Step-up PSU with TL499A

[Kain]

I have been trying to build a low power step up PSU for my PIC programmer. It is supposed to take 5V DC and convert it into 20VDC. The load takes about 50mA. I tryed to achieve it with TL499A, and on first sight I think the schematic and the board look good. However, when I built it it seems that something is shorting up - I think the chip is not working properly for some reason. I know that it is shortening because the current protection of my PSU goes on even when i set it to upper limit, such as say 250mA...

Circuit.pdf

PCB.pdf

 

[audioguru2]

Hi Kain,
I think your TL499A is working normally, but you are using it near its limit. To get "about" 50mA out (maybe 60mA?) at 20V, the 5V input will draw an average of 200mA (or 240mA) plus more for its efficiency loss. TI's application note shows its efficiency of between 65% and 80% with only a 3V input. so if its efficiency in your application is 75%, then its 5V average input current is between 267mA and 320mA.

It draws a short peak input current much higher than its average input current calculated above. So it definately needs a big capacitor on its input to ground, so that your 5V supply doesn't current-limit. ;D

Hey! 3000 posts for me! ;D ;D ;D

[move]3000 posts!    ;D ;D ;D                3000 posts!    ;D ;D ;D                3000 posts!    ;D ;D ;D                3000 posts!    ;D ;D ;D                [/move]

View attachment 37230

 

[Kain]

Congratulations for the 3000 posts (already 3004). Is there going to be a party too? ;D About the step-up PSU - i tried setting the current limiter to 350mA and yet exactly the same as before happens. Sometimes actually the board fires up, BUT I am unable to regulate the output trough the trimpot while I am actually supposed to be able. The output simply stayed at 4.65 - 4.7V which is even lower than the input... I really hate when something doesn't work

 

[Kain]

I just tried it - doesn't help at all. You ment the capacitor to be between the input and ground, correct? I used 1000uF cap to try it  ???

 

[audioguru2]

Hi Kain,
I didn't check your pcb layout, but it sounds like your TL499A is busted.

 

[Kain]

Hum... Am i supposed to solder those with low voltage soldering iron?

 

[audioguru2]

Hi Kain,
You decided to use tiny little SMD parts, so have fun trying to solder them! ;D
You have the TL499A running close to its power limit so maybe a little SMD one overheats.

 

[Kain]

Well, to be honest it never even started working on first place. I changed the chip once too and guess what - no change at all. I do not think I have overheated the part either - the soldering was really quick  :

 

[MP1]

Kain,
Since this is for a PIC programmer, is this supply to be used with a graphic display? If so, the voltage should be a negative voltage if I am guessing your application correctly. Graphic displays such as Sharp LM24014H and Epson SED1330. Just a thought.

MP

 

[Kain]

Actually the reason why I am making it is because I want to power my programmer stright from the USB port which can supply 5V @500mA max. My programmer requires 18V @ 50mA so I wanted to boost the USB's 5V to 18 or 20V and use a single cable to connect the programmer to any PC (no power adapters).