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# TPS54521: Okay to use in drop out?

Part Number: TPS54521

Hello,

I have an application that needs Vin=12V-16.8V, Vout=12V, Iout=4.5A.

Of course it is not possible to have 12Vout when Vin approaches 12V, unless the part supports 100% duty cycle.

However, it is not essential in our application for the output to be a perfect 12V; some output sag at low input voltage is acceptable (-7.5%)

It appears this part has a maximum duty cycle of 98%, so doing some quick math I think the output voltage sag would be:

• (DC%) * (Vin - I*R)
• DC%=0.98
• Vin=12V
• I=4.5A
• R=57m + 25m (FET + Ldcr)
• Vout = ~11.4V

This ignores the 2% part of the switching cycle, but I think that's okay for this approximation.

1. Do you think 11.4Vout is realistic?
2. Will the part still behave normally when operating in "drop out" like this?
3. Is there a better part to consider? Cost is priority, efficiency a close second.

Thank you!

• Hi, Sir

1. IC cannot achieve 11.4Vout.

2. No, when Vout < target Vout(12V), FB voltage < Vref, Comp pin voltage will be charged high, IC will think there is overload condition, then trigger OCP and hiccup.

3. As I know, there is no part can meet your requirement.