Philips Semiconductors Interconnectivity ISP1122 Power Management Design Guide (Last updated as at 4 May 1999) ____________________________________________________________________________________ Philips Semiconductors - Asia Product Innovation Centre Visit http://www.flexiusb.com Interconnectivity Page 2 of 13 Power Management Guide Introduction to Overcurrent Protection The USB Specification Revision 1.1 requires overcurrent protection in both individual and ganged protection schemes. The protection device must protect equipment from damage; for example, PCB traces from burning, or power supply from overloading. In addition, the protection device must not cause a nuisance trip during a hotplug event. Lastly, when a fault occurs on a port, the adjacent ports must remain functional. There are two types of overcurrent detection practices in the industry. 1. Protection using polyswitch for easy implementation 2. Use of overcurrent detection IC; for example, Texas Instruments TPS2014 and Micrel MIC2526 PolySwitch Implementation Using PolySwitch devices in an individual port protection scheme provides an easy design for a downstream power connection. The implementation of such technique is illustrated below. This technique uses the characteristics of a temperature-dependent resistor device--polymeric PTC device--a series element in a circuit. This device protects the circuit by going from a low-resistance to a high-resistance state when there is an overcurrent: it increases in resistance, and thus reduces the current in the circuit to a value that can be safely carried by any of the circuit elements. The increase in resistance corresponds to the rapid increase in the temperature of the device, caused by the I2R heating within the device. 1:ypical PTC Application In USB Spec. Rev. 1.1, there is a condition that states that the host must be informed of any overcurrent event. The notification of the overcurrent event can be implemented through the techniques as shown as follows: ____________________________________________________________________________________ Philips Semiconductors - Asia Product Innovation Centre Visit http://www.flexiusb.com Interconnectivity Page 3 of 13 Power Management Guide A pull-down resistor of high resistance (1M) is connected from the PolySwitch to ground where this end must be connected to the overcurrent detection pin of the USB hub controller. This high ohmic resistor prevents the Vbus from floating during the overcurrent event as the current is cut off from the local power supply. The trade-off for using PolySwitch is the response time, typically 100 ms, when there is an overcurrent situation. Overcurrent Detection IC The preceding diagram shows the implementation of device with built-in overcurrent circuitry. A typical example is from the solution offered by Texas Instruments. TPS2014 and TPS2015 provide overcurrent protection by switching to a constant current source whenever the trip threshold current is exceeded. The controller is notified of a fault condition when the overcurrent logic output is set to low. The switch remains in this constant current state until either the fault is removed or the switch is disabled. ____________________________________________________________________________________ Philips Semiconductors - Asia Product Innovation Centre Visit http://www.flexiusb.com Interconnectivity Page 4 of 13 Power Management Guide The overcurrent circuitry has the advantages of fast trip time, fault reporting capability and low onresistance when compared to PTC resistors. The typical trip time is approximately 10 ms. However, the cost to implement such circuitry block would usually result in higher cost of discrete device building. Overcurrent Detection Techniques from Philips Semiconductors 10mV 85mV 10mV USB downstream connector Z ferrite bead USB Spec 4.75V(min) DC 5.1V (Min: 4.85V Max:5.35V) Over current detection Load Voltage drop across PMOS 100mA 15mV 500mA 75mV 750mA 1.125mV ISP1122 assume Rds=150mohms Philips ISP1122 has a different approach to the implementation of overcurrent detection. As shown in the preceding diagram, the integrated overcurrent detection circuit of ISP1122 senses the voltage drop across the power switch or an extra low-ohmic sense resistor. The reference voltage is VSP/BP (self-powered mode) or VCC (bus-powered mode). Apart from the power switch and the sense resistor (self-powered, global overcurrent detection only) no other external components are required. ____________________________________________________________________________________ Philips Semiconductors - Asia Product Innovation Centre Visit http://www.flexiusb.com Interconnectivity Page 5 of 13 Power Management Guide USB Spec. Rev. 1.1 mentions that for a self-powered hub, the minimum voltage observed at downstream must be at least 4.75 V. From the preceding diagram, a 5.1-V power supply with a tolerance of 3% to satisfy the specification. The voltage drop usually include a typical voltage drop of 10 mV across the PCB traces, a 75-mV drop across the PMOS transistor, and a 10-mV drop across the ferrite beads. As for global overcurrent detection 10mV 85mV 10mV USB downstream connector Z ferrite bead USB Spec 4.75V(min) DC 5.1V (Min: 4.85V Max:5.35V) Over current detection Load Voltage drop across PMOS 100mA 15mV 500mA 75mV 750mA 1.125mV ISP1122 assume Rds=150mohms The integrated overcurrent detection circuit of ISP1122 senses the voltage drop across the power switch or an extra low-ohmic sense resistor. The reference voltage is VSP/BP (self-powered mode) or VCC (bus-powered mode). Apart from the power switch and the sense resistor (self-powered, global overcurrent detection only) no other external components are required. The ISP1122 has several modes of operation, each corresponding with a different pin configuration. Modes are selected using the pins INDV, OPTION and SP/BP, as shown in the following: Mode selection table Mode 0 1 2 3 4 5 6 7 INDV 0 0 0 0 1 1 1 1 OPTION 0 0 1 1 0 0 1 1 SP/BP_N 0 1 0 1 0 1 0 1 PSW1,2,3,4_N GL1,2,3,4_N GL1,2,3,4_N GL1,2,3,4_N GL1,2,3,4_N PSW1,2,3,4_N PSW1,2,3,4_N PSW1,2,3,4_N GL1,2,3,4_N PSW5_N GPSW_N GPSW_N GPSW_N GL5_N Inactive PSW5_N Inactive GL5_N OC1,2,3,4_N Inactive Inactive Inactive Inactive OC1,2,3,4_N OC1,2,3,4_N Inactive OC1,2,3,4_N OC5_N GOC_N GOC_N Inactive GOC_N Inactive OC5_N Inactive OC5_N Inactive OCX_N pins need to be tied to Vcc ____________________________________________________________________________________ Philips Semiconductors - Asia Product Innovation Centre Visit http://www.flexiusb.com Interconnectivity Page 6 of 13 Power Management Guide The following table explains the mode configuration in the physical configuration of the hub implementation. Mode Self-/BusPowered Power Switch control GoodLink Overcurrent Protection Remarks 0 1 2 3 4 5 6 7 Bus-powered Self-powered Bus-powered Self-powered Bus-powered Self-powered Bus-powered Self-powered Ganged Ganged Ganged Inactive Individual* Individual Individual* Inactive Yes* Yes* Yes* Yes No No No Yes Global Global Inactive Global Individual* Individual Inactive Individual Ganged and bus powered Ganged and self powered No overcurrent No power switch control Individual and bus powered Individual and self powered No overcurrent No Power switch control * Without Port 5 Mode 0 - Bus powered, Global overcurrent protection, and GoodLink The circuitry shows a classical example of a Philips hub implementation on mode 0 configuration. To provide global overcurrent protection of 500 mA to all downstream ports, a PMOS transistor of Rds 55m will be required. ____________________________________________________________________________________ Philips Semiconductors - Asia Product Innovation Centre Visit http://www.flexiusb.com Interconnectivity Page 7 of 13 Power Management Guide Mode 1 - Self-powered, Global overcurrent protection, and GoodLink ____________________________________________________________________________________ Philips Semiconductors - Asia Product Innovation Centre Visit http://www.flexiusb.com Interconnectivity Page 8 of 13 Power Management Guide Mode 2 - Bus-powered and GoodLink ____________________________________________________________________________________ Philips Semiconductors - Asia Product Innovation Centre Visit http://www.flexiusb.com Interconnectivity Page 9 of 13 Power Management Guide Mode 3 - Self powered, Global overcurrent protection, and GoodLink ____________________________________________________________________________________ Philips Semiconductors - Asia Product Innovation Centre Visit http://www.flexiusb.com Interconnectivity Page 10 of 13 Power Management Guide Mode 4 - Bus powered, Individual overcurrent protection ____________________________________________________________________________________ Philips Semiconductors - Asia Product Innovation Centre Visit http://www.flexiusb.com Interconnectivity Page 11 of 13 Power Management Guide Mode 5 - Self-powered and Individual overcurrent protection ____________________________________________________________________________________ Philips Semiconductors - Asia Product Innovation Centre Visit http://www.flexiusb.com Interconnectivity Page 12 of 13 Power Management Guide Mode 6 - Bus powered, No overcurrent protection ____________________________________________________________________________________ Philips Semiconductors - Asia Product Innovation Centre Visit http://www.flexiusb.com Interconnectivity Page 13 of 13 Power Management Guide Mode 7 - Self powered, Individual overcurrent protection, and GoodLink ____________________________________________________________________________________ Philips Semiconductors - Asia Product Innovation Centre Visit http://www.flexiusb.com