Designing Adjustable Voltage SMPS

[Warpspeed]

Yes, you just need to put "something" in series with the load that is both fully adjustable and controls the maximum current that can flow to the load.

 

[fethiyeli]

What about feeding the shut down pin with clock signal ? Am i able to achive very low voltages by using shut down pin with clock signal or something in order to eliminate the one of the pulses at output ? The logic is to eliminate one of the pulses. If done and successfull, so the only one pulse will try to regulate the output and that pulse will also try to increase its duty cycle.

 

[KlausST]

Hi,

Just build a fixed output voltage SMPS and then "manipulate" the feedback by adding/subtracting a fixed current directely at the Fb pin.

Usually there is a resistor divider connected to the FB pin. R1 to output voltage, R2 to GND.
The needed current is R1/delta_V.

Sadly you didn't post any circuit we can refer to...
Example: Lets say you designed an SMPS with max output voltage of 30V, and R1 is 100k.
Then if you want the output voltage to be reduced to 25V, then you need to 'add' a current of (30V - 25V) / 100k = 50uA

Klaus

 

[fethiyeli]

what about pulse skipping method ? Am i able to control light loads with pulse skipping method ? I am gonna share the schematic soon.

 

[KlausST]

Hi,

As said before: first design an SMPS with fixed output voltage.
Usually you don't need to add extra effort (In adding pulse skipping), because it's done by the circuit itself.

Read the datasheet.
* some will drop switching frequency
* some will use pulse skipping
* some will use burst mode
* some need minimum load current

Klaus

 

[max.wangxin.sh]

I hope LTC8390+LTC3070 is helpful. just FYI.