boost converter

[justDIY]

does anyone have a simple circuit for a 2-3v to 5v boost converter?

I'm looking at Fairchild's FAN5333 boost converter, marketed as a constant current serial led driver, but should be easy enough to use as a low power voltage regulator.

I want to power my 5v led spinner from a pair of AAA batteries and I need 250-300ma of current peak, but not continuous. The batteries don't have to last forever, so efficiency isn't a huge concern, just small size.

https://www.fairchildsemi.com/pf/FA/FAN5333B.html

 

[ronsimpson]

JustDIY,

I like the FAN5333. You are aware there are several versions. Depending on who you get it from there are versions with feedback voltages from 0.1V to 5.0V. And there are some options on peak current limit.

Very simple. Low price. Of the many I tried this is one of the best. I have not tried it with 10 LEDs but with 1 LED you can drive large currents.

 

[ronsimpson]

Look at the FAN5336.
The FAN5333 is built to regulate current. If you are driving the LED direct use it. If you are powering something at 5 Volts use the FAN5336 or similar because it is built to regulate voltage.
The question is do you want to regulate at 5 volts or do you want to regulate at 300mA?

 

[Hero999]

This scematic comes with LTSpice.
https://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1035,P1449,D2742

 

[justDIY]

Look at the FAN5336.
The FAN5333 is built to regulate current. If you are driving the LED direct use it. If you are powering something at 5 Volts use the FAN5336 or similar because it is built to regulate voltage.
The question is do you want to regulate at 5 volts or do you want to regulate at 300mA?

@ronsimpson

i want to regulate to ~5v, with variable current

From what I see in the datasheet, replacing the single current sense resistor with a voltage divider should have it regulating voltage instead of current. I calculated 100k and 6.65k will give me the 0.315v feedback voltage required by the FAN5333B, resulting in 5.05v which is close enough.

Any flaws in that thinking?

The 5336 is nearly twice the price and in a much harder to hand solder package - no hot air reflow station for me yet.

@hero

thanks for the schematic ... Linear isn't an option right now - the vendor I'm ordering from doesn't stock their parts, and the LT1300 is $5+ elsewhere, versus less than a buck for the Fairchild part. National is also off the table for similar reasons.

 

[Hero999]

You can build a cheap boost converter from a couple of transistors and a ferrite core.

Refer to this thread.
https://www.electro-tech-online.com/threads/help-on-simple-dc-dc-convertor-step-up.24941/

Obviously you'll need to change the component values for the 1.8V to 5V conversion you require.

Edit:
You'll also probably want to use a Schottky for DI and something like ZXTN19020CFF ZXTN19020CF, ZXTN25020CFH or ZTX869 for Tr1,

 

[ronsimpson]

FAN5332 & FAN5331
Vfb=1.25
More stable in 5 volt mode.

 

[justDIY]

FAN5332 & FAN5331
Vfb=1.25
More stable in 5 volt mode.

Does the higher feedback voltage offer more noise immunity or something, making it more accurate for regulating voltage instead of current?

The 5332 looks good; I'll grab a few of both the '32 and '33, and experiment!

 

[ronsimpson]

The parts with 0.1V to 0.3V feedback include an extra amplifier and are not as stable.