TPS202x/3x and TPS204x/5x USB Power Distribution Application Report 1998 Mixed-Signal Products SLVA049 IMPORTANT NOTICE Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinue any product or service without notice, and advise customers to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability. TI warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with TI’s standard warranty. Testing and other quality control techniques are utilized to the extent TI deems necessary to support this warranty. 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Copyright 1998, Texas Instruments Incorporated Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 TPS202x/3x and TPS204x/5x Power Distribution Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 USB Power Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 USB Power Distribution for Self-Powered Hubs (Including Hosts) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Self-Powered Hub Port Protection Topologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Ganged Port Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Individual Port Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Output Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Ferrite Beads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Transient OC Pin Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Self/Bus Powered Hybrid Hub . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 List of Figures 1 Maximum Voltage Drops and Droops on the USB Power Distribution System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Self Powered Hub . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 Simplified Ganged Port Protection Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4 TPS2024 in a Ganged Port Protection Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5 Four-Port Ganged Hub Using 33 µF Tantalum Electrolytic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6 Four-Port Ganged Hub Using 100 µF Aluminum Capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 7 Simplified Individual Port Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8 TPS2044 in a Individual Port Protection USB Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 9 TPS2044 Four Port Hub Using 33 µF Tantalum Output Capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 10 TPS2044 Four Port Hub Using 100 µF Aluminum Electrolytic Output Capacitors . . . . . . . . . . . . . . . . . . . . . . . 11 11 R-C Filter for the OC Pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 12 Self/Bus Powered Hybrid Hub . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 List of Tables 1 2 3 4 5 Texas Instruments Power Distribution Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Voltage Droop Results for a Ganged Port Protected Self-Powered Hub Topology . . . . . . . . . . . . . . . . . . . . . . . . . 8 Voltage Droop Results in Individual Port Protected Self-Powered Hub . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Output Capacitors Used in Testing Voltage Droop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Ferrite Beads Used in Testing Voltage Droop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 TPS202x/3x and TPS204x/5x USB Power Distribution iii iv SLVA049 TPS202x/3x and TPS204x/5x USB Power Distribution ABSTRACT The USB specification, revision 1.1, defines the power distribution requirements for the self-powered hubs (including hosts), bus-powered hubs, and functions. This application report demonstrates how Texas Instruments power management products meet or exceed voltage regulation, voltage droop, and EMI requirements in the USB power distribution system. Introduction The Universal Serial Bus (USB) interface is a 12-Mb/s or 1.5-Mb/s, multiplexed serial bus for low to medium bandwidth PC peripherals (e.g., keyboards, printers, scanners, and mice). The four-wire USB interface provides dynamic attach-detach (hot plug-unplug) of peripherals. Two lines are provided for differential data and two lines are provided for 5-V power distribution. USB data is a 3.3-V level signal, but power is distributed at 5 V to allow for voltage drops in cases where power is distributed through more than one hub. Each function must provide its own regulated 3.3 V from the 5-V input or from its own internal power supply. The USB specification defines the following five classes of devices, each differentiated by power consumption requirements: • Bus-powered hubs • Self-powered hubs • Low power, bus-powered functions • High power, bus-powered functions • Self-powered functions Self-powered and bus-powered hubs distribute data and power to downstream functions. This application report describes power distribution solutions for both types of hubs and functions. 1 TPS202x/3x and TPS204x/5x Power Distribution Switches TPS202x/3x and TPS204x/5x Power Distribution Switches Texas Instruments offers a variety of power switch devices for USB applications: the TPS202x, TPS203x, TPS204x, and the TPS205x. All devices are high-side switches with built-in overcurrent protection that includes open-drain, active-low overcurrent flag output for overcurrent reporting. The switches provide power switching and maintenance-free fault protection; they are controlled by 3.3-V and 5-V logic level compatible enables. For added flexibility in interfacing with hub controllers, the devices are available in active-low or active-high enables. The TPS202x (active-low) and TPS203x (active-high) families of devices are 50-mΩ high-side N-channel MOSFET switches that are offered at different current limit levels. The TPS202x/3x families are ideal for ganged-port protected power distribution system topologies. The TPS2041/2/4 and TPS2051/2/4 series of devices are 135-mΩ high-side N-channel MOSFET switches rated for a fixed continuous current of 0.5 A with typical current limit of 0.9 A. The TPS204X series differs in the number of power switches available in each packaged device: TPS2041 (Single), TPS2042 (Dual), TPS2044 (Quad). The TPS2041 series is ideal for individual port protection of one-port (TPS2041), two-port (TPS2042), or four-port (TPS2044) topologies. The TPS204X devices are enabled with an active-low 3-V or 5-V logic enable. The TPS2042 and TPS2044 devices incorporate a dual thermal trip to allow fully independent operation of the dual power distribution switches. In over-current or short-circuit conditions, the junction temperature rises. Once the die temperature rises to an internal set trip point, the thermal sense circuitry determines which switch is in an over-current condition and turns that power switch off, isolating the fault without interrupting operation of the adjacent power switch. The second thermal trip point is a higher temperature point that turns off both switches. Active-high versions are also available: TPS2051, TPS2052, and TPS2054 as single, dual, and quad power switches. Table 1 lists characteristics of the TPS20xx family of devices. Table 1. Texas Instruments Power Distribution Switches TI PART NUMBER NO. OF SWITCHES ENABLE LOGIC ON RESISTANCE CONTINUOUS CURRENT CURENT LIMIT TPS2041 Single Active-low 135 mΩ 0.5 A 0.9 A TPS2051 Single Active-high 135 mΩ 0.5 A 0.9 A TPS2042 Dual Active-low 135 mΩ 0.5 A 0.9 A TPS2052 Dual Active-high 135 mΩ 0.5 A 0.9 A TPS2044 Quad Active-low 135 mΩ 0.5 A 0.9 A TPS2054 TPS2022† Quad Active-high 135 mΩ 0.5 A 0.9 A Single Active-low 50 mΩ 1.0 A 1.5 A TPS2032† TPS2023† Single Active-high 50 mΩ 1.0 A 1.5 A Single Active-low 50 mΩ 1.5 A 2.2 A TPS2033† TPS2024† Single Active-high 50 mΩ 1.5 A 2.2 A Single Active-low 50 mΩ 2.0 A 3.0 A TPS2034† Single Active-high 50 mΩ 2.0 A 3.0 A † Product Preview, release 1Q99 2 SLVA049 Definitions Definitions Bus-powered hub – Bus-powered hubs (BPH) obtain all power from upstream ports and often contain a nonremovable function. One typical BPH application is a keyboard. The total current drawn by a BPH is the sum of the hub controller current, nonremovable function, and downstream port and is no greater than 500 mA or 5 unit loads. Each port must distribute 1 unit load. Current limit – USB specification requires SPHs to limit current to downstream ports and report over-current conditions to the controller. Downstream port – A port that is electrically further from the host and generates downstream data traffic from the hub. Dynamic attach and detach – To attach or remove devices while host is in operation (hot-plug, unplug event). High-power, bus-powered function – All power to these devices comes from the upstream port. Its current draw can be 1 unit load before configuration and up to 5 unit loads after configuration. Host – The host computer system where the USB host controller is installed. Inrush current – The current surge into uncharged downstream input capacitors. Low-power, bus-powered function – All power to these devices comes from the upstream port. Its current draw is less than 1 unit load. Power Switching – USB specification requires BPHs to power switch downstream ports to limit aggregate current draw of the controller, function, and ports to less than 100 mA before enumeration. Self-powered function – May draw up to 1 unit load from the upstream port. The remainder of power is provided from a local power supply. Self-powered hub – Self-powered hubs (SPH) have a local power supply that power nonremovable functions and the downstream ports. Typical self-powered hubs are PCs, monitors, printers, and stand alone hubs. The SPH is required to have current limit and report overcurrent conditions. The SPH must be able to supply 5 unit loads to all external downstream ports even when the hub is in a suspended state. A battery-powered SPH may configure for 1 unit load or 5 unit loads of output current per port. Unit load – One unit load is 100 mA. Upstream port – The port on a device that is electrically closer to the host and generates upstream data traffic from the hub. USB – Universal Serial Bus Voltage droop – The momentary drop in voltage under transient load conditions. Voltage regulation – The difference in DC voltage levels under continuous load and no-load conditions. TPS202x/3x and TPS204x/5x USB Power Distribution 3 USB Power Distribution USB Power Distribution The USB power distribution requirements are designed so that 4.35 V is the minimum voltage supplied to downstream functions. The USB power distribution system is supplied power from a host or self-powered hub and distributed downstream. Resistive voltage drops are associated with each connection, PCB trace, cable, or other component on the power distribution system. USB specification 1.1 dictates the minimum voltages on the downstream ports of the self-powered hubs, bus-powered hubs, and on a downstream function when operating in continuous load current states and transient states during hot-plug events (see Figure 1). The minimum dc voltage on self-powered hubs at the board side of the connection is 4.75 V; it is 4.4 V on a bus-powered hub. Bus-powered hubs are allowed a 100-mV voltage drop leaving a 250-mV voltage drop for the cable. During hot-plug connections, the self-powered hub voltage is allowed to drop 330 mV to 4.42 V and 4.07 V on the bus-powered hub. Downstream functions should have a dc voltage of 4.35 V. All hubs and functions must be able to provide configuration information with a minimum voltage of 4.4 V at the connector end of the upstream cables. The specification also dictates that all downstream devices (self-powered hubs, bus-powered hubs, bus-powered functions, and self-powered functions) can only sink (draw) current from the source voltage. All USB downstream devices must be able to enter into the suspend state to reduce current consumption. Suspend current is a function of unit load allocation. All devices initially default to low power and are limited to 500 µA of suspend current. Devices configured for high power and enabled as a wake up source may draw 2.5 mA. Configured bus-powered hubs may consume 500 µA of suspend current per port and 500 µA for the hub and nonremovable function for a total of 2.5 mA. When determining the suspend current, the current through the pull-up (1.5 kΩ) and pull-down (15 kΩ) resistors on the data lines must be included. Figure 1 summarizes the voltage drops and droops in the USB power distribution system. Host or Self-Powered Hub Bus-Powered Hub Low-Power Bus-Power Function 4.75 V/4.42 V Minimum 4.4 V/4.07 V Minimum 4.35 V Minimum Figure 1. Maximum Voltage Drops and Droops on the USB Power Distribution System 4 SLVA049 USB Power Distribution for Self-Powered Hubs (Including Hosts) USB Power Distribution for Self-Powered Hubs (Including Hosts) Self-powered hubs (SPH) have a local power supply that powers nonremovable functions and the downstream ports (Figure 2). The hub must supply 5.25 V to 4.75 V on the board side of the downstream connection under full-load and no-load conditions and is required to have current limit and report overcurrent conditions to USB software. Typical SPHs are desktop PCs, monitors, printers, and stand alone hubs. Power for the hub controller may be supplied by the local power supply or by the upstream voltage bus for loads up to 100 mA. The advantage of using the power from the upstream port is that communication to the hub controller is possible and the distinction between an unconnected and unpowered device can be made. Most hub controllers use 3.3-V supply voltage, and a low drop-out (LDO) voltage regulator is needed to regulate the 5-V bus voltage to 3.3 V. The number of downstream ports supported by self-powered hubs is limited by the current the local power supply can support and by safety concerns. Each port must be capable of supplying 5 unit loads and must limit current when the aggregate current drawn exceeds 5.0 A. The current limit should exceed the continuous current level of 500 mA by enough to ensure that dynamic-attach events and startup currents do not trip the overcurrent protection. Figure 2 shows a typical SPH. Upstream Data Port Upstream Voltage Bus 1 Unit Load (max) LDO Regulator TPS7133 TPS7233 Hub Controller TUSB2070 TUSB2040 TUSB2140 Downstream Data Port LDO Regulator TPS7133 TPS7233 On/Off On/Off Local Power Supply Current Limit TPS2041/51 TPS2042/52 TPS2044/54 TPS2022/23/24 TPS2032/33/34 Current Limit TPS2041/51 TPS2042/52 TPS2044/54 TPS2022/23/24 TPS2032/33/34 Nonremovable Function 500 mA/Port Downstream Voltage Bus 500 mA/Port Downstream Voltage Bus Figure 2. Self Powered Hub TPS202x/3x and TPS204x/5x USB Power Distribution 5 Self-Powered Hub Port Protection Topologies Self-Powered Hub Port Protection Topologies The self-powered hub power distribution system can be designed with two distinct topologies for port protection: individual port protection or ganged port protection. The ganged port protection is the most economical and the least flexible topology for fault isolation. The ganged topology uses a common current limit element for two or more ports. In a ganged solution, more current flows through the current limit device. The resistance of the current limit device must be low, so as not to exceed the maximum voltage drop limit under full operating conditions. Fault isolation is another limitation of a ganged solution. To isolate the fault in a ganged solution, power will be interrupted to the adjacent port. This may not be desirable in some USB applications. An individual port protection topology provides a more robust configuration and a better end-user experience. The individual port protection is the most flexible topology for fault isolation in the USB power distribution system. The individual port protection topology uses current limit elements for each port, making isolating faults with minimal interruption to the adjacent ports possible. Ganged Port Protection When designing a ganged topology a low-resistance switch is needed to minimize voltage drop in the hub, because current for all downstream ports passes through the current limit device (see Figure 3). The TPS202x power switches are low 50-mΩ devices with current limit and overcurrent response flag as required in USB self-powered hubs. The current limit level of the devices has been optimized for ganged two (TPS2022), three (TPS2023), and four (TPS2024) port protection topologies. The TPS2022 is ideal for a two-port ganged solution; it is rated for a continuous current of 1 A and has a current limit of 1.5 A. Using ferrite beads with a resistance of 10 mΩ or less, a two-port TPS2022 ganged port solution can be designed with 5.05 V ±3% power supply with up to 88 mΩ of trace resistance on the higher current path. If more trace resistance is required, use the following equation to calculate the required source voltage for a tighter tolerance power supply. + 4.75 V ) n 0.5 A n + number of ports R SWITCH + 50 mW V PS ǒ R TRACE ) RSWITCHǓ ) 2 0.5 A R BEAD In a manner similar to the two-port ganged protection solution, a three- or fourport ganged solution can be designed with the TPS2023 or TPS2024 devices, respectively. By using the above equation, the right power supply can be calculated for a specific number of ports, power switches, and trace resistances. VPS is the minimum voltage of the power supply at full load over the temperature range and line regulation. The output voltage of the hub must be greater than 4.75 V and no greater than 5.25 V over all legal load conditions on the upstream side of the connector. 6 SLVA049 Self-Powered Hub Port Protection Topologies R(TRACE) R(SWITCH) L1 + POWER SUPPLY I(T) I(1) 4.75 V L2 – L3 + I(2) 4.75 V L4 – L5 + I(3) 4.75 V L6 – L7 + I(4) 4.75 V L8 – Figure 3. Simplified Ganged Port Protection Topology The typical voltage droop on a two-, three- and four-ganged port self-powered hub was measured using different types (e.g. tantalum and aluminum) and values of output capacitors with the legal USB hot plug load of 10 µF and 44 Ω. Tantalum, aluminum electrolytic, and ceramic capacitors were used for the 10-µF load capacitors to investigate the effect on the drooping. The 10-µF ceramic capacitor resulted in the largest output voltage droop and the aluminum electrolytic gave the smallest droop. Typical USB applications use aluminum electrolytic capacitors of 10 µF or less, and voltage droop results will be shown using a 10-µF, 50-V capacitor. Voltage droop on the hub output varied with the length of cable and the equivalent series resistance of the load and output capacitors. Figure 4 shows the TPS2024 in a ganged-port protection topology, and Table 2 lists voltage droop results for a ganged topology. Figure 5 shows scope traces for a four port ganged hub using 33-µF tantalum capacitors, and Figure 6 shows the same data using 100-µF aluminum electrolytic capacitors. TPS202x/3x and TPS204x/5x USB Power Distribution 7 Self-Powered Hub Port Protection Topologies TPS2024 Local Power Supply GND OUT IN OUT IN OUT EN OC Figure 4. TPS2024 in a Ganged Port Protection Topology Table 2. Voltage Droop Results for a Ganged Port Protected Self-Powered HubTopology GANGED 33 µF 47 µF 2 3 4 120 110 68 µF 100 mF ALUMINUM 120 230 150 100 Ch. 1 Ch. 2 Figure 5. Four Port Ganged Hub Using 33 µF Tantalum Capacitors 8 SLVA049 Self-Powered Hub Port Protection Topologies Ch. 1 Ch. 2 Figure 6. Four-Port Ganged Hub Using 100 µF Aluminum Electrolytic Individual Port Protection Individual port protection has two advantages over ganged port protection in the USB power distribution system: individual port fault isolation, and better voltage droop performance. In a ganged-port protection system, a fault on a single port will interrupt service to an adjacent port. In an individual port protection system, a fault can be isolated without interruption to the adjacent ports. Also, individual port protection provides better voltage droop performance by the resistive isolation of the power switches between ports, reducing charge sharing and voltage droop during hot plug events. The TPS2042 is an ideal device for a two-port hub requiring an individual port protection solution; the device is rated for a continuous current of 0.5 A per switch with a typical current limit of 0.9 A. The TPS2042 also features independent thermal protection for each switch, which disables the switch in overcurrent when an over temperature condition occurs. A two-port hub can be designed with up to 71 mΩ of trace resistance on the higher current path using a 5.05-V ±3% power supply and two ferrite beads on each port with a resistances of 10 mΩ or less. If more trace resistance is needed, a tighter tolerance power supply should be used. + 4.75 V ) n 0.5 A n + number of ports R SWITCH + 135 mW V PS R TRACE ) 0.5 A R SWITCH )2 0.5 A R BEAD Using the above equation the power supply tolerance can be calculated for a three- and four-port individually protected hub using the TPS204x power switch devices. The tolerance of the power supply must be such that the voltage will not exceed 5.25 V or drop below 4.75 V over all conditions. The same 10-µF/44-Ω USB load was used for the voltage droop test on the individually protected part. Figure 7 shows a simplified individual port protection schematic, and Figure 8 shows the TPS2044 in an individual port protection USB application. TPS202x/3x and TPS204x/5x USB Power Distribution 9 Self-Powered Hub Port Protection Topologies R(TRACE) R(SWITCH) L1 + POWER SUPPLY I(T) I(1) 4.75 V L2 R(SWITCH) – L3 + I(2) 4.75 V L4 R(SWITCH) – L5 + I(3) 4.75 V L6 R(SWITCH) – L7 + I(4) 4.75 V L8 – Figure 7. Simplified Individual Port Protection TPS2044 GND1 GND2 OUT1 Local Power Supply IN1 IN2 OUT2 OUT3 OUT4 EN1 OC1 EN2 OC2 EN3 OC3 EN4 OC4 Figure 8. TPS2044 in an Individual Port Protection USB Application 10 SLVA049 Self-Powered Hub Port Protection Topologies Table 3 lists the voltage droop in an individual port protected SPH. Figures 9 and 10 show scope traces of the TPS2044 four-port hub using 33-µF tantalum and 100-µF aluminum electrolytic output capacitors respectively. Table 3. Voltage Droop Results in Individual Port Protected Self-Powered Hub VOLTAGE DROOP on a SELF-POWERED HUB GANGED 33 µF 47 µF 2 3 4 100 100 68 µF 100 mF ALUMINUM 110 150 140 90 Ch. 1 Ch. 2 Figure 9. TPS2044 Four-Port Hub Using 33 µF Tantalum Output Capacitors Ch. 1 Ch. 2 Figure 10. TPS2044 Four-Port Hub Using 100 µF Aluminum Electrolytic Output Capacitors TPS202x/3x and TPS204x/5x USB Power Distribution 11 Self-Powered Hub Port Protection Topologies Output Capacitance A minimum of 120-µF low equivalent-series-resistance (ESR) output capacitance is required per hub (i.e., a four-port hub can use a minimum of 33 µF per port) to bypass the Vbus power lines. The hot plug droop voltages should be considered when choosing the type of capacitor. The ESR of the output capacitor affects the Vbus voltage droop during hot plugging when the maximum allowable USB load of 10 µF in parallel with 44 Ω, or a high-power bus-powered device, transitions from low-power mode to high-power mode. The cable length and load capacitor ESR determines the peak inrush current and the maximum voltage droop. For low voltage applications, solid tantalum capacitors take up less physical space for the same capacitance and voltage rating than aluminum electrolytic capacitors, and do not suffer from electrolyte dry out, which limits the life expectancy of aluminum electrolytic capacitors. To meet the USB specification, tantalum capacitors as low as 33 µF, 47 µF, and 68 µF can be used in four-, three-, and two-port hubs, respectively. If through-hole components are preferred for the output capacitors, 100-µF aluminum electrolytic capacitors with less than 1 Ω of ESR can be used to meet the voltage droop requirement. Table 4 summarizes the data for capacitors used to test voltage droop. Table 4. Output Capacitors Used in Testing Voltage Droop VALUE ESR AT 100 kHz CAPACITOR TYPE PART NUMBER MANUFACTURER 33 µF 0.90 Tantalum 293D336X0010D2T Vishay Sprague 47 µF 0.70 Tantalum 293D476X0010D2T Vishay Sprague 68 µF 0.70 Tantalum 293D686X0010D2T Vishay Sprague 100 µF 0.46 HFQ Radial Aluminum Electrolytic ECA-1AFQ101 Panasonic Ferrite Beads Ferrite beads should be used in series with each Vbus, ground, and data line to reduce EMI and voltage droop during hot-plug events. Table 5 lists the ferrite beads used in the testing of the TPS20xx parts. The ferrite beads that were used have low dc resistance that has minimal impact on voltage regulation. Table 5. Ferrite Beads Used in Testing Voltage Droop IMPEDANCE (Ω) AT 100 MHz DC RESISTANCE (mΩ) FERRITE BEAD PART NUMBER MANUFACTURER 80 10 Ferrite bead surface mount ACB-1812-5.0A Associated Components Technology 80 0.9 Ferrite bead surface mount SMB40 Allied Components 95 0.9 Ferrite bead surface mount 2743037447 Fair-Rite Transient OC Pin Response The hot-plug connection of the legal USB load of a 10-µF and 44-Ω load to an enabled power switch device may cause a false overcurrent response when the inrush current flows through the device, charging the down stream capacitor. An RC filter must be connected to the OC pin shown in Figure 11 to reduce the erroneous OC responses. Also, using low ESR, large value, electrolytic capacitors on the output of the devices reduces the pulse width or eliminates the false OC response by minimizing the current flow through the device during the hot-plug events, thereby providing a lower impedance energy source. 12 SLVA049 Self-Powered Hub Port Protection Topologies The pulse width of the erroneous OC response is typically less than 200 µs without an output capacitor on the device when hot plugging a 10-µF aluminum electrolytic capacitor. If the hot-plug load is greater than 10-µF, the OC pulse width will be greater. Since the OC response is dependent on the many components in the power distribution, it is recommended to test the power distribution system with only a pullup resistor to determine the necessary filter to attenuate the glitch. Once the pulse width of the glitch is known, the filter resistance and capacitance values can be calculated using the equation below. For most applications 0.5 to 1 ms is sufficient. As a precautionary note, the pullup resistance should not be so large as to cause a false overcurrent response at startup when the hub controller voltage reaches its final voltage well before the OC pin voltage. Tfilter Tstartup + Rfilter Cfilter + (Rpullup ) Rfilter ) (1) Cfilter V(pullup) R(FILTER) TPS2042 R(FILTER) GND Voc OC1 IN OUT1 EN1 OUT2 EN2 OC2 To USB Controller C(FILTER) Figure 11. R-C Filter for the OC Pin Self/Bus Powered Hybrid Hub Many available self-powered hubs are designed to switch to bus-powered mode when local power is not available. The TPS2041 and TPS2021 can be used in the upstream vbus power line, as shown is Figure 12, to prevent power flow upstream. A disable signal should be used to turn off the power switch when the local power supply is present. TPS202x/3x and TPS204x/5x USB Power Distribution 13 Self-Powered Hub Port Protection Topologies Upstream Data Port Upstream Voltage Bus 1 Unit Load (max) Power Switch TPS2041 On/Off Hub Controller TUSB2070 TUSB2040 TUSB2140 Downstream Data Port LDO Regulator TPS7133 TPS7233 On/Off On/Off Local Power Supply Nonremovable Function Current Limit TPS2041/51 TPS2042/52 TPS2044/54 TPS2022/23/24 TPS2032/33/34 Current Limit TPS2041/51 TPS2042/52 TPS2044/54 TPS2022/23/24 TPS2032/33/34 Figure 12. Self/Bus Powered Hybrid Hub 14 SLVA049 500 mA/Port Downstream Voltage Bus 500 mA/Port Downstream Voltage Bus Summary Summary Texas Instruments power management products meet or exceed voltage regulation, voltage droop, and EMI requirements in the USB power distribution system. TPS202x/3x and TPS204x/5x USB Power Distribution 15