# How "well regulated" must the input voltage to a DC-DC boost controller be?

According to its datasheet, the TPS43061 "is a low Iq current-mode synchronous boost controller with wide-input voltage range". My intent is to use this controller to boost and regulate the stepped-down and rectified mains voltage from 12.6 V to 15 V, supplying up to 6 A output.

Under "Power Supply Recommendations" in Section 10 of the datasheet, it says that "[the] input supply should be well regulated." How regulated is "well" regulated? There doesn't appear to be a line regulation parameter in the electrical characteristic section.

My guess is that the "well regulated" part means that the input voltage ripple should not exceed some designed amount, and indeed the minimum input voltage is used in calculating the values for various external components.

Can I assume, living in the United States, that the mains voltage will be stable enough to follow this recommendation, assuming that the output of the rectifier has enough capacitance?

• Slightly odd setup there - where's the 12V input from? Why not just have a normal mains-driven 15V 6A psu? Jan 15, 2018 at 15:09
• I am trying to design the 15V 6A power supply. The 12V is the output of a bridge rectifier following a 12.6VAC transformer secondary. Jan 15, 2018 at 16:07
• "The 12V is the output of a bridge rectifier following a 12.6VAC transformer secondary." Peak output voltage will be ~12.6V*1.414 - 2 diode voltage drops = ~17V. A boost regulator can only raise voltage not lower it to 15V, so you have a problem! Jan 15, 2018 at 16:53
• @BruceAbbott Good point, I forgot about that. My question bout the "well regulated" supply input still stands though, since I could still (in theory) use this chip to boost to 20V or 24V or something. Jan 15, 2018 at 16:59

Power Supply Recommendations

The device is designed to operate from an input voltage supply range between 4.5 and 38 V. This input supply should be well regulated. It is important to remember a boost topology requires an input current greater than the output current. The power supply must then be capable of supporting a current approximately equal to IOUT × VOUT / (VIN). If the input supply is located more than a few inches from the TPS4306x converter, additional bulk capacitance may be required in addition to the ceramic bypass capacitors. An electrolytic capacitor with a value of 100 μF is a typical choice.

If the input supply isn't well regulated, under some loading conditions its voltage may drop too low and cause the booster circuit to try and take significantly more current from the input supply (in order to maintain the boosted output voltage on its own load) and this rapidly spirals into either failure of the input supply or some kind of oscillation on the booster output.

In other words, when using a voltage booster either take care to prevent sudden changes in output load current inflicting even greater changes on the current taken from the input voltage supply or use a powerful input supply that has inherently good regulation.

• So if I am using an unregulated DC voltage source like rectified mains voltage, I should just take care to have enough capacitance? Is there a better way to pre-regulate the input supply? Jan 15, 2018 at 16:30
• @ZacCrites if I were you I'd probably simulate a bridge rectifier with a varying load to get the capacitance required. LTSpice is free and fairly easy to use if somewhat limited but yes, it boils down to choosing a bigger capacitor usually. Jan 15, 2018 at 16:58

Ok, so the regulator will have it's own bandwidth, probably not higher than few kiloherz. Obviously you will want any fluctuations of the input to be lower than that. But more accurate answer requires some specifics: power, current, input/output voltage, load properties.

But the bottom line actually should be "relax, just do it". A chance to screw up if you work according to the application notes is very low.

Pay attention! Your input will be DC, nothing to do with mains!!!

• The input will be DC, but it will be unregulated. My questions is about whether or not the rectified mains voltage is stable enough to be considered "well regulated", as recommended in the datasheet. Jan 15, 2018 at 16:09
• Well. Like i said, more details required. But in your case you can solve anything with additional capacitance. Mains will provide enough power. And- it's not regulated, at least not the common way
– user76844
Jan 15, 2018 at 16:11