# Need Help! LM2623 Converter

Discussion in 'The Projects Forum' started by Etronic, Sep 19, 2012.

1. ### Etronic Thread Starter Active Member

Oct 7, 2011
127
2
Hi

The schmatic i enclosed has a 5V output.I need a 9V ouput and a 3.3V output.Got this from TI.

I want to use this circuit to boost the output at the voltages above.

There will be two separate outputs using two separate converters.

1.I need to know at what FQ is the switching circuit set at in the enclosed schmatic. this set the Mosfet to ture on/off.They state to have it set to 300KHZ to 2MHZ.
2.What schottky diode are they using in this circuit at 3A.
3 The voltage input i need to use is 1.2 and 3.6 for each separate circuit.
But i don't know how to set the input voltage as they suggested in the
Design notes on page 10 in the Datasheet.
4. I will also need to change the voltage divider resistors for the two voltage outputs that i need to use.

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2. ### crutschow Expert

Mar 14, 2008
20,812
5,915
1. R3 sets the switching frequency. Look at the middle-left graph on page 6 of the data sheet for frequency versus input voltage for various values of R3.

2. Any 3A Schottky with a voltage rating of at least 15V.

3. Are you referring to Vdd? It must be between 3V to 5V.

For 3.3V output you can connect Vdd directly to the output as shown.

For 9V output you can connect Vdd to the output through a voltage divider. For example 1870Ω connected to the output, in series with 1500Ω to ground will give 4V at the resistor junction, which is connected to Vdd. You should decouple Vdd with a 0.1μF ceramic cap to ground.

4. The reference voltage at VF is typically 1.24V. The voltage formula is VF = (Vo x RF2)/(RF1 + RF2).

For 3.3V output you can use RF1 = 221kΩ and RF2 = 133kΩ.

For 9V output you can use RF1 = 102kΩ and RF2 = 16.2kΩ

3. ### Etronic Thread Starter Active Member

Oct 7, 2011
127
2
Thanks for helping me. I will try those values for the voltage divider

This coverter on its Vdd has a input voltage of 0.8V to 14V.
But has a start-up voltage of 1.1V.

Last edited: Oct 15, 2012
4. ### Etronic Thread Starter Active Member

Oct 7, 2011
127
2
Hi

I been trying to get this circuit to work.
Its the same boost converter. but? uses 1 cell.From TI LM2623 circuit cookbook.An1221.
I put it on a breadboard to test. But,I can't get it work.
I want 3.3 volts on the output.The input is a 1.2V cell.
the load on the output is between 15-25mA.

I tryed connecting the parts about 20 times.No luck.
I never have this problem building circuits.But this one is confusing the hell
out of me.

Can someone draw me a drawing of this circuit on a breadboard and post it here.I may be connecting some parts wrong.
i want to see someone else's drawn circuit on a breadboard.