You could at least test this in the real circuit to see if the FB voltage is contributing to the output voltage error. Keep the meter leads as close to the ground plane as possible and away from the switching nodes. The RC will remove the noise from switching, and also keep the meter from interfering with the FB pin. Add a resistor from the FB pin to a resistor, say 10K and connect the other side of the resistor to a capacitor going to ground, say 1 uF ceramic, and use a volt meter to measure the capacitor voltage to see what FB voltage is. One thing you can look at on your actual circuit is the FB voltage. And if the resistors were worst case the voltage would be down near 1.19. Just a quick look at the output voltage with just the min FB reference voltage and perfect resistors says the output can be 1.215V. Have you done a worst case analysis of what the output voltage can be just considering the min max values of the FB reference voltage and the tolerances of the feedback resistors. This is likely why the output voltage is falling with increased load. If the inductor is going into saturation it is losing inductance and consequently will have less energy for each switching cycle. You stated the output voltage is falling with increased load current.
Say choose an inductor that is spec'ed for saturation current of 2.5 or 3.0 A. So you need to choose an inductor that has margin. But if the average output current is 1A the peak current in the inductor may be 1.3 or 1.5 A. One can calculate this based upon VIN and VOUT and use the expected duty cycle percentage to figure out how much time the drive into the inductor is ON. The current in the inductor will have an average value of 1A, but it will have a higher peak value since it is being driven only part of the switching cycle. The inductor should have a saturation current much higher than the output current.
If you go to the inductor data sheet at this linkĪnd look at the plot for saturation current on page 2 for the inductor you have chosen it shows it has lost 40% of its inductance when 0.8 A is flowing through it.Īlso notice on page 1 in the table the saturation current value for this inductor is only 0.65 A I believe you need to change to a better inductor.
0 kommentar(er)
