OSD335x Power Inputs and Outputs
Published On: June, 29, 2017 By: Eshtaartha Basu | Updated: July 18, 2018 by Cathleen Wicks
Introduction
When it comes to booting a device, power circuitry is the first thing that comes to mind. To have stable and predictable operation from any electronic device, we need to make sure the power circuitry is well designed and is able to meet the power requirements of the device under normal as well as extreme operating conditions.
The Power Circuitry is a complex topic, so we have split the discussion into five parts. They are:
- Power Inputs and Outputs: This article will focus on input and output power considerations for the OSD335x.
- Ground Connections: This article will focus on ground connections and important layout pour considerations for the OSD335x.
- Power Management: This article will focus on topics related to control and management of power for the OSD335x.
- Clamping Circuit: This article will focus on clamping circuit which can be used to prevent certain power down issues that may arise with the OSD335x.
- ESD Protection: This article will focus on steps needed to provide Electro Static Discharge (ESD) protection to the OSD335x based Printed Circuit Boards (PCB).
This article is the first part of the OSD335x Reference Design Lesson 1 Power circuitry articles. We recommend you to read other parts of power circuitry article as well which are available at links provided above. As we discuss the power circuitry, we will build the schematic and layout the corresponding traces. All the components used in this article can be found in the provided library . The components will be introduced after relevant discussion as we proceed with the article.
Prerequisite
This article is a part of the broader OSD335x Reference Design Series: Lesson 1 which consists of a sequence of articles designed to help you build the bare minimum circuitry required to boot the OSD335x.
We recommend reading the introduction article Introduction to Bare Minimum Circuitry to Boot OSD3358 before this one. This article builds upon the foundation outlined in it.
All design files for this lesson can be downloaded here.
This articles as well as the entire OSD335x Design Tutorial can be downloaded here.
Power Input
The OSD335x can be powered from 5V DC power supply (generally sourced from an AC adapter), from a standard USB port(5V) or using a standard single cell (1S) Li-Ion/Li-Polymer (LiPo) battery.
VIN_AC
While this port is called VIN_AC, it is a DC power input generally powered from a 5V AC adapter. By default, this input has 2A current limit and can be used as primary power input. For our design, we will use a DC barrel connector to source this input since DC barrel connectors are used by many other electronic devices and adapters compatible with this type of connector are easily available. The DC barrel connector device we’re using can be found under the name PJ-102B_POWER_CON in the provided library. Let’s connect this pin directly to the input pins Y5 and Y6 of the OSD335x.
VIN_USB
This input of the OSD335x can be powered from the VBUS line of the USB client connector at 5V. By default, the input current limit of this pin is 500mA which is also the standard output current limit for a USB 2.0 host port. However, through software configuration of the power management IC (PMIC) inside the OSD335x, the current limit can be raised to 1.3A.
The USB client connector used in this design can be found under the name 10118192-0001LF in the provided library. The VBUS pin of this connector should be connected to the pins Y8 and Y9 on the OSD335x.
VIN_BAT
VIN_BAT pin can act as either a battery input or output. It acts as a battery input when the OSD335x is running on battery power. It acts as a battery output when the OSD335x is charging the connected battery (more information on charging given in the below perk). This input should be powered by a single cell (1S) Li-Ion or Li-polymer battery with voltage range of 2.75V to 5.5V.
In this lesson, we will primarily use VIN_AC and VIN_USB to source power to the OSD335x. However, in the future we may want to use a (1S) Li-ion or Li-Polymer battery to power our design. Hence, we will add thru-hole test points for battery power inputs so that we can connect a battery later if necessary. Thru-hole test points can be found under the name TESTPAD/W_HOLE_1X1 in the provided library.
Input Power Schematics
Based on the description above, let’s update our schematics with all the input power connections as shown in Figure 1 (Updates made to the schematics are shown using dotted lines).
It is a good idea to add test points to all the input power rails so that we can easily test the voltages during debug. For this design, we have used thru-hole test points for all the input power rails. But, for your design, feel free to use surface test pads to save board area.
Supply pins are also added to input power rails so that we can see where these voltages are used elsewhere on the schematic.
The reason behind the presence of resistor R1 is explained under the ESD (Electro Static Discharge) section of the article OSD335x Power Management (coming soon).
Input Power Layout
Now that we have completed the schematics for power input, let’s begin the layout. As per the guidelines in the introduction article, we will use 6mil (approx. 0.15mm) traces for signals and at least 15mil (approx. 0.40mm) traces for power traces and connect them to the OSD335x power pins using pours so that there is good connection to the BGA balls. While we could use pours for the entire power connection, in this design we do not have any peripherals with high current draw so we can use traces to make layout easier. The layout is shown in Figure 2.
The components are placed and routed in a specific manner to accommodate future components and to facilitate easy routing as we go ahead with the articles
Power output
The PMIC and LDO inside the OSD335x generate many different power outputs. Some of the outputs are only for internal use within the System in Package (SiP). However, others provide power that can be used by the systems external to the SiP. Please go through the OSD335x power app note before budgeting power for your design. The maximum power output of each of these pins can be found in the OSD335x datasheet. The following power outputs can be used for external devices:
- SYS_VOUT: Shared supply sourced by the PMIC. This rail also supplies power to the AM335x, DDR3 and TL5209 LDO inside the SiP. This output is not regulated. It merely reflects the voltage of the input power source that is being used to power the PMIC. Therefore, when using a battery, it is necessary to make sure that any components that use the SYS_VOUT power output can operate on a voltage between 3V and 5V since the PMIC will switch to a different power input when charging the battery.
- SYS_VDD1_3P3V: Dedicated 3.3V supply rail for external circuitry. Powered by the TL5209 LDO and enabled by LDO4 of the PMIC. This power output will be connected to the power plane of our layout.
- SYS_VDD2_3P3V: Dedicated 3.3V supply for external circuitry directly supplied by LDO2 of the PMIC.
- SYS_RTC_1P8V: 1.8V output powered by LDO1 of the PMIC. It is also used internally to power the RTC of the AM335x.
- SYS_VDD_1P8V: 1.8V output powered by LDO3 of the PMIC.
- SYS_ADC_1P8V: 1.8V output powered by LDO3 of the PMIC and filtered for analog applications. Internally, it supplies power to the AM335x ADC.
It is a good idea to add test points to all the power output pins as shown in Figure 3 so that we can probe the voltages during debug.
Test points on internal power rails
The OSD335x provides external access to critical internal power rails. These pins should be used for testing/monitoring purposes only. They shouldn’t be used to power external circuitry. Test points need to be added to these power rails so that internal voltages can be looked up in case of power-up issues. You can use either thru-hole test points or test pads, whichever makes your routing easier. For this design, we will be using thru-hole test points.
The OSD335x pins that provide access to internal power rails are VDDSHV_3P3V, VDDS_DDR, VDD_MPU, VDD_CORE and VDDS_PLL.
If you’re curious about how the internal power rails are used within the SiP, you can find more information below:
- VDDSHV_3P3V: Dedicated 3.3VDC to power the AM335x I/O. It is supplied by the TPS65217C LDO4.
- VDDS_DDR: Dedicated 1.5VDC supply to power the AM335x DDR3 interface and DDR3 device.
- VDD_MPU: Dedicated 1.1VDC supply to power the AM335x MPU domain.
- VDD_CORE: Dedicated 1.1VDC supply to power the AM335x CORE domain.
- VDDS_PLL: Filtered 1.8VDC to supply power to the AM335x PLLs and oscillators.
You can also refer Texas Instruments (TI) Power Hookup Application Note for in depth information.
Schematics for power output pins and test points on internal power rails
Let’s add test points and supply pins suitably as shown in Figure 3 (Updates made to the schematics are shown using dotted lines). Thru-hole test points can be found under the device name TESTPAD/W_HOLE_1X1 in the provided library.
Layout for test points on internal power rails
The test points were placed and routed to accommodate future components and facilitate easy routing as shown in Figure 4.
Analog reference input and ground
The OSD335x has an Analog-to-Digital Converter (ADC) interface that can be used for things like monitoring voltages and interfacing with analog sensors. To use the ADCs, the Analog Power and Ground must be connected appropriately. The interface can tolerate inputs up to 1.8V depending on the analog voltage reference VREFP. Internally, the OSD335x connects the VREFN pin of the AM335x to analog ground, so the range of ADC is analog ground to VREFP. Generally, VREFP is connected to SYS_ADC_1P8V but it can be set to a lower voltage using a voltage divider.
Since the voltage reference, VREFP, needs to be a clean as possible, we want to put a resistor footprint between VREFP and the power connection. This gives the option of putting a resistor or ferrite bead to help suppress noise. Also, we want to put bypass capacitors between VREFP and analog ground to help suppress noise. These connections can be seen in Figure 5 (Updates made to the schematics are shown using dotted lines).
If you do not need to use the ADC interface in your application, then VREFP should be shorted with AGND.
Layout for Analog Connections can be made as shown in Figure 6.
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