What the 3V3 pin can give

The front page and the small print

The ME6211's data sheet says 500 mA on its first page. That is what the part can deliver with its input at 4.3 V, only a volt above its output.

A regulator of this kind drops the difference between its input and its output across itself, and every milliamp through it turns that drop into heat. From 5 V, the drop is 1.7 V. The SOT-23-5 package it comes in is rated 300 mW, and 300 mW over 1.7 V is about 176 mA.

Slide the current up. Below about 50 mA the regulator barely notices; past about 175 mA it is running hotter than its package is rated for. The data sheet gives the 300 mW without a temperature, so this book reads it as the limit on a bench at room temperature, not in a warm enclosure.

What actually takes current

• The TK97 itself: 60 µA for the regulator, and 0.33 mA for each L line held LOW. With all six held LOW, about 2 mA.
• A sensor or an I2C block in the socket: a few milliamps.
• A display with its backlight: more, and its own book is the place to look. Anything that stays under about 150 mA leaves room.

An Uno's own 3.3V pin is good for 50 mA. That is one reason a 3.3 V block beside an Uno is better off in the TK97's socket than on the Uno's 3.3V pin.

Where the 5V comes from

Whatever the socket takes, the 5V pin gives, plus a little. On USB that is shared with your whole board; a computer's port promises 500 mA. A 5V pin that sags under load shows up here as a 3V3 pin that sags too, once the 5V falls within about a quarter of a volt of 3.3 V.