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MCP2200T-ISS bảng dữ liệu(PDF) 6 Page - Microchip Technology |
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6 / 44 page MCP2200 DS22228B-page 6 2011 Microchip Technology Inc. 1.5.2.1 Internal Power Supply Details MCP2200 offers various options for power supply. To meet the required USB signaling levels, MCP2200 device incorporates an internal LDO used solely by the USB transceiver, in order to present the correct D+/D- voltage levels. Figure 1-2 shows the internal connections of the USB transceiver LDO in relation with the VDD power supply rail. The output of the USB transceiver LDO is tied to the VUSB line. A capacitor connected to the VUSB pin is required if the USB transceiver LDO provides the 3.3V supply to the transceiver. FIGURE 1-2: MCP2200 INTERNAL POWER SUPPLY DETAILS The provided VDD voltage has a direct influence on the voltage levels present on the GPIO pins (Rx/Tx and RTS/CTS). When VDD is 5V, all of these pins will have a logical ‘1’ around 5V with the variations specified in Section 3.1, DC Characteristics. For applications that require a 3.3V logical ‘1’ level, VDD must be connected to a power supply providing the 3.3V voltage. In this case, the internal USB trans- ceiver LDO cannot provide the required 3.3V power. It is necessary to also connect the VUSB pin of the MCP2200 to the 3.3V power supply rail. This way, the USB transceiver is powered up directly from the 3.3V power supply. 1.5.2.2 USB Bus Powered (5V) In Bus Power Only mode, all power for the application is drawn from the USB (Figure 1-3). This is effectively the simplest power method for the device. FIGURE 1-3: BUS POWER ONLY In order to meet the inrush current requirements of the USB 2.0 specifications, the total effective capacitance appearing across VBUS and ground must be no more than 10 µF. If it is not more than 10 µF, some kind of inrush limiting is required. For more details on Inrush Current Limiting, search for that subject in the current Universal Serial Bus Specification . According to the USB 2.0 specification, all USB devices must also support a Low-Power Suspend mode. In the USB Suspend mode, devices must consume no more than 500 µA (or 2.5 mA for high powered devices that are remote wake-up capable) from the 5V VBUS line of the USB cable. The host signals the USB device to enter Suspend mode by stopping all USB traffic to that device for more than 3 ms. The USB bus provides a 5V voltage. However, the USB transceiver requires 3.3V for the signaling (on D+ and D- lines). During USB Suspend mode, the D+ or D- pull-up resistor must remain active, which will consume some of the allowed suspend current budget (500 µA/2.5 mA). The VUSB pin is required to have an external bypass capacitor. It is recommended that the capacitor be a ceramic cap, between 0.22 and 0.47 µF. Figure 1-4 shows a circuit where MCP2200’s internal LDO is used to provide 3.3V to the USB transceiver. The voltage on the VDD affects the voltage levels onto the GPIO pins (Rx/Tx and RTS/CTS). With VDD at 5V, these pins will have a logic ‘1’ of 5V with the variations specified in Section 3.1, DC Characteristics. LDO 3.3V USB Transceiver D+ VDD VUSB D- IN OUT VDD VUSB VSS VBUS |
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