Original question:
UCC28951: Is it possible to design Constant Current using UCC28951?
Hello, my name is Hanif Faalih from Universitas Gadjah Mada. Maybe sounds familiar since i asked you in this forum about soft starting a month ago.
Now, i consider to make CC/CV method in my system. I already read the reference that discussed the same topic, but i still confuse the low pass filter role in comparator and when they called "the lowest error, win and control the output".
This is the schematic that i got from this forum.
Hello Hanif
Apologies for the delay in responding here.
Probably the best thing to do is to first ignore the current error amplifier - (disconnect the diode at ICOMP !)
In this case, and assuming the load current is fairly low (50% of Imax for example), the VCOMP output from the error amplifier will settle to a voltage such that the PWM comparator puts out the duty cycle so that VOUT_FB = VOUT_REF. The VCOMP output forward biases the diode in its output and pulls down on the EA+ pin. The linear operating range of the EA+ pin is 0.8 to 2.8V . In this condition, the voltage error amplifier has control of the output, the system is operating in CV mode, and of course the other error amplifier is disconnected.
Now, with the ICOMP diode still disconnected and the current still at 50% of Imax - the ICOMP output will be high because the IOUT_FB signal is less than the IOUT_REF signal. If you then connect the diode it will be reverse biased because the ICOMP signal is high and the VCOMP signal is being held in the range 0.8 < VEA+ < 2.8 (minus a Vf drop)
If the load current increases steadily, then the IOUT_FB input to the current error amplifier will eventually reach the IOUT_REF voltage. When this happens, the output of the current error amplifier will drop as it comes out of saturation and it will pull low enough to forward bias the diode in its output. At this time, the load current equals the current limit, or current regulation setpoint and the current error amplifier will pull low enough to be able to affect the PWM comparator duty cycle. This means that the current amplifier will control the PWM duty cycle so that the current does not increase past the regulation setpoint. As the PWM duty cycle drops, so will the output voltage. When the output voltage drops, the voltage error amplifer output will go high and this will reverse bias the diode in its output. The system is now operating in CI mode.
The on-board error amplifier of the UCC28950 is configured as a voltage follower, so that its COMP output is the same as its EA+ input.
The low pass filter on the CS signal input to the controller is used to filter any leading edge spikes on the CS signal waveform. If unfiltered these spikes can cause erratic duty cycle changes because the PWM comparator can trip on the noise spike instead of on the current sense signal. Typically a 1k Ohm resistor with at 330pF capacitor is used - but this can vary from design to design.
Regards
Colin
