The picture shows a portable device, powered by a 7.4V lithium battery, and supports USB charging. A dual power switching circuit is designed as follows: When in use, when the USB is plugged in, it can be switched normally, but when the USB is unplugged, the VGS of the mos tube is powered down very slowly, causing the system to temporarily power down. How do you solve this problem?
Maxwell Posted on January 18, 2021
For problems like yours, you should focus on the switching speed and pay attention to the power failure during the switching process. For example, the switching speed of the 5 voltage comparison circuit is fast, and the subsequent power supply switching speed is fast. Work hard, such as the comparison circuit should not exist on the connection. Any capacitor storage affects the switching speed. In addition to the fast switching speed of the switch tube, it also requires continuous power supply to the load. The size of the load is usually difficult to design with voltage difference. If the load is small, increasing the capacitor capacity may provide Short-term power supply, if the load is large, the large-capacity capacitor will not help, if the voltage is the same, the power supply in parallel with the battery is appropriate.
Vanessa Posted on January 18, 2021
It should be that the discharge time of C183 is a bit longer, you can try a small one.
Angelina Posted on January 18, 2021
C183 100nF is too large, and R250 1M is also too large. After SWITCH_T does not output high level, the discharge of G pole is too slow. You can adjust these two values, you can also increase the capacitance on the system side, or you can modify the design of the MOS tube switch and use a triode.
