

I’d like not having to use a comparator or similar to control EN1 if possible.

If there is some built in hysteresis on the EN1 input, would it be possible for you to specify the minimum hysteresis?

  • Hi Albin,

    The LM5140-Q1 EN inputs are actually semi precision, even though we don't publish it in the datasheet. Here are the approximate limits:

    Instead of 2V max, EN rising threshold is 1.61 max, 1.45min (avg 1.53V)

    Instead of 0.8V min, EN falling threshold is 1.04Vmin, 1.2V max (avg 1.12V)

    If these levels are unsuitable or the hysteresis is too wide, you can add an input supervisory IC (e.g. TPS3710-Q1) to control EN as needed.

    Regards,

    Tim