Output filter capacitor
A capacitor should be used on the Vbus output of each downstream port. Tests were carried out to determine the effects of "capacitor output" geometry versus "ferrite output" with respect to meeting voltage droop and EMI requirements. Far better droop performance was obtained with the output capacitor (150 µF) downstream of the ferrite bead due to the lowered effective impedance as seen by the output connector. The "ferrite output" configuration, with the ferrite bead "downstream" of the output capacitor, is more commonly seen in EMI protection geometry, but measurements indicated that EMI protection was excellent in either geometry.
A ferrite bead in series with the Vbus pin is usually necessary to lower EMI radiation down the USB cable. The series resistance of the bead and its RF impedance are the important criteria. A "bead-on-a-wire" configuration is selected primarily for its low series DC resistance. These devices are available in both through-hole and SMD (bead-on-a-strap) configurations. Any ferrite bead with 50 ohms or more impedance at 100 mHz will work. The "Capacitor output" geometry, where the output capacitor follows the ferrite bead, is the preferred geometry for excellent droop characteristics at the downstream port.
The USB specification 1.1 requires overcurrent protection in either an individual or ganged protection scheme. A UL-recognized safety device facilitates agency testing and provides high reliability. Along with meeting safety requirements, the protection device should protect the equipment from damage (for example, protect PCB traces from burning). In addition, the protection device must not nuisance trip (such as during a hot-plug event). Using PolySwitch devices in an individual port protection scheme provides an optimal design for a downstream power connection. When a fault occurs on a port and that port's PolySwitch device trips, the adjacent ports remain functional. Individual port protection also allows the designer to select a smaller and faster-acting device.
The hub controller supplies a software-controlled on/off signal from the host. Power to downstream ports must have on/off signal-switching capability to power off all downstream ports when the hub comes out of power-up, or when it receives a reset on its root port. A low RdsOn MOSFET must be selected for power switching. Selection of a MOSFET should include a 50-milliohm-or-less RdsOn resistance, logic-level gate, and a 3-A or higher current rating. A P-Channel MOSFET is usually employed as the initial logic level of most control microprocessors' output pins and is weakly pulled high. The P-Channel MOSFET is turned on by driving its gate low; the default state of the MOSFET should be off. A weak external pull-up may be used if desired.
As with any powered port, short circuits and equipment damage can occur and should be a concern. During a short-circuit event, current can get high enough to cause damage to the Bus-Powered Hub. PolySwitch devices are appropriate in individual or ganged port protection schemes and provide Bus-Powered Hub designs protection at the port to protect the MOSFET during a fault.
Section 188.8.131.52 of the USB specification requires inrush-current limiting when switching downstream power. This applies to Bus-Powered Hubs, where the Vbus voltage could droop significantly if measures are not taken to "soft start" the downstream power. A simple resistor capacitor (RC) network will provide an inexpensive solution to limiting inrush-current. Figure 7 shows Vbus rising sharply without soft start, whereas Figure 8 shows a much more gradual rise with soft start implemented.
Pulse-width modulation of the PWR_ON- pin may be used in place of the RC network to provide the necessary soft start. The control microprocessor is programmed to drive the pin to zero in a stepwise fashion over a few milliseconds. Figure 9 illustrates the behavior of the pulses.
In order to meet the voltage drop, droop, and EMI requirements in Specification 1.1, careful PCB layout is necessary. The following guidelines must be considered:
- Keep all Vbus traces as short as possible and use at least 50-mil, 1-oz. copper for all Vbus traces.
- Avoid vias as much as possible. If vias are necessary, make them as large as feasible.
- Place the output capacitor and ferrite beads as close to the USB connectors as possible.
- If using a Vbus switch, place it close to the USB connectors.
- Use a separate ground and power planes if possible.
- If using separate planes, separate the power plane into switched (downstream) and unswitched (logic or upstream) sections.
The USB specification states a minimum port-output voltage (VOUT) in two locations on the bus, 4.75 V out of a Self-Powered Hub Port and 4.40 V out of a Bus-Powered Hub Port. Careful PCB design layout and propor device selection can easily achieve these requirements.
The following voltage droop analysis is divided into two parts, Standard USB voltage droop and short- circuit voltage droop.
Standard USB voltage droop
Voltage droop occurs during a hot-plug event as the result of the connection of a peripheral and its uncharged-input bulk capacitance to a USB port. For a few tens of µs the host must supply high current into the peripheral until its bulk capacitance is charged to Vbus, creating a capacitive divider between the host's bulk capacitance and that of the just-plugged-in USB peripheral. A standard test has been developed by the USB Implementor's Forum (USB-IF) to ensure compliance with the USB specification's limitation of voltage droop to 0.330 V maximum on a hub. Six different PolySwitch devices were tested. Worst case results for each device are shown in Table 1.
Table 1. Standard Voltage Droop test results.