RT9712 Просмотр технического описания (PDF) - Richtek Technology

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производитель

RT9712

90mΩ, 1A/1.5A High-Side Dual Power Switches with Flag

Richtek Technology 

RT9712

Output Filter Capacitor

A low-ESR 150μF aluminum electrolytic or tantalum

between VOUT and GND is strongly recommended to meet

the 330mV maximum droop requirement in the hub VBUS

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

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

transients. Ferrite beads in series with VBUS, the ground

line and the 0.1μF 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 ).

The following calculation determines VOUT (MIN) for multi-

ple ports (NPORTS) ganged together through one switch (if

using one switch per port, NPORTS is equal to 1) :

VOUT (MIN) = 4.75V − [ II x ( 4 x RCONN + 2 x RCABLE ) ] −

(0.1A x NPORTS x RSWITCH ) − VPCB

Where

RCONN = Resistance of connector contacts

(two contacts per connector)

RCABLE = Resistance of upstream cable wires

(one 5V and one GND)

RSWITCH = Resistance of power switch

(90mΩ typical for RT9712A/B/C/D)

VPCB = PCB voltage drop

The USB specification defines the maximum resistance

per contact (RCONN) of the USB connector to be 30mΩ

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.

If the hub consumes the maximum current (II) of 500mA,

the maximum resistance of the cable is 90mΩ.

The resistance of the switch can be defined as follows :

RSWITCH = { 4.75V − 4.4V − [ 0.5A x ( 4 x 30mΩ + 2 x

90mΩ) ] − VPCB } ÷ ( 0.1A x NPORTS )

= (200mV − VPCB ) ÷ ( 0.1A x NPORTS )

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

the maximum resistance for the switch is 250mΩ for four

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

maximum 100mΩ 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°C when output short circuit occurs (VOUT < 1V) as

shown in Figure 2.

VOUT Short to GND

1V

VOUT

IOUT

Thermal

Shutdown

OTP Trip

120 °C

Point

100 °C

100 °C

IC Temperature

80 °C

Figure 2. Short Circuit Thermal Folded Back Protection

when Output Short Circuit Occurs (Patent)

Copyright ©2012 Richtek Technology Corporation. All rights reserved.

DS9712-04 November 2012

is a registered trademark of Richtek Technology Corporation.

www.richtek.com

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