TPS55340: How to make isolated buck?

Thanks for fast reply. 

Thanks for reference to the other chips. 

Can any of these chips make an adjustable-voltage buck/stepdown?

Hi Johny,

Can you describe your application requirements in detail?

Input voltage range:

Output voltage range:

Output voltage adjustable?

Output current:

Isolation required?

Best Regards,

Feng Ji

Input voltage range: 12V to 24V DC

Output voltage range: up to 5V

Output voltage adjustable? Yes

Output current: up to 4A or 5A

Isolation required? 150V

Priorities: Low BOM cost, small inductor

Hi Johny,

I think https://www.ti.com/tool/PMP40569 is a quite similar to your application.

The voltage stress of the flyback will be much higher than 24V. LM5157 or LM5158 will leave you more margin than TPS553440.

What is the minimum output voltage? And how will you adjust the output voltage?

Best Regards,

Feng Ji

by "stress" do you mean losses heat?

how is voltage adjusted on the LM5157? Resistor? PWM?

The PMP40569 doesn't meet my requirements: 36~57-Vdc input (i need 12V-24V), 5-V outuput (i need adjustable)

Hi Johny,

The voltage stress means Absolute Maximum Ratings of each pin must be satisfied.

PMP40569 should be a good reference for you.

Typical controllers are designed for fixed output voltage. You may adjust the output voltage with additional circuit.

Best Regards,

Feng Ji

What is the adjustable voltage circuit?

Hi Johny,

There are many ways to adjust the output voltage. You may refer to Methods of output-voltage adjustment for DC/DC converters.

Best Regards,

Feng Ji

Thanks for that useful article. Applying a DAC output to the FB pin might be my chosen method. 

How to combine that with feedback from the feed-back optocoupler from the secondary?

The TPS55340 datasheet states:

To set the output voltage, select the FB pin resistor RSH and RSL according to Equation 3.

What do Rsh and Rsl stand for?

Your article mentions:

The main disadvantage of the VSET pin in devices without an I2C bus is that the output voltage cannot be changed during operation because the resistor value is only measured at startup

Can the TPS55340 Vout be adjusted during operation?