Application Note 17 Micrel Application Note 17 Universal Serial Bus Power Management by Kevin Lynn Functions Functions are endpoint low- or high-power devices and do not repeat data or provide downstream power. Introduction Power management and distribution is a major factor in correctly designing USB (Universal Serial Bus) peripherals. Proper methods of designing USB peripheral power distribution are crucial to ensure full compliance with the USB specification, including compliance with electromagnetic interference (EMI) and voltage regulation requirements. High-power functions may initially draw 100mA at 4.4V from an upstream self-powered port, increasing after enumeration up to 500mA. If, during enumeration, there is insufficient power available from upstream, the remainder of the function is not powered and a power limit warning message is sent. Self-powered functions may draw 100mA from upstream with the remainder from a local power supply. Low-power functions may draw up to 100mA from an upstream cable. Device Classes USB defines several device classes, differentiated by their power requirements or capabilities. The most important device classes are hubs and functions. Hubs Hubs provide for distribution of data and power to downstream (away from the host) devices and communicate with the host. Hubs may be either locally powered (self-powered), bus-powered from an upstream cable (toward the host), or a combination of the two. USB Power Distribution Connection between devices are made using cables which are either detachable (such as a USB modem) or permanently attached (such as a keyboard). High speed (>1.5Mb/ s) peripherals require shielded cable, while low-speed peripherals may operate with unshielded cable to minimize cost. The upstream port of a hub is, by definition, a highspeed port and requires a shielded cable. Figure 1 shows the voltage regulation requirements of a USB system based on current USB requirements. Self-powered hubs have a local power supply. Examples are PCs (host), stand-alone hubs, monitors, printers, scanners, or docking stations, which may draw up to 100mA (one unit load) from an upstream port. Self-powered hubs are required to limit and report overcurrent conditions and may supply up to 500mA to each downstream port. Self-Powered Hub Requirements USB self-powered hubs are required to provide a minimum of 4.75V at downstream ports under all legal load conditions. Self-powered hubs may draw up to 100mA operating current from an upstream hub. To comply with UL defined safety requirements, current from any port must be limited to less than 25VA (5A for 5V ports). If such an overcurrent condition occurs, even if it is only momentary, it must be reported to the hub controller. Detection of overcurrent must disable all affected ports. If the Bus-powered hubs obtain all power from the bus (a cable to an upstream self-powered hub) and may supply 100mA or more to each downstream port if the input power budget is not exceeded. Bus-powered hubs may draw 100mA at start-up from an upstream hub, increasing up to 500mA after enumeration, apportioning the power in increments of 100mA per downstream port plus 100mA internally. Bus-powered hubs must provide power switching for downstream ports on a per port basis or have a single switch for all of the ports (gangmode power control). 4.75V min. at 500mA 4.4V min. at 100mA USB SelfPowered Hub Power Supply USB Bus-Powered Hub Hub LowPower Device 4.85V to 5.25V HighPower Device Figure 1. Typical USB Voltage Distribution System Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com October 1998 1 Application Note 17 Application Note 17 Micrel overcurrent condition has caused a permanent disconnect (such as a blown fuse), the hub must report it upon completing its reset or power-up. Overcurrent protection may be implemented on all downstream ports in aggregate (ganged), or on an individual (per port) basis. Overcurrent limiting devices may include polyfuses, standard fuses, or a solid-state switch. A practical limit is seven downstream ports per protection device. Nominal Supply Minimum Maximum Maximum Voltage Tolerance Voltage Voltage RO N 1% 4.8V 4.9V 40mΩ 2% 4.75V 4.95V 0mΩ 4.85V 3% 4.7V 5V — 4% 4.66V 5.04V — 5% 4.61V 5.09V — 1% 4.85V 4.95V 140mΩ 2% 4.8V 5V 40mΩ 4.90V 3% 4.75V 5.05V 0mΩ 4% 4.7V 5.1V — 5% 4.66V 5.15V — 1% 4.9V 5V 240mΩ 2% 4.85V 5.05V 140mΩ 4.95V 3% 4.8V 5.1V 40mΩ 4% 4.75V 5.15V 0mΩ 5% 4.7V 5.2V — 1% 4.95V 5.05V 340mΩ 2% 4.9V 5.1V 240mΩ 3% 4.85V 5.15V 140mΩ 5.00V 4% 4.8V 5.2V 40mΩ 5% 4.75V 5.25V 0mΩ 1% 5V 5.1V 440mΩ 2% 4.95V 5.15V 340mΩ 5.05V 3% 4.9V 5.2V 240mΩ 4% 4.85V 5.25V 140mΩ 5% 4.8V 5.3V — 1% 5.05V 5.15V 540mΩ 2% 5V 5.2V 440mΩ 3% 4.95V 5.25V 340mΩ 5.10V 4% 4.9V 5.3V — 5% 4.85V 5.36V — 1% 5.1V 5.2V 640mΩ 2% 5.05V 5.25V 540mΩ 5.15V 3% 5V 5.3V — 4% 4.94V 5.36V — 5% 4.89V 5.41V — Voltage Regulation Requirements The dc output voltage, measured at the board side of the selfpowered hub downstream connector, must remain below 5.25V and above 4.75V under all legal continuous load conditions. The internal power supply tolerance dictates the allowable component voltage drops within a self-powered hub. Table 1 depicts the range of resistance of the overcurrent protection device, assuming a 30mV (or 60mΩ) drop across the PCB components. For example, Table 1 shows that using a 5V, 3% power supply allows a maximum resistance of 140mΩ for the overcurrent protection device. A 5.1V, 3% supply allows for a 340mΩ device. If it is expected that the PCB component voltage drops will be other than 30mV, then these values can be scaled appropriately. The components that make up the voltage drop include the circuit trace, overcurrent protection device, and output filter ferrite beads. The printed circuit board power and ground traces, solder connections, and ferrite beads on both power and ground output lines may total 60mΩ, dropping a total of 30mV at 0.5A. Additional traces, ferrite beads, and protection devices are needed for each output port. Figure 2 represents the component voltage drops of a typical self-powered hub. Transient Droop Requirements USB supports dynamic attachment (hot plug-in) of peripherals. A current surge is caused by the input capacitance of the downstream device. Ferrite beads are recommended in series with all power and ground connector pins. Ferrite beads reduce EMI and limit the inrush current during hotattachment by attenuating high-frequency signals while dc current passes freely. The simplest ferrite beads consist of a small ferrite tube on a tinned solid copper wire. The resistance of the ferrite bead wire should be as low as possible, with a large solder-pad to minimize connection resistance. USB requires that momentary droop of the 4.75V at a buspowered hub’s upstream cable connector never goes below 4.42V. 500mA max. (+) Power Supply 4.85V to 5.25V 3.5A max. (–) (trace resistance) 0.01Ω 33µF 5mV 0.1µF 0.01Ω 1/2 MIC2526 Switch 0.14Ω Table 1. Maximum Allowed On-Resistance with 30mV PCB Voltage Drop* * Shading represents USB-compliant conditions. Ferrite Bead (solder joint resistance) 0.02Ω 10mV 70mV 33µF* 0.02Ω 5mV 4.75V min. 0.01µF 500mA max. 10mV USB Self-Powered Hub Cable Connector Downstream Peripheral Port 0.03Ω V BUS 15mV 0.03Ω GND 15mV * Tantalum or 100µF electrolytic, per port Figure 2. Self-Powered Hub Per-Port Voltage Drop Application Note 17 2 October 1998 Application Note 17 Micrel VBUS (Port 2) 100mV Bulk capacitance of at least 120µF per hub is required. This bulk capacitance provides the short-term transient current needed during a hot-attachment event. A four-port hub with a 33µF, 16V tantalum capacitor mounted close to each downstream connector (see Figures 5, 6, 7, and 8) should have hot-plug droop much less than 330mV. If electrolytic capacitors are substituted, 100µF, 10V units should provide similar transient droop protection. Figure 3 shows the waveforms for a typical hot-plug event using the circuit of Figure 6. The hub output capacitor supplies the bulk of the inrush current to the downstream unit load (44Ω, 10µF). This current exceeds 2A for less than 10µs. The adjacent port voltage droop of 142mV is well within the 330mV specification limit. output ports to isolate transient droop during hot-attachment. Ground planes are good thermal radiators and provide EMI suppression. Conductor Width (inches) Resistance (mΩ/inch) 1 ⁄2 oz/ft2 0.025 0.050 0.100 0.200 39.3 19.7 9.8 4.9 1 oz/ft2 0.025 0.050 0.100 0.200 19.7 9.8 4.9 2.5 2 oz/ft2 0.025 0.050 0.100 0.200 9.8 4.9 2.5 1.2 3 oz/ft2 0.025 0.050 0.100 6.5 3.2 1.6 142mV 2.57A peak 330mV droop specification limit IOUT (Port 1) 500mA/div. Table 2. Trace Resistance Figure 2 shows the various voltage drops for a self-powered hub caused by printed circuit trace resistance, solder joints, power switches, and ferrite beads on power and ground leads. Traces which carry the combined current from the input should be made heavier to minimize voltage drops, or separate wide traces should be laid out directly from the input filter capacitor to each switch input pin. 100mA steady load TIME (10µs/div.) Figure 3. Typical Hot-Plug Event Detach Transient Surge When current in a wire is interrupted, the inductance of the wire may cause a voltage spike as the magnetic field collapses. To reduce these spikes, which generate EMI, and to prevent damage to components, 0.01µF, 25V ceramic bypass capacitors should be installed directly from VBUS pin to the ground pin at each port. Voltage Drop Analysis Adding the drops to the minimum output voltage of 4.75V shows that a minimum input of 4.85V is needed to ensure adequate output. The overall voltage drop to each port, caused by the printed circuit board and overcurrent protection device, is recommended to be less than 100mV. Ground traces are as important as power traces, as all voltage drops are in series. The voltage drop across a self-powered hub board has three components: board (VPC), protection (VP), and output filter (VFB). VPC is the trace resistance of the power and ground paths, approximately 15mΩ each, times 0.5A. VPC = 2 × 0.015Ω × 0.5A = 15mV VFB is the resistance of two ferrite beads and their solder joints, approximately 15mΩ each × 0.5A VFB = 2 × 0.015Ω × 0.5A = 15mV VPC + VFB = 15mV + 15mV = 30mV The maximum voltage drop recommended across a selfpowered hub is 100mV, so the loss in the protection device (VP) should not exceed 70mV: VP =100mV – 30mV = 70mV At an output port current of 500mA, the per-port protection device’s resistance may be up to 140mΩ: VP = 140mΩ × 0.5A = 70mV Printed Circuit Layout The power circuitry of USB printed circuit boards requires a customized layout to maximize thermal dissipation and to minimize voltage drop and EMI. Printed circuit power and ground traces should be wider than traces used in a normal digital layout to reduce their in-circuit resistance. Each solder or header connection may be expected to contribute up to 10mΩ, emphasizing the importance of trace resistance reduction. Table 2 shows typical resistance in mΩ/inch for standard conductor widths and thickness. Through-hole via connections may each have 15mΩ resistance. If a power trace traverses through a board, use multiple vias to reduce the interconnect resistance. Placing traces on both sides of the board, connected with multiple vias, can cut the trace resistance in half. Solder plating on the solder-side also reduces trace resistance. Short and wide same-side traces generally reduce voltage drop. Ground planes should have a separation line between October 1998 Conductor Thickness 3 Application Note 17 Application Note 17 Micrel 350mV(max) 100mV(max) From Upstream USB Port Cable Connector VBUS 0.03Ω 4.75V min. 500mA max. 15mV GND 0.03Ω 15mV Maximum Cable Cable Resistance Connector 0.03Ω 0.190Ω 95mV 15mV 0.190Ω 0.03Ω 95mV 15mV 1/2 MIC2526 Switch 400mA max. (trace resistance) 0.015Ω 4.7µF to 10µF 0.14Ω 6mV 0.1µF 0.015Ω 56mV 0.015Ω 6mV USB Cable Assembly VBUS 1.5mV 1mV 120µF 2 of 4 Downstream Peripheral Ports Cable Connector Ferrite Bead (solder joint resistance) 0.010Ω 4.40V min. 100mA max. 0.01µF 0.010Ω 0.015Ω 1mV 1.5mV 0.010Ω 0.015Ω 1mV 1.5mV GND VBUS 4.40V min. 100mA max. 0.01µF The upstream USB port can supply 500mA max. Up to 100mA is allocated to a processor in the bus-powered hub. 0.010Ω 0.015Ω 1mV 1.5mV GND Additional Ports USB Bus-Powered Hub Figure 4. Bus-Powered Hub Voltage-Drop Analysis 5V LDO regulators can provide regulated voltage output with as little as 5.35V input under full load. Micrel’s MIC29301 LDO regulator is rated for continuous 3A output with a typical 3.8A current limit and short-circuit limit of less than 5A. MIC29301 has a logic-level, noninverting, output-enable pin that allows the output to be turned on or off, and an open-drain error flag that conducts to ground if the device is in current limit or undervoltage lockout (low input—excessive dropout). Figure 5 shows a stand-alone self-powered hub using an MIC29301-5.0. Integrated High-Side Power Switches Component Selection Due to the limited voltage drop budget allowed for selfpowered hubs, each component must be evaluated for its contribution to voltage drop. Polyfuse Protection Limitations Most resettable Polymer Positive Temperature Coefficient Fuses (PPTC—polyfuse) are not suitable as the sole overcurrent protection in USB self-powered hubs. A self-resetting 2.5A PPTC may take up to 100 seconds to trip at 5A. The voltage drop due to the higher on-resistance of lower currentrated polyfuses placed on individual ports may exceed the 100mV self-powered hub drop budget. Polyfuse Resistance Increases For hubs with a regulated power supply, integrated power switches provide an economical, robust power management solution. Micrel’s MIC2505/25 single and MIC2506/26 dual switches react quickly to overloads, providing safety margin and overall power control and have an error flag output. The enable feature is available as inverting or noninverting logic, allowing flexibility in choice of hub controllers. All integrated switches are guaranteed to survive an output short circuit with a 5.25V input. Table 3 shows the characteristics of Micrel’s USB power switches. Polyfuses may permanently increase resistance after a trip event or exposure to high temperature, such as wavesoldering. A typical 0.11A polyfuse specified to pass 0.67A at 70°C, requires 5 seconds to trip with a 5A load. Its pretripped on-resistance ranges from 50mΩ to 100mΩ at 25°C ambient, rising permanently to approximately 170mΩ after an initial trip. This exceeds the 140mΩ resistance recommended by USB. Polyfuses require additional components to be switched off or reliably report overcurrent. Power Regulators Part Number Switches Linear regulators may be used to provide cost-effective closely-regulated standard voltages from unregulated power supplies. Low-dropout (LDO) regulators provide currentlimited, ±2% regulated output over a wide range of input voltage and output loads. LDO regulators have a logic enable pin and an error flag output that indicates current limiting or undervoltage. Logic IOUT RON MIC2505-1 single noninverting 2.0A 35mΩ MIC2505-2 single inverting 2.0A 35mΩ MIC2506 single noninverting 1.0A 70mΩ MIC2525-1 single noninverting 0.5A 140mΩ MIC2525-2 single inverting 0.5A 140mΩ MIC2526-1 dual noninverting 0.5A 140mΩ MIC2526-2 dual inverting 0.5A 140mΩ Table 3. Micrel Switches Application Note 17 4 October 1998 Application Note 17 Micrel Individual Port Control Figure 6 in the circuit examples section shows a self-powered hub using two MIC2526 dual 500mA switches to control four individual ports. If the USB controller has a single output enable or overcurrent detection pin, the respective inputs or outputs may have a common connection. An external digital latch on each enable pin could be used to disable individual ports during an overcurrent condition, allowing the other ports to continue to operate. Individual- and Ganged-Port Switching Individual-port switching offers several advantages over ganged-port switching. If a fault should occur on one port, the other ports remain unaffected, while a ganged-port switch would disable all ports. Individual-port control also offers superior transient droop performance since ports have isolation from each other. Individual-port control advantages apply to both self-powered and bus-powered hubs. (+) 4.45V to 5.25V 3.5A max. (–) Ferrite Bead 10k 33µF MIC5203-3.3 MIC5207-3.3 LDO Regulator VBUS IN D+ D– D+ 3.3V USB Controller V+ OUT MIC2526 ON/OFF OVERCURRENT 1.0 µF GND GND VBUS ENA 10k 4.7 µF 33µF* OUTA FLGA 0.1 µF 0.01µF IN FLGB GND ENB OUTB GND 0.1 µF D+ 33µF* 0.01µF MIC2526 ENA * 33µF, 16V tantalum or 100µF, 10V electrolytic per port IN FLGB GND ENB OUTB Downstream USB Port 2 500mA max. VBUS OUTA FLGA D– GND GND Bold lines indicate 0.1" wide, 1-oz. copper high-current traces. Downstream USB Port 1 500mA max. VBUS D+ D– D– 0.1 µF D+ 33µF* 0.01µF D– GND Downstream USB Port 3 500mA max. VBUS D+ 0.01µF 33µF* D– GND Downstream USB Port 4 500mA max. Figure 6. Regulated-Input Self-Powered Hub Ferrite Bead MIC29301-5.0 LDO Regulator 5.4V 5.4V 20VA 47µF VBUS D+ MIC5203-3.3 MIC5207-3.3 100mA LDO Regulator max. IN OUT D– GND 10k IN EN V+ GND VBUS 5.0V D+ FLAG GND 0.01µF 33µF D– GND 33µF 3.3V USB Controller 1.0 µF OUT ON/OFF Downstream USB Port 1 4.75V min 500mA max. VBUS OVERCURRENT D+ 33µF 4.7 µF 0.01µF D– GND Downstream USB Port 2 4.75V min 500mA max. D+ D– Bold lines indicate 0.1" wide, 1-oz. copper high-current traces. VBUS GND D+ 10k 33µF 0.01µF D– GND Downstream USB Port 3 4.75V min 500mA max. VBUS D+ 0.01µF 33µF D– GND Downstream USB Port 4 4.75V min 500mA max. Figure 5. Simple Stand-Alone Self-Powered Hub October 1998 5 Application Note 17 Application Note 17 Micrel Inrush Current When power switches are energized, output capacitors may draw high inrush current causing an upstream voltage droop which could affect other circuits. USB requires that hot-attach or switched events present one unit load maximum (44Ω min. with 10µF max.). High inrush current may also cause EMI or damage tantalum capacitors. Micrel’s power switch products eliminate these problems by providing soft start which limits inrush current. Micrel’s MIC2526 high-side switches have a charge pump which slows turn-on time between 1 and 2ms. Suspend Power When a hub is suspended, downstream ports are still powered. When downstream devices are suspended, the maximum allowed average current per port is 500µA including the data line pull-up resistors and power switch operating current. Voltage-Drop Regulation A 350mV maximum total drop is allowed from an upstream self-powered hub connector to bus-powered hub downstream socket connector, as shown in Figure 4. This drop includes the cable, connectors, switch, and board. An attached cable may have 20mΩ per IDC header, or soldered connection, instead of 30mΩ per USB cable connector contact, providing a larger voltage margin for the hub components. Standard length cables have a wire size chosen to produce a voltage drop of less than 190mV for two conductors at 500mA current. Shorter cables, using the same gauge wire, produce a smaller voltage drop. Figure 4 shows the component voltage drops for a typical bus-powered hub. Ganged Overcurrent Limiting Figure 7 shows a ganged-output, self-powered hub using a single MIC2505 2A switch to protect up to four outputs. Output filters with two ferrite beads per port satisfy USB requirements by preventing hot-attach transients at one port from causing transient voltage droop of more than 330mV on adjacent ports. Bus-Powered Hub Requirements USB specifies that bus-powered hubs have power switching for their downstream ports, so that downstream current may be enumerated and controlled. Port power is switched under host software control on an individual port basis or by a single switch for all ports (gang-mode power control). Port power is not switched off during a reset, so that attached devices may be monitored. The upstream cable connector must receive a minimum of 4.75V, while each downstream port provides a minimum of 4.40V at 100mA. Bus-powered hubs obtain all power from an upstream self-powered hub. They may draw up to 100mA upon power up. After configuration of internal functions and downstream ports, a maximum of 400mA is allocated up to a maximum of four downstream ports. Each port must be capable of supplying a minimum of 100mA. Transient Regulation Bus-powered hubs have similar requirements to that of selfpowered hubs. USB requires that voltage transients at a 4.4V peripheral port must never go below 4.07V. Layout, ferrite beads, and both capacitor value and type affect transient regulation. (+) 4.45V to 5.25V 3.5A max. (–) Ferrite Bead VBUS IN D+ D– GND 10k 33µF MIC5203-3.3 MIC5207-3.3 LDO Regulator D+ 3.3V USB Controller V+ OUT GND VBUS 1.0 µF ON/OFF OVERCURRENT MIC2505 EN 10k 0.1µF 4.7 µF 33µF* OUT FLG IN GND OUT GATE Downstream USB Port 1 4.75V min. 500mA max. VBUS D+ 33µF* GND D– GND IN 0.1µF D+ D– 0.01µF 0.01µF D– GND Bold lines indicate 0.1" wide, 1-oz. copper high-current traces. Downstream USB Port 2 4.75V min. 500mA max. VBUS D+ 33µF* 0.01µF D– GND * 33µF, 16V tantalum or 100µF, 10V electrolytic per port Downstream USB Port 2 4.75V min. 500mA max. VBUS D+ 0.01µF 33µF* D– GND Downstream USB Port 3 4.75V min. 500mA max. Figure 7. Ganged-Switch Self-Powered Hub Application Note 17 6 October 1998 Application Note 17 Micrel Specifying an attached bus-powered hub cable simplifies the voltage drop equation by reducing cable length and the downstream connector resistance. A keyboard with a 1m cable would be a typical example of an attached cable buspowered hub. Since the keyboard would plug into the monitor or self-powered hub near the user, it does not need a longer cable. Lower Cable Resistance USB Cables USB specifies the construction and wire gauge of cables used to connect host to hub to peripherals. USB also specifies a maximum cable power wire resistance of 190mΩ and a maximum cable length of 5m (3m for a low-speed attached subchannel cable) to minimize voltage drop and transmission time. Connector Resistance The standard mated USB connector is assumed to have 30mΩ maximum contact resistance per pin after 1500 insertions, including the solder connections to the PC board and cable wires. Since the voltage cannot be measured at the contact point, USB voltages are usually measured at the circuit board side of a connector, between the power pin and ground pin (not power supply ground). Detachable Cables Table 6.2 in the USB specification shows resistance per meter for various AWG wire sizes. A keyboard-based buspowered hub, with a short 2m, 22AWG attached cable, as opposed to the standard 3.1m, 22AWG detachable cable, has a cable resistance of 114mΩ per conductor. Four-Port Bus-Powered Hub Voltage Drop See Figure 4 for the various voltage drops for a bus-powered hub. There are two components of voltage drop: High Current in Upstream Cable Use the 500mA maximum to compute the voltage drops in the upstream cable connector, cable wires, and downstream cable connector. Good layout should produce less than 35mV drop in the 500mA section of the hub printed circuit board. A ganged-output (MIC2526) has a typical on-resistance of 145mΩ, including lead solder joints, for a 400mA voltage drop of 58mV. Subtracting the above voltage drop (93mV) from the 100mV maximum, leaves 7mV for each 100mV output filter and connector. VCON = 0.5A × 30mΩ × 4 pins = 60mV VWIRE = 0.5A × RWIRE × 2 × wire length V0.5 = VCON + VWIRE = 50mV + VWIRE Detachable cables have different style connectors on each end to prevent incorrect connections (for example: the downstream port of one hub to the downstream port of a second hub). A standard USB cable has a Series A connector on the upstream end and a Series B connector on the downstream end, with 28AWG data wires and 90Ω ±15% impedance. Cables are not allowed to have the same series (A or B) connector on both ends to ensure proper data and power flow. The maximum voltage drop allowed across a bus-powered hub and its cable is 350mV. A bus-powered hub may draw 500mA, producing a 250mV drop in a standard 190mΩ maximum cable and its connectors, leaving 100mV margin for a bus-powered hub circuit board, switch, and connectors. Ferrite Bead 4.45V Upstream VBUS 500mA max. VBUS D+ MIC5203-3.3 MIC5207-3.3 LDO Regulator IN D– GND D+ 3.3V USB Controller V+ OUT GND VBUS 10k 1.0 µF ON/OFF OVERCURRENT MIC2525 EN 10k 0.1µF 4.7 µF D– 33µF* OUT FLG IN GND OUT NC D+ 0.01µF NC D– GND Downstream USB Port 1 4.4V min. 100mA max. VBUS 0.1µF D+ 33µF* 0.01µF D– GND GND Bold lines indicate 0.1" wide, 1-oz. copper high-current traces. Downstream USB Port 1 4.4V min. 100mA max. VBUS D+ 33µF* * 33µF, 16V tantalum or 100µF, 10V electrolytic capacitor per port. 0.01µF D– GND Downstream USB Port 1 4.4V min. 100mA max. VBUS D+ 0.01µF 33µF D– GND Downstream USB Port 1 4.4V min. 100mA max. Figure 8. Ganged-Output Bus-Powered Hub October 1998 7 Application Note 17 Application Note 17 Micrel Although Micrel’s USB switches are compliant to IEC standards for ESD protection, USB controllers may need additional ESD protection to assure a robust design. For USB controller data lines, ESD protection should also be provided protection that can be added inexpensively with external transient protection devices such as the SurgeX™ protector from Cooper/Bussman, telephone (314) 394-2877. Self-Powered Hub with Individual Power Switches Circuit Examples Figure 5 shows a minimal self-powered hub, using Micrel’s MIC29301-5.0 3A LDO linear regulator to provide 5.0V ±2% (4.9V to 5.1V) from an unregulated >5.35V, 3.5A transformerrectifier-capacitor input. The MIC29301-5.0BT has an enable pin to control ganged power, up to 3.5A, to seven downstream ports each drawing up to 500mA. The MIC29301 limits current to less than 4A with less than 5A short-circuit current. An error flag goes low if the device is in current limit, thermal shutdown, or has an excessive output voltage drop. If heavy loads cause the thermal shutdown to activate, a slow thermal oscillation may occur as the regulator heats up under load and cools down when shut-off. The hub controller may be programmed to shut down the LDO regulator after the error flag has been low for a preset time and then to periodically reenergizing the LDO regulator enable to determine if the excessive load has been removed. When a local 5V +5⁄–3% power supply, such as a standard personal computer, provides operating voltage from 4.85 to 5.25V, protection devices with less voltage drop than a polyfuse or LDO regulator are needed. Micrel’s integrated high-side power switches have low on-resistance, built-in current limiting, thermal shutdown, and logic-level enable. The worst-case voltage drop recommended across the selfpowered hub board, MIC2526, and ferrite beads is 100mV, guaranteeing 4.75V at the downstream output port. Figure 6 illustrates a self-powered hub with up to four downstream 500mA switched ports, using two MIC2526 dual 0.5A power switches. Figure 7 shows a four-port self-powered hub, using a single MIC2505 2A power switch to provide ganged power from a 5V +5⁄–3% regulated local supply. Output Filter Output ports must have a 120µF bulk capacitance per hub. Tantalum capacitors across the power and ground lines, near each downstream connector, reduce EMI and decouple voltage droop caused when downstream cables are hotattached. Ferrite beads in series with Vbus, the ground line and the 0.01µF bypass capacitors at the power connector pins are recommended for EMI and ESD protection. Flag Transient Filtering Ganged-Output Bus-Powered Hub Figure 8 shows a single MIC2525 140mΩ switch providing ganged power switching to four downstream ports. The single power switch controls all downstream ports, with soft start limiting the inrush current to the four downstream 33µF capacitors when the switch is enabled. A low-cost MIC5203 (SOT-143) or MIC5207 (TO-92) 3.3V low-dropout (LDO) regulator provides power from the bus to the USB controller. The transient in-rush current to downstream capacitance may cause a short-duration (10µs to 20µs) error flag, which may cause erroneous overcurrent reporting. A simple 1ms RC low-pass filter (10kΩ and 0.1µF) in the flag line (see Figures 5, 6, and 7) eliminates short-duration transients. Electrostatic Discharge Protection Summary Flyback transients and electrostatic discharge (ESD) may exceed the maximum operating voltage ratings of ICs. Micrel’s MIC2526 has been tested and certified to exceed the requirements of IEC 1000-4-2 (EN 50082-1) for 15kV air-discharge and 8kV contact discharge with the output filters shown in the circuit examples (120µF, 0.01µF, and ferrite beads). Application Note 17 Micrel’s logic-controlled high-side power switch integrated circuits are ideal for USB power switching applications. Each offer superior protection and performance relative to polyfuses. In addition, proper PCB layout techniques are highly recommended to ensure that both USB voltage regulation and transient regulation specifications are satisfied. 8 October 1998 Application Note 17 Micrel Printed Circuit Layout Hints Definitions • Power Traces wide, heavy copper short, straight traces “star” from input filter capacitor same side, no vias • Ground wide ground plane same side, no vias • Switches wide power input traces 0.1µF ceramic input bypass close to IC wide power output traces IC placed near output port “star” power traces from input capacitor power traces on same side, no vias • Output Filter close to output port 120 µF tantalum, 10V 0.01µF ceramic 25V bypass ferrite bead on power lead ferrite bead on ground lead connector shield to ground plane • Thermal Management ground plane under switch wide traces to all IC pins thermal paste or adhesive under IC maximum copper area, both sides air circulation inside and outside Controller A microprocessor which implements the Universal Serial Bus protocols. Host The root hub host computer system where the Universal Serial Bus host controller is installed. This includes the host hardware platform (CPU, bus, etc.) and the operating system in use. Host Controller The host’s Universal Serial Bus interface. Hub A Universal Serial Bus device which repeats data and provides additional connections to the Universal Serial Bus. Bus-Powered Hub Receives operating power from the upstream port, up to 500mA. Root Hub A Universal Serial Bus hub directly attached to the host controller. This hub is attached to the host; tier 0. Self-Powered Hub Draws less than 100mA from upstream port, provides downstream power from 5V +5/–3% internal power supply and up to 500mA per port, 3.5A per protection device. Port Point of access to or from a system or circuit. For Universal Serial Bus, the point where a Universal Serial Bus is attached. Downstream Port The direction of data flow away from the host. A downstream port is the port on a device electrically farthest from the host that generates downstream data traffic from the hub. Downstream ports receive upstream data traffic. Root Port The upstream port on a hub. Upstream Port The direction of data flow toward the host. An upstream port is the port on a device electrically closest to the host that generates upstream data traffic from the hub. Upstream ports receive downstream data traffic. Unit Load Not more than 100mA power drain under normal operating conditions, with a 4.4V bus. Universal Serial Bus (USB) A collection of Universal Serial Bus devices, software, and hardware that allows them to connect the capabilities provided by functions to the host. October 1998 9 Application Note 17 Application Note 17 Application Note 17 Micrel 10 October 1998 Application Note 17 October 1998 Micrel 11 Application Note 17 Application Note 17 Micrel MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB USA http://www.micrel.com This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc. © 1998 Micrel Incorporated Application Note 17 12 October 1998