# Understanding LDO Dropout

Disclaimer: Super newbie and first time poster here.

The MCP1700 LDO regulator datasheet states on p3 that the Droptout voltage is 350mV for Vout > 2.5V @ I=250mA. I thought great I can power my ESP32 from an 18650 battery. 3.7V - .35V = 3.35V. Works out perfectly! Well it doesn't. The regulator's Vout kept dipping below 2.5V and the ESP32 kept browning out.

I couldn't understand this until I went back and read the datasheet, carefully this time, and saw this note 5: Dropout voltage is defined as the input to output differential at which the output voltage drops 2% below its measured value with a VR+ 1V differential applied.

My question: If I need a whole extra 1V for the regulator to properly work, then why tell me that the drop out is 350mV? What's the significance of this information? I came across the same thing with multiple regulators so I must be missing something.

Thanks!

• Do you know the actual load current? Have you decoupled the input and output as required? Commented Apr 29, 2020 at 9:13
• The 1 V extra is just a reference point to measure the dropout voltage. With a 3.3 V regulator, then 3.3 V + 1 V = 4.3 V is applied. Then the output voltage is measured, it will be around 3.3 V. Let's say it is 3.305 V. Now we lower the input voltage slowly until Vout has dropped 2%, that is when Vout = 3.305 V - 2% = 3.239 V. Then Vdropout = Vin - Vout. Commented Apr 29, 2020 at 9:14
• The LDO output is stable when using only 1µF output capacitance. What input/output capacitors did you use? .. as @PeterSmith also asked. An oscillating output may have fried your ESP32. Commented Apr 29, 2020 at 9:14
• The ESP draws current in a spikey manner- have you confirmed the actual input voltage dips under load with an oscilloscope? Commented Apr 29, 2020 at 9:32
• Yep I used capacitors just like described in the datasheet. 1uF Cin and 1uF Cout. I also tried adding an electrolytic capacitor 47uF. Commented Apr 29, 2020 at 19:04