Hello There!
I am doing my new design to use a LT1930 SEPIC circuity to drive a pump(with 13.6 ohms motor resistance), and I do meet a problem on that, the circuity is as blow:
I was first trying to leave the pump un-connected and use a mod wire to short the pump just to test the SEPIC circuity, and it runs good - the FB voltage was lined up to the number which appears on its datasheet, and the output voltage is just as anticipated 6.61V. Then I tried to applied the pump on it, the pump can still running but the FB voltage was only 0.8V and the output voltage was only 4.7V, and this two numbers they are unstable on the multimeter. This is the problem.
One thing I currently know is that when you applied 6.61 voltages across the pump, its peak-start-current can go up to 2.5-3.2A, which is greater than Switch Current Limit on Lt1930's datasheet(see the following picture), but I am not sure if that is where this problem comes from.
I tried to add a FET instead of R19 and make pump soft-start, after doing so, the feedback voltage can reach around 1.0V, but there is still a gap between the standard value on the datasheet.
I tried to connect and un-connect the pump manually to mimic a longer soft start, with DCDC always on, and I got the same result with the FET soft-start. So I assume the problem is not here.
Does anyone can give me an idea how to solve this problem without switching the pump itself?
Best,
Matt