# Boost converter 3.3 to 5V MAX756

I want to step up my power supply from 3.3V to 5V. For this I choose the MAX756 for his price and his availability. But I don't understand the pinouts...

• Pin 1 : I can leave it on fly ;
• Pin 2 : To ground ;
• Pin 3 : to GND with 0.1µF capacitor ;
• Pin 4 : On fly ?? ;
• Pin 5 : Connect to Vin so directly to my 3.3 input voltage ? ;
• Pin 6 : My output voltage ?? ;
• Pin 7 : Ground ;
• Pin 8 : ??? ;

There is also this schematic that I don't really understand too...

Here you can find the datasheet : MAX756-MAX757.pdf

Thanks!

EDIT

Okay, so thanks to your help I have this now: It should be okay.

I have doubts about the capacitor (C2), do I still need a 100uF capacitor tied to the ground just after my schottky diode (on pin8:LX)? Same question about the capacitor of 150uF tied to the ground between Vin and pin5:LBI ?

Otherwise everything seems okay to me can you confirm ? Huge thanks to you guys !

EDIT2

Here my final schematic:

I think everything is okay now !

• Is it really a signal you want to change? The MAX756 is for a 5V power supply where you have some lower voltage. If you want to change a 3.3V output signal to 5V (say from a microcontroller to some peripheral) then you want something like BSS138 cdn.sparkfun.com/datasheets/BreakoutBoards/BSS138.pdf Perhaps you could describe your actual problem circuit, it would be clearer how to help you. – jonathanjo Mar 17 at 23:16
• My bad, I'm talking about power supply, not a signal. I have a 3.3V supply where I need to get a 5V one. I edited my question – Martin Mar 18 at 9:42

There is also this schematic that I don't really understand too...

First, take a look at the internal diagram given in the datasheet:

The IC is is a simple boost (step-up) converter. Here's a simple diagram:

Image Source (There's a detailed explanation about how a boost converter works)

Pin 1 : I can leave it on fly ;

Nope! Here is what the datasheet says: "Do not leave the control inputs floating." (at p.8, the last sentence before the section Design Procedure).

Pin 4 : On fly ?? ;

If you want to have a low battery indicator then this output can be used. If not then leave unconnected.

Pin 5 : Connect to Vin so directly to my 3.3 input voltage ? ;

Yes.

Pin 6 : My output voltage ?? ;

Yes.

Pin 8 : ??? ;

Refer to the schematic given at p.6 of the datasheet. But yours doesn't require a divider to connect to pin-2. Instead, pin-2 goes to ground.

EDIT as per OP's UPDATE

do I still need a 100uF capacitor tied to the ground just after my schottky diode (on pin8:LX)? Same question about the capacitor of 150uF tied to the ground between Vin and pin5:LBI ?

Yes. Both of them are required.

• I edited my question with your answer and the one of @jonathanjo :) – Martin Mar 18 at 18:42
• @Martin please see my edit. – Rohat Kılıç Mar 18 at 19:18
• see my edit too please, I think everything is fine now ;) ! – Martin Mar 18 at 21:42
• @Martin looks like so. – Rohat Kılıç Mar 18 at 22:07
• thanks you very much for your time !! – Martin Mar 18 at 22:36

The circuit is a boost switched-mode power supply. Basically it stores energy in the inductor and the capacitor, switching back and forth with a transistor at high speed according to a clever signal which depends on the output voltage. (Ask a question about "boost switching regulator" if you want more on this). The basic circuit is like this (which you can see from the block diagram of the datasheet).

simulate this circuit – Schematic created using CircuitLab

Referring just to the datasheet and the typical application circuit:

From datasheet.

• "If the low-battery comparator is not used, connect LBI to VIN and leave LBO open"
• Connect /SHUTDN to VIN, "Do not leave the control inputs floating"
• FB is 3/5 on the 767, connect to GND for 5V.
• Use the other component values they tell you, but pay attention to the notes:
• "Best performance is obtained by using specialized low-ESR capacitors,"
• Inductor which has extra notes in the datasheet: "For highest efficiency, limit L1’s DC resistance to 0.03Ω or less."
• "For optimum performance, a switching Schottky diode, such as the 1N5817, is recommended."

After edits to question:

Yes, you definitely need both C1 and C2.

• I edited my question with your answer and the one of @Rohat Kılıç ;) – Martin Mar 18 at 18:41
• Updated answer: yes you definitely need C1, C2, C3. – jonathanjo Mar 18 at 22:20
• Ok thanks for your help mate it's done now :) !! – Martin Mar 18 at 22:35