Tool/software:
Hello Expert,
Do you mind helping to have a view on attached schematic and layout?
VIN: 48V to 120V
Vout; 24V
Iout; 2.5A to 3A. (max power is 36W and normally is 15W)
The customer follows WEBENCH to create the circuit, and the final circuit can not output stable voltages.
The VS pin did not find a minimum protection threshold of 4.52V at start-up during the previous test, but it can be seen that VDD drops slowly to 7.8V turn-off immediately after 21V turn-on, then recharges the capacitor to 21V and turns it off again after turn-on. to keep cycling through this process. The VS divider resistors have tried to use 62K, 18K, 53K, 15K, 68.1K, 17.8K, and the system cannot run at 50V using 68.1K, 17.8K (there is no further VS voltage for this set of parameters). with 62K, 18K, 53K, 15K, the start-up voltage can be adjusted down to 45V and is essentially normal at light loads. But it does not start up with 1.4A load.
Next, adjust Rtl down to 1.5K by increasing the value of the feedback current, and adjust RFB3 to 100K and RFB4 to 22K. When the proportional link is increased, it can be loaded to 1.5A and CC constant current mode is entered, at a voltage of only 10.5V, and it is likely that the load will not start at 1.5A. After adjusting RCS, the system will not start as if RCS is less than 0.2R, i.e. scaling up the load output current is not valid. The volume RCS voltage is less than 1.5 V at start-up failure and does not reach the current limit.
Here are some curves tested.
VDD: Purple line
VOUT: Yellow line
...
DRV: The purple line.
