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LM5019 Constant on time Buck controller needs no “added ESR”?

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treez

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The LM5019 is a Buck controller using constant off time control.
We want it to do 24V to 16V at 50mA.
Page 10 and 13 of its datasheet express that "ESR" should sometimes be added to the output capacitance (via an added “ESR resistor”). This is in order to create a ramp voltage at the switching frequency which can be used by the PWM comparator to help give relatively stable regulation of Vout.

.........Though i can certainly see the reasoning behind this, ...for our application, i don’t see the point in doing it. All we want is a 16V DALI bus which will be loaded with 50mA at the most. I appreciate that the LM5019 will operate in Burst mode if we don’t add the “external ESR” to the output capacitor, but i don’t see the problem with that...do you? Even if there’s one volt pk2pk of vout ripple, this is of no concern to us.


LM5019 datasheet
https://www.ti.com/lit/ds/symlink/lm5019.pdf
 

7.3.1 explains the operation. It doesn't have the usual analog feedback, it deliberately oscillates around the reference voltage to turn the output device on and off. If you don't allow a small amount of ripple back to the comparator, it gets stuck in on or off state. The resistor is essentially an RC filter to remove the ripple from the output if a clean output is needed. If you short the resistor it will kill the ripple to the feedback input.

Brian.
 
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I’m confused. If you don’t care about output ripple then add the esr. This type of control “likes” esr and ripple so you’ll reduce risk by adding it in.
 

Thanks, and i presume at startup, the vout divider voltage is below the reference voltage for a time interval, and so then the LM5019 just oscillates on and off with the TON and TOFF times as given on page 11?
 

I would guess so, it just seems to pulse on/off continuously from it's timer until the comparator starts sensing the ripple and takes over.

Brian.
 
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