2020. 1. 15. 11:06ㆍDigital/회로
This article takes a look at STUSB4500 which is a standalone USB Power Delivery controller from STMicroelectronics.
The STUSB4500 is a Power Delivery (PD) controller for sink devices such as printers, cameras, point-of-sale, and healthcare devices. It supports configurable PD profiles for up to three different sink devices. These profiles, referred to as Power Data Objects (PDOs) in the datasheet, are stored in an on-chip non-volatile memory. This allows the device to function as a standalone solution where PD contracts can be negotiated without necessarily requiring that an MCU is included in the design.
The STUSB4500 includes features that are intended to simplify the overall circuit design, such as on-chip drivers for the PMOS transistors switching the VBUS power line. It can monitor the VBUS pin and discharge it through either internal or external paths when needed. Additionally, the device withstands high voltages up to 28 V on the VBUS pins and has a short-to-VBUS protection on the CC pins.
In the rest of the article, we’ll briefly review some of these features. If you're interested, you can learn more about the pinning of the USB Type-C standard.
Why USB Type-C?
The USB Type-C standard allows devices to choose the level of power flow through the interface via a protocol called USB Power Delivery.
In the figure below, you can see an example of USB power delivery wherein the sink requests 9V bus voltage (left) and 5V bus voltage (right) from the source. The VBUS voltage is then adjusted as needed.
Figure 1. Image courtesy of Richtek.
The STUSB4500 is designed to detect the connection between two USB Type-C ports as a standalone solution. It determines the cable orientation to re-route the USB data through the cable accordingly.
When connected to a sink device that supports the PD capability, the STUSB4500 manages USB PD contract negotiations and configures the incoming VBUS power path depending on the sink requests. As mentioned above, the device incorporates several features to facilitate the design.
Functional Block Diagram
The functional block diagram of the STUSB4500 is shown in Figure 2.
Figure 2. Image courtesy of STMicroelectronics.
The CC1 and CC2 Pins
The CC1 and CC2 pins are used to determine the cable attachment and the plug orientation. Moreover, the PD negotiations are performed over these pins. As shown in the example schematic of Figure 3, these two pins are connected to the CC1 and CC2 pins of the USB Type-C standard.
Figure 3. Image courtesy of STMicroelectronics. Click to enlarge.
The CC1DB and CC2DB Pins
These two pins are used to enable the “dead battery” mode of operation. When the dead battery mode is supported, the CC1DB and CC2DB pins should be connected to CC1 and CC2 pins, respectively (See Figure 3). This creates a pull-down path on the CC1/CC2 pins and establishes a source-to-sink connection even when the sink has a dead battery.
The DISCH Pin
There are circumstances that we need to rapidly discharge the VBUS line, e.g. when the cable is disconnected or the sink requests a lower voltage on the VBUS. The DISCH pin can be configured to implement a discharge path for the VBUS line on the side of the sink devices (See Figure 3).
The DISCH pin can be configured as either an input or an output. When used as an input pin, the STUSB4500 creates an internal discharge path. The maximum discharge current through the chip is about 500 mA, hence a current limiting series resistor needs to be included (R2 in Figure 3). When a higher discharge current is required, we can configure the DISCH pin as an output to control an external discharge path.
The ATTACH Pin
This pin is asserted to indicate a valid source-to-sink connection.
The POWER_OK2/POWER_OK3 Pins
These two pins work with VBUS_EN_SNK to deliver power after USB PD negotiations. POWER_OK2 is a high-voltage drain output via an external PMOS transistor. POWER_OK3 is a low-voltage open-drain output.
If the PD profile of the sink number (2 or 3) matches that of the source, POWER_OKx activates the power path of this sink device. Figure 3 shows how these two pins can be used. In this figure, the two sink devices PDO2 and PDO3 can be activated by the POWER_OK2 and POWER_OK3 pins, respectively.
POWER_OK2 is a high-voltage open drain output and is capable of directly driving a PMOS transistor to enable the power path of PDO2. However, POWER_OK3 is a low-voltage open drain output. I believe that’s why the power path of PDO2 is controlled by a PMOS directly connected to the POWER_OK2 pin while the circuitry related to the PDO3 power path is a little bit more complicated. If you have any idea about the circuitry connected to the POWER_OK3 pin, share it with us in the comments below.
The GPIO Pin
This is a general purpose pin. It could be configured to indicate a hardware fault detection or to specify how much current is advertised by the USB Type-C source. There are other configuration options for this pin. Please refer to the datasheet for more information.
The VBUS_EN_SNK Pin
This pin enables the VBUS power line when a source is connected. Figure 3 shows how this pin can be used. Note that, to enable the power path of any of the VSNK, PDO2, or PDO3 sink devices of Figure 3, we need to activate the VBUS_EN_SNK pin.
The A_B_SIDE Pin
This output pin indicates the cable orientation.
The VBUS_VS_DISCH Pin
This is an input pin that can be used to monitor the VBUS voltage and discharge it when needed. Unlike the DISCH pin, the VBUS_VS_DISCH pin is connected to the receptacle side of the design (See Figure 3). The maximum discharge current of this pin is 50 mA and a current limiting serial resistor is recommended by the datasheet.
The VREG_1V2 and VREG_2V7 Pins
These pins are included to connect an external decoupling capacitor to the chip’s internal 1.2-V and 2.7-V regulators.
The I²C Interface Pins
The STUSB4500 supports an I2C connectivity over the SCL, SDA, ALERT, ADDR0, and ADDR1 pins.
In this article, we tried to provide you with a basic understanding of the STUSB4500 functionality. We examined the device pinout referring to the example schematic given in the device datasheet. Now, you should be able to more easily grasp the details provided by the datasheet. Remember that you also need to fully understand the decision algorithm that the STUSB4500 uses to determine which sink device should be powered. You can find the details on page 11 of the datasheet.
If you have any experience with this PD controller or other similar parts, please let us know in the comments below.