TPS55010 Shorter Transformer

Hi Kevin,

There are a few of reasons the two transformers you linked in may not work well with the TPS55010. First the inductance needs to be relatively low for a flybuck to meet the minimum peak to peak ripple requirement. Equation 14 in the datasheet can be used to calculate the maximum inductance based on the minimum ripple current requirement. Second it is important to make sure the saturation rating for the transformer is high enough. The 4.7uH LPD5030 may be good enough but it depends on the application conditions to determine the inductance and actual saturation current rating needed. I didn't see a saturation rating on the 750110055 but the high inductance of this transformer will most likely make it not an option anyway.

The last limitation would be the 1:1 turns ratio. The output voltage is just the primary side regulated voltage scaled by the transformer turns ratio. The primary side regulated voltage is essentially the output voltage of a normal buck converter and the highest we recommend this voltage being is 80% of the input voltage. However operating as close as possible to 50% duty cycle is ideal. A 1:1 transformer may work for an application that takes a 5 V input to ~3.3 V isolated with the primary side set near 3.3 V but it would not work for a 5 V input to ~5 V isolated for example. The highest possible primary voltage is 4 V limiting the isolated voltage to 4 V - Vf, where Vf is the forward voltage of the schottky diode on the isolated side. A 1:2 transformer is best for 5 V to 5 V output where the primary side is set near 2.5 V.

Table 4 in the datasheet should provide a good reference for specifying a custom transformer for any application. Let me know if you have any questions on this.

You may also find the PMP6838 reference design of interest for an example of very small transformer. However this uses an inductor rated for only 100 V. To get the required isolation, a customized version of the either of the transformers you linked in could likely work.

Best Regards,
Anthony

I also wanted to suggest that you look at the SN6501. This would be a better option if this is powering a sensitive analog part of the system where keeping noise low is important. I'm not completely familiar with the ECG application but I'm thinking this might be a concern for parts of this system.

The tradeoff made with using the SN6501 instead is it is an open loop push-pull power supply and depending on how tight of regulation is required, an LDO may be needed. The TPS55010 on the other hand has regulation on the primary side of the transformer and may not require post regulation, again depending on the required regulation. If the isolated power is needed for something digital instead where low noise is less of a concern, the TPS55010 will work fine.