RT9712

11

DS9712-04 November 2012

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Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Output Filter Capacitor

A low-ESR 150

μF aluminum electrolytic or tantalum

(Per USB 2.0, output ports must have a minimum 120

μF

of low-ESR bulk capacitance per hub). Standard bypass

methods should be used to minimize inductance and

resistance between the bypass capacitor and the

downstream connector to reduce EMI and decouple voltage

droop caused when downstream cables are hot-insertion

line and the 0.1

μF bypass capacitors at the power

connector pins are recommended for EMI and ESD

protection. The bypass capacitor itself should have a low

dissipation factor to allow decoupling at higher frequencies.

Voltage Drop

The USB specification states a minimum port-output

voltage in two locations on the bus, 4.75V out of a Self-

Powered Hub port and 4.40V out of a Bus-Powered Hub

port. As with the Self-Powered Hub, all resistive voltage

drops for the Bus-Powered Hub must be accounted for to

guarantee voltage regulation (see Figure 7-47 of Universal

Serial Specification Revision 2.0 ).

Where

(two contacts per connector)

(one 5V and one GND)

(90m

Ω typical for RT9712A/B/C/D)

The USB specification defines the maximum resistance

Ω

and the drop across the PCB and switch to be 100mV.

This basically leaves two variables in the equation: the

resistance of the switch and the resistance of the cable.

the maximum resistance of the cable is 90m

Ω.

The resistance of the switch can be defined as follows :

90m

= (200mV

If the voltage drop across the PCB is limited to 100mV,

the maximum resistance for the switch is 250m

Ω for four

ports ganged together. The RT9712A/B/C/D, with its

maximum 100m

Ω on-resistance over temperature can fit

the demand of this requirement.

Thermal Shutdown

Thermal protection limits power dissipation in the

RT9712A/B/C/D. When the operation junction temperature

exceeds 120

°C (typ.), the OTP circuit starts the thermal

shutdown function and turns the pass element off. The

pass element turns on again after the junction temperature

cools to 80

°C. The IC lowers its OTP trip level from 120°C

to 100

shown in Figure 2.

Figure 2. Short Circuit Thermal Folded Back Protection

when Output Short Circuit Occurs (Patent)

1V

Thermal

Shutdown

OTP Trip Point

100 C

°

100 C

°

80 C

°

IC Temperature

120 C

°