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I need a low cost, low power (50mA) isolated power supply. The SN6501 looks great, but I have trouble finding an isolation transformer for output voltages >5V. I think that with a primary side voltage of 5V +-2% and a secondary side voltage of 15V + headroom for a LDO on the isolated side I need a 1:3.3 transformer. Coincidentally this is the turn ratio of ADSL transformer. But are they really suitable?
Hi Lukas,
There is no reason you cannot step the voltage up on the secondary side of the transformer. The only issue will be for you is that you will be trading off on current driving capabilities. The device is only capable of driving a continuous power of 250mW (so at a 5V VIN you are limited to 50mA continuous). Therefore, you will not be able to generate a 50mA continuous supply at 15 volts on the secondary with this device.
If your current requirement can be lowered, there are multiple topologies shown on page 22 of the datasheet that can step up the output voltage of the secondary. Are you just looking for transformer part numbers? If so, I would recommend contacting any one of the transformer manufacturers that we have listed on page 22 of the datasheet.
Another option is to look at the TPS55010 which is a higher power isolated DC/DC converter with additional features.
http://www.ti.com/product/tps55010
Thanks,
John
I saw the TPS55010 but the problem is that it is a QFN which causes various problems (AOI etc.). Is it possible / recommended to boost the SN6501 output current with external FETs?
Hi Lucas,
The D1 and D2 outputs of the device are going to go from High Impedance to GND. Are you thinking that you would use this to drive a P-Channel FET? Not sure if I am following what you were thinking.
Thanks,
John
I thought using a NMOS on each output (with adequate pullup resistor on the Gate). But this seems to be to much trouble and similar devices from other manufacturers are able to handle more output power.
Thanks for the help!
Hi Greg,
You are correct, I made a mistake (I just took power and divided by voltage which makes the incorrect assumption that the full 5V is dropped over the low side drivers... which is incorrect). Assuming that your ambient temperature is below ~95°C, you can continuously draw 350mA of current and not exceed the 150°C max junction temperature. I apologize for the mistake.
In terms of the typical curves in the datasheets, one of the caveats is that these are only typical and do not show worst case which is what you have to design for. How much current at 15V do you need?
Thanks,
John
Hi Greg,
I agree, for typical condition the device can handle quite a bit of power. The most common feedback we get with this device is the want to handle more power. We appreciate the feedback and will use this for future devices.
In terms of the higher output voltage diagrams on page 21 of the datasheet, I agree that it is important for the schematics that charge different output caps on each output cycle of the SN6501 to have equal loading. I appreciate the detailed explanation.
Please let us know if you need anything else.
John
Hi Greg,
I am hoping it will be more bullet-proof! :)
Two other things we often hear are the ability to have a soft-start and a current limit. I believe with the current limit the device will be more robust, whereas the soft start is more of a nice to have feature.
Thanks,
John