# Can this this dual output DC-DC converter be used for negative supply?

An application requires a -5 V negative power supply with max 550 mA. I can't find a reliable dedicated positive to negative converter for such currents in the market at a reasonable price.

I came up with this DC-DC converter and here is the datasheet.

Can I just ignore the +Vout and only use -Vout for a circuit which requires -5 V and max. 550 mA where ambient temperature will not exceed 40°C? And if I use an op-amp with such a negative supply should it be dual type?

If a DC-DC module is isolated, then you can convert a positive output into a negative output by simply connecting the positive output pin to GND:

This is why you wont find isolated modules that specify they are positive in, negative out. Because they can be either.

The part you link to is indeed isolated, so you could use say TEN 8-1211 in the configuration pictured on the right to get a negative output (connect +Vout and -Vin to GND, giving -Vout as -5V).

Given you only need ~3W, lower power and smaller isolated DC/DC modules with regulated 5V output are readily available (e.g. TDN 5-2411W).

• If line power is acceptable, one could use the most ubiquitous supply of all: the USB 5V charger cube. Just hack the USB cable. Jan 27 at 22:25
• @Tom Carpenter Thank you for the answer. This is interesting. I was planning to use negative supply for a laser diode driver which requires -5V such as i.stack.imgur.com/Xqt04.png . And as you see anode of the diode is grounded. Let me know you still think it is fine with isolated DC-DC single supply Jan 27 at 23:05
• @user1245 See no reason why a single supply wouldn't work when wired for negative output. You might want to get a supply of higher voltage than you need them use a negative LDO linear regulator to filter noise. Jan 27 at 23:16
• @TomCarpenter I see. So regarding single the DC-DC converter to obtain -5V I can just connect the +Vout to the Vin ground and his will give me a -5 V at the output's ground. Correct? Just asking to be sure I understand. I actually can also use lower than -5V. They are either 5V or 12V. How can I use LDO in this case? I cannot find -4.7V or -4V LOD or even -3.3V LDO. Or and DC DC converter which is between 12V and 5V. Jan 27 at 23:49
• @user1245 Requires? The diagram shows the anode connected to ground. But why does the anode HAVE to be connected to ground? you could just connect the anode to +5V Jan 27 at 23:50

You can. But some modules needs extra minimum load.

Some dual output power supplies are unstable when leaving one side as open. For a little more detailed information, check this FAQ About DC-DC Converters article. Thus, you may need to connect some load resistor to the unused positive side.

Check the datahseet of your DC-DC module carefully. There might be minimum required load current for stable operation.

I have no expeirence with your specific model, but with some other dual supply modules. To use TDK +/- 12V DC-DC modules, I should connect 100 ohm / 3W resistor for +12V to use -12V. If not, I got + 13.5V / - 10.5V.

• There is sort of a caveat in the datasheet: Regulation – Load variation 0 – 100 % dual output models: 1 % max. (balanced load) 5 % max. (Load cross variation 25 % / 100 %). Not quite sure what this means with no load on the first output - the simple way out would be dissipating 1 W in a 25 Ω resistor. Feb 1 at 8:30

To address the second part of the question, any single-rail op amp can be used with a negative supply. Conceptually there’s a 5V difference between the two rails, one is designated as ‘ground’ and the other as relative to ground. Within the circuit the components don’t (for many purposes) know or care which rail is ground, they only see the voltage differences. So as far as the op amp and other components are concerned, you can use a positive-rail circuit but connect the positive rail to ground.