ISP1122 power management design guide - Philips Semiconductors / NXP Semiconductors

Philips Semiconductors

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ISP1122

Power Management Design Guide

(Last updated as at 4 May 1999)

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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:

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A pull-down resistor of high resistance (1MΩ) 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.

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The overcurrent circuitry has the advantages of fast trip time, fault reporting capability and low on-

resistance 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

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.

DC 5.1V

(Min: 4.85V

Max:5.35V)

10mV 85mV 10mV

ferrite

bead

USB

downstream

connector

Over current

detection

ISP1122

USB Spec

4.75V(min)

Load Voltage drop

across PMOS

100mA

500mA

750mA

15mV

assume Rds=150mohms

75mV

1.125mV

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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

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 INDV OPTION SP/BP_N PSW1,2,3,4_N PSW5_N OC1,2,3,4_N OC5_N

0 0 0 0 GL1,2,3,4_N GPSW_N Inactive GOC_N

1 0 0 1 GL1,2,3,4_N GPSW_N Inactive GOC_N

2 0 1 0 GL1,2,3,4_N GPSW_N Inactive Inactive

3 0 1 1 GL1,2,3,4_N GL5_N Inactive GOC_N

4 1 0 0 PSW1,2,3,4_N Inactive OC1,2,3,4_N Inactive

5 1 0 1 PSW1,2,3,4_N PSW5_N OC1,2,3,4_N OC5_N

6 1 1 0 PSW1,2,3,4_N Inactive Inactive Inactive

7 1 1 1 GL1,2,3,4_N GL5_N OC1,2,3,4_N OC5_N

Inactive OCX_N pins need to be tied to Vcc

DC 5.1V

(Min: 4.85V

Max:5.35V)

10mV 85mV 10mV

ferrite

bead

USB

downstream

connector

Over current

detection

ISP1122

USB Spec

4.75V(min)

Load Voltage drop

across PMOS

100mA

500mA

750mA

15mV

assume Rds=150mohms

75mV

1.125mV

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The following table explains the mode configuration in the physical configuration of the hub

implementation.

Mode Self-/Bus-

Powered Power Switch

control GoodLink Overcurrent

Protection Remarks

0Bus-powered Ganged Yes* Global Ganged and bus powered

1Self-powered Ganged Yes* Global Ganged and self powered

2Bus-powered Ganged Yes* Inactive No overcurrent

3Self-powered Inactive Yes Global No power switch control

4Bus-powered Individual* No Individual* Individual and bus powered

5Self-powered Individual No Individual Individual and self powered

6Bus-powered Individual* No Inactive No overcurrent

7Self-powered Inactive Yes Individual 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 55mΩ will be required.

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Mode 1 – Self-powered, Global overcurrent protection, and GoodLink

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Mode 2 – Bus-powered and GoodLink

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Mode 3 - Self powered, Global overcurrent protection, and GoodLink

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Mode 4 - Bus powered, Individual overcurrent protection

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Mode 5 – Self-powered and Individual overcurrent protection

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Mode 6 - Bus powered, No overcurrent protection

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Mode 7 - Self powered, Individual overcurrent protection, and GoodLink