Ch340 Datasheet (I) 1 usb to Serial Port Chip ch340

CH340 Datasheet (I)
13 
Prioritized choice for diode is a low-power Schottky diode, such as BAS70, BAT54, B0520, etc.
NMOS tubes with low power and capacitance are preferred, such as 2SK3018, 1012, etc. 
Generally, it is not recommended that CH340 and MCU are powered separately. If it is really necessary, it is 
recommended to use CH340K or CH343 (a USB to UART chip which has VIO power pins and supports
I/O independent power supply). 
7.8. Connect to MCU and supply power separately, avoiding inward current poured 
backwards 
(figure below) 
The figure above shows the reference circuit of MCU connecting to CH340K through TTL UART to realize 
USB communication under dual power supply mode. The CH340K is powered by the USB bus, VBUS 
(VCC), and the MCU uses another power supply VDD, which supports 5V, 3.3V or even 2.5V and 1.8V. 
The EPAD of CH340K is an optional GND pin, which can be easily connected to GND or not connected 
according to PCB routing. 
The TXD, RTS# and RXD pins of CH340K have built-in diodes (as shown in the figure) to prevent current 
backward, and a weak pull-up resistor of about 75K
Ω
is built to maintain the high level in default or idle 
state (not shown in the figure), which can not only realize low-level drive and weak high-level drive, but also 
reduce the current backflow when CH340K and MCU supply power independently. CH340K can completely 
prevent the internal current of MCU power supply to power-off CH340K, but also reduce the external 
current of CH340K power supply to power-off MCU (no more than 150
μ
A
）。
In addition, CH340X and CH340C, CH340N with the last three digits of the batch number greater than B40 
can also completely prevent the MCU power supply from injecting the current of the power-off CH340, so 
as to avoid the CH340 wasting the current of the MCU power supply after the USB power is off. 
If it is necessary to completely prevent the CH340K power supply from injecting the current to the 
power-off MCU, refer to the figure in Section 7.7 and add NMOS and diode. 
When it is used for communication baud rate above 120Kbps, it is recommended to enable the built-in or 
external 2K 
Ω
~ 22K 
Ω
pull-up resistor for the RX pin of MCU, or select other models of USB to UART 
chip with VIO power supply pin to support I/O independent power supply. 
The DTR# pin of CH340K is an ordinary push-pull output, and the CTS# pin is an ordinary input with 
built-in pull-up resistor. Neither of these two pins has built-in diodes and does not have the function of 
preventing current backflow. Generally, they are not used to connect MCU. 
DTR # can be used to control the power switch of VCC supplying power to VDD. As shown in the figure 

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