Boost Switching regulators - Page 1
Forum section: electronics / projects

author: Kay
author_rank: Contributor
timestamp: November 29, 2014, 06:55:21 am
content: I have an application where I would like to set the output voltage of a switching boost IC such as AAT1217, TLV61220,..at 3.3V and I-out max=150mA.The typical input voltage to the boost IC would be Vin<=3.3 from a Li-fepo4 battey, single cell.But when the battery is under charge, the Vin to the boost IC can go as high as 3.7V.I have tested few boost IC and it seems to work except the Vout changes a little when Vin>3.5v, which is not a problem at the output, but my question is , if this may cause damage to the IC, inductor, or excess EMI, or any other issue in the long run, any idea??The Speic ICs are expensive and don't seem to have high efficiencies.thanks

author: tautech
author_rank: Super Contributor
timestamp: November 29, 2014, 07:53:38 am
content: AAT1217 datasheet says VINmax 6 VTLV61220 datasheet says VINmax 7.5 VSo no problem for both of these.

author: Kay
author_rank: Contributor
timestamp: November 29, 2014, 09:21:00 am
content: Hi tautech,thanks for the reply,the recommended operating range in AAT1217 datasheet in the Features, first page, says Vin=0.5 to Vout.my case , Vin=3.7, it is > 3.3= voutgenerally when Vin>Vout, won't that cause any issues, damage or instability in boost regulators?

author: Zero999
author_rank: Super Contributor
timestamp: November 29, 2014, 11:56:42 am
content: Quote from: Kay on November 29, 2014, 09:21:00 amIdeally you need a different topology, one that can reduce the voltage as well as boost it, such as SEPIC. As the input voltage is only 0.4V greater than the output, you might just get away with it but it's something you need to test.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2014, 05:32:50 pm
content: You could burn the extra 0.7V with a PN diode instead of a schottky diode. If you don't mind the drop in efficiency.The chip will simply throttle down as it attempts to regulate the output voltage.A SEPIC topology probably would've been a better choice though, besides which, it would be fine for any lithium type cell (up to 4.20V for LiPoly?).Tim

author: Kay
author_rank: Contributor
timestamp: November 30, 2014, 01:30:28 am
content: Hi Tim,The cell is Li-fepo4, it offers a lot more cycles compared to Li-ion, and better for solar charging because of less self discharge rate.if the cell is not under charge, the output voltage of the cell is <3.3V so in most of the time, the input to the boost is less than 3.3, only when it is under charge the voltage go up to 3.7V max.The SEPIC I have seen is expensive and don't offer the the type of efficiency compared to boost.It seems if I increase the Vout to 3.5V, then it would be safer as the 0.2V extra is equal to Vdiode, but that 0.2V equals some efficiency drop,I tested some boost that use an external diode for when the input voltage is Vin>Vout+Vdiode, the switcher simply stops switching and then Vout becomes Vin-Vdiode,the boost IC that have their own rectifying diode such as TLV61220 behave differently though. even when they stop switching the Vout is still under some regulation,

author: Zero999
author_rank: Super Contributor
timestamp: November 30, 2014, 02:29:41 pm
content: I suppose the question is what's the absolute maximum voltage which can be tolerated on the 3.3V rail? If it's greater than 3.7V minus the resistive losses in the wiring and switch at the current drawn, then it shouldn't cause a problem.

author: Kay
author_rank: Contributor
timestamp: December 01, 2014, 05:36:46 am
content: The rail can do up to 3.8V.in terms of possible issues at the switcher some ICs act differently though, just checked FAN4860, Vout=5,the output becomes a ramp from 0 to 5V, with an average at 0.6V when Vin>5.1,I attached a diode from SW to Vout then the Vo=Vin-0.3, a DC signal, it is still switching but at a very low frequency