Intro

When I searching for most cheap and available method to get isolated power for isolating interfaces (UART, RS232, RS485, CAN...) I found DC-DC modules converters. These converter relatively small, cheap and super common (especially 5V->5V):

Usage Example (Isolated UART)

The idea is that left side (insolated) powering from 3.3V/5V and other side (isolated) get 3.3V/5V or other voltage from DC-DC converter:

For test purposes I soldered capacitors according to the datasheet close to the pins and thin wires for power bench and multimeter connection.

🔺 Issue Description

DC-DC 3.3V

No Load	With Min Load
VIN = 3.281V	VIN = 3.299
VOUT = 3.680V	VOUT = 3.545V

Note: As you can see even at minimum load current the VOUT is higher by 3.545-3.300=0.245V which close to the IC typical maximum voltage 3.6V

DC-DC 5.0V

So, high output voltage when no load and voltage depends on current.

❔ Causing

In the datasheet we can see not only MAX current, but also MIN current, which for B0303S (3.3V) is 30.3mA and for B0505S (5V) is 20mA, which could be significant for many circuits:

🔧 Issue Fix

Additional Load Resistor (RLOAD)

Resistor can always maintain required minimum current:

Additional Voltage Regulator (LDO)

When you need stable voltage on isolated side consider small (SOT-23) and cheap voltage regulator (XC6206)in, so you can install it with 3.3V or 5V DC-DC and get 0.2-0.3A, which is more enough for many ICs.

✔ Fixed

I recommend to use these setup if you need stable voltage 3.3V on the isolated side:

• B0303 + LDO 3.3V
• B0505 + LDO 3.3V
• B0305 + LDO 3.3V

Conclusions

• Output voltage of the isolated DC-DC converter depends on output current
• Exact voltage value depends on load current and DC-DC manufacturer
• Maximum output voltage may damage sensitive components, since using it without appropriate load cause higher VOUT than you expected