[Power supply column] Part 3: When the input voltage is increased, the output voltage becomes unstable...

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Basic Power SupplyAnalog Devices

This is Sasaki in charge of power supply dock technology.

In this column, I would like to introduce some examples of problems related to power supplies that I encountered multiple times during various customer support sessions.

One day, I received the following inquiry from a customer.

I should be satisfied with the specs...

I am having trouble with the output power becoming unstable when I increase the input voltage of the DC/DC converter. . .

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The output voltage is unstable and fluctuates at a low frequency and does not meet the power requirements of the FPGA
Input voltage and output voltage are within the specified range
No problems with circuit constants for peripheral circuits

Then visit the customer immediately.

When observing the output voltage and switching waveform of the DC/DC converter with an oscilloscope...

Article header library 127841 pic01 1 — Ch1: Output voltage waveform, Ch3: Switching waveform

The switching waveform is broken. This is the reason for the unstable output voltage.
This power supply IC cannot lower the output voltage if the input voltage is 7V or higher under the current circuit conditions.

e? However, the data sheet for this product says that it can be input up to 28V.
Are you saying the datasheet is wrong?

Switching DC/DC converters have a minimum on time that is determined by the input/output voltage difference and switching frequency.
This time it seems to be in an unstable state because it does not meet the regulations.

Cause of unstable output voltage → Deviation from Minimum On Time regulation

For switching DC/DC converters, the average voltage during on/off switching operation (PWM) is the output voltage. When obtaining a lower output voltage from a higher input voltage, the switching on time is shorter. Minimum On Time is how much you can shorten the on time.

This time, the on-time was extremely short, deviating from the minimum on-time regulation, and the output voltage became unstable.

When I tried lowering the input voltage a little (slightly longer switch on time), I was able to get a normal waveform within the Minimum On Time specification.