an 006 may 1998 1 advanced solutions for usb protections abstract the aic1526 is a dual high - side power switch for self - powered and bus - powered usb applications. this application note describes how the aic1526 provides the power switching and over - current prote ction. t he items such as bus - powered hubs and self - powered hubs are first introduced, after which , the power distribution of usb specification (version 1.0) and the key points of aic1526 application will be followed. introduction the universal ser ial bus (usb) interface is a 12 mb/s, multiplexed serial bus designed for low to medium speed pc peripherals. the usb interface consists of four lines (see figure 1). two data lines(d+,d - ) are used for differential data transmission and the other t wo power lines are for power distribution of the 5 volts. the usb interface is conceived for hot - plug - unplug of peripheral devices to achieve the real ?plug - and - play? function. ? 2 power lines: -- 5v -- gnd ? 2 data lines -- d+ -- d- 5v d+ d- gnd figure 1. usb connector hubs provide distributi ons of data and power to downstream (away from the host) devices and communicate with the host. a hub consists of one upstream port and several downstream ports (see figure 2). by connecting the serial connecting hubs, the number of devices can be increase d up to the maximum of 127 devices.
an 006 2 mother board(host) hub hub device downstream connector upstream connector device device device figure 2. connections of usb self - powered hubs have a local power supply. the hubs in pcs (host), stand - alone hubs, and monitors are usually self - powered hubs. the self - powered hubs are re quired to limit and report over - current conditions and have the ability of supply ing up to 500ma to each downstream port (see figure 3). bus - powered hubs obtain all power from upstream self - powered hubs, and may supply 100ma or more to each downstream port if the input power budget does not exceed. the bus - powered hub must provide s power switching for downstream ports on each port basis or have a single switch for all of the ports (gang - mode power control).
an 006 3 self- powered(sp) hub/host bus- powered(bp) hub down- stream data ports upstream data ports 5v 3.3v overcurrent report down- stream vbus local power supply hub controller over- current limit down- stream data ports upstream data ports 5v from upstream port 3.3v turn on/off control 5v-3.3v regulator hub controller power switch 500ma/port ( max) down- stream vbus 4 ports(max) 100ma each figure 3. self - powered hub & bus - powered hub usb power distribution voltage requirements: the self - powered hub has an internal power supply. the dc output voltage, measured at the board side connector of the self - powered hub, must in the range of 4.75v ~ 5 .25v. the bus - powered hub obtains all power from an upstream self - power hub. the upstream cable connector must receive a minimum of 4.65v, while each downstream port provides a minimum of 4.40v at 100ma. output current limits: because the power is supplie d from an internal power supply in self - powered hub, the over - current protection is required on the output ports for safety reason. ul limits require ports to deliver no more than 5a (including short circuit currents). the power distribution for a self - pow ered hub can be designed either with one over - current protection device each output port or by ganging multiple ports to one over - current protection device. the minimum output current limit of each output port is 500ma for the self - powered hubs, but the maximum output current limit depends on the numbers of output ports of a self - powered hub. if there is only one output ports of a self - powered hub, the maximum output current limit is 5a for this hub to supply maximum current 5a to ganging multiple downst ream ports. in the same way, if a self - powered hub has two output ports, the maximum output current limit is 5a/2=2.5a for
an 006 4 each output port to supply maximum current 2.5a to downstream ports. table 1 shows the minimum and maximum output current limit of ea ch port for different number of ports of self - powered hub. a self - powered hub with 4 output ports is the most popular type in recent market , and the output current limit is typically 700ma~1.25a. a bus - powered hub receives power from upstre am port up to 500ma. the output current of each port doesn?t have to be limited, but needs to be turned on or turned off. usb current limits requirements per ports ? minimum current per port is 500ma. ? if a channel has n ports, maximum current per port is 5 a/n.(n 7) minimum current/port individual port maximum current i ndividual port total maximum current combining all ports 1 port 500ma 5.0a 5 .0 a 2 ports 500ma 2.5a 5.0a 3 ports 500ma 1.6a 5.0a 4 ports 500ma 1.25a 5.0a 5 ports 500ma 1.0a 5 .0a 6 ports 500ma 0.83a 5.0a 7 ports 500ma 0.71a 5.0a a hub with 4 ports is the most widely used. table 1. the minimum and maximum current limit transient requirements: when a function or hub is plugged into the network, inrush current and voltage d roop is induced. the target maximum droop in the hub v bus is 330mv or about 10% of the nominal signal swing from the function. in order to reach this target, the usb specification (version 1.0) about the load and the hub port v bus power lines must meet the following conditions (see figure 4): the hub port v bus power lines must be bypassed with no less than a 120 m f tantalum capacitor. the maximum load that can be placed at the downstream end of a cable is 10 m f in parallel with 44 w . aic 1526 out gnd 120 m f 10 m f 44 w suddenly plug in! figure 4. capacitors of power lines and devices
an 006 5
an 006 6 application of aic1526 figure 5 shows that the aic1526 contains 2 high - side power switches. each switch provides controllable power switching, over - current protection, thermal shutdown, and under voltag e lockout (uvlo). figure 6 shows a typical application circuit. each port has an enable pin, a flag pin, and an output pin: l the enable pin (ctl) is the input turn on/off signal of the port. ctl signal is a ctive low for aic1526 - 0, and active high for aic 1526 - 1. l the flag pin (flg) is the indication of normal/fault. if the operation is normal, the flag signal is high. either one of input under voltage, output current limit and thermal shutdown occurs, the flag signal is pulled low. l the output pin is the con trolled output to downstream devices. the power switch between the "in" pin and the "out" pin is a low on - resistance (typical 150m w ) mosfet. driver flag thermal shutdown uvlo turn on/off control current limit in(5v) ctla flga outa ctlb flgb outb aic1526 driver flag thermal shutdown uvlo turn on/off control current limit current sense current sense figure 5. the block diagram of aic1526
an 006 7 ctla flga ctlb flgb outa outb gnd v bus d+ d- gnd 10 k 10 k usb controller philips intel winbond ?. on/off on/off over current over current vcc=5.0v 0.1 m f v bus d+ d- gnd in figure 6. the ty pical application circuit of aic1526 voltage and current transient in order to provide a good power quality for suddenly plugged/unplugged downstream devices, the voltage and current transient must be controlled. the ferrite beads and th e bypass capac itors can be used to reduce current surge and voltage spike , as well as decrease electromagnetic interference (emi). when the downstream device is suddenly plugged in to the network, the voltage transient on the input capacitance of the downstream device will induce the current surge. the ferrite beads are recommended in series with all power and ground connector pins (see figure 7). the ferrite bead is similar to a high permeability inductance wherein the high frequency impedance attenuates the high frequency current, and the dc current passes freely. when the downstream device is suddenly unplugged, the current transient of the wire inductance causes the voltage spike. a bypass capacitor (0.01 m f / 25v) should be installed from v bus pin to ground pin at each connector as closer as possible (see figure 7). ferrite bead bulk capacitor bypass capacitor 120 m f 0.01 m f 0.1 m f ferrite bead cable connector one port of aic1526 in out gnd 5v i nput 5v gnd usb hub figure 7. ferrite beads and bypass capacitors of usb hub
an 006 8 error flag signal filtering the output current limit of each port is 1.0a for aic1526. if the continuous curre nt is above 1.0a, the flag signal will be pulled low to tell the hub controller the operation is abnormal. however, the current surge caused by hot - plug - in is often excessive to trigger the flag signal to be pulled low. if the inrush current is still larg e enough to trigger the flag even the ferrite beads are already added, it's necessary to add the low - pass filter between the flag pin to the hub controller. figure 8 shows that the low - pass filter with time constant is equal to 1ms. through the equation (1 ), we can obtain that the hub controller will receive 4.75v after 52 m s since the flag signal starts to be pulled low. t/1ms 5e 4.75 - = s 52 t m = t (1) because the transient period during hot - plug is always 20~30 m s, the rc filter preven ts the error flag from sending to the hub controller. 5 v : flag : 10 k 10 k 0.1 m f usb controller figure 8. low - pass filter to prevent the error flag summary because usb is a hot - plug / unplug system, its components are subject to electrostatic discharge (esd). the ai c1526 is designed to withstand human body model esd test. to assure robust and maintenance - free protections, it's necessary to place low esr ceramic bypass capacitors as closely as possible between the in and out pins to increase the esd immunity. to preve nt excessive voltage droop during hot - plug, the power lines must be bypassed with an output tantalum capacitor. besides, ferrite beads and small bypass capacitors are recommended in series with all power and ground connector pins to reduce inrush current a nd decrease emi.
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