Whether MC34063 Can Work in CC Mode?

[Anil Thomas]

Hi Friends,
I designed one circuit with MC 34063, Which is having input voltage 20-30, and output 18v, 700mA.(Buck Converter). But If the input voltage is 30v I am able to get 700mA. While input voltage is decreasing output current also decreasing. For 20 V input I am able to get only 400 mA. What I can do to get constant current..? Help me..

 

[FvM]

I presume you are operating 34063 as voltage reguator and are not asking for constant current but for increasing the maximum output current at lower input voltage?

To understand why the output current is limited, we have to refer to your circuit details.

 

[mrflibble]

If your input has limited current capability then you get exactly the situation you describe. Power being voltage x current and all that.

 

[Orson Cart]

the max pwm of this chip is 5/6 so at 20V in the max out is 16.66 volts, not the 18 volts you are after, this is the reason for the lesser current you are seeing.

 

[FvM]

the max pwm of this chip is 5/6 so at 20V in the max out is 16.66 volts, not the 18 volts you are after, this is the reason for the lesser current you are seeing.

That's obviously the problem.

Although the duty cycle can go as high as 87% at lower switching frequencies (review data sheet Figure 1, Output Switch On–Off Time versus Oscillator Timing Capacitor ), the output saturation voltage must be also taken into account, it's about 1.6 V in emitter follower configuration (no external switch transistors used).