RT9741 datasheet(9/11 Pages) RICHTEK

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RT9741

DS9741-01 March 2015

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

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.

The following calculation determines VOUT(MIN) for multiple

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 (typical 100m

Ω for

RT9741A and 120m

Ω for RT9741C)

VPCB = PCB voltage drop

The USB specification defines the maximum resistance

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

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 90m

Ω. The

resistance of the switch is defined as follows :

RSWITCH = { 4.75V

− 4.4V − [ 0.5A x ( 4 x 30mΩ + 2 x

90m

Ω) ] − 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 250m

Ω for four

ports ganged together. The RT9741, with its maximum

120m

Ω on resistance can fit the demand of this

requirement.

Thermal Considerations

For continuous operation, do not exceed absolute

maximum junction temperature. The maximum power

dissipation depends on the thermal resistance of the IC

package, PCB layout, rate of surrounding airflow, and

difference between junction and ambient temperature. The

maximum power dissipation can be calculated by the

following formula :

PD(MAX) = (TJ(MAX)

− TA) / θJA

where TJ(MAX) is the maximum junction temperature, TAis

the ambient temperature, and

θJAis the junction to ambient

thermal resistance.

For recommended operating condition specifications, the

maximum junction temperature is 125

°C. The junction to

ambient thermal resistance,

θJA, is layout dependent. For

SOT-23-3 package, the thermal resistance,

θJA, is

243.3

°C/W on a standard JEDEC 51-7 four-layer thermal

test board. The maximum power dissipation at TA = 25

°C

can be calculated by the following formula :

PD(MAX) = (125

°C − 25°C) / (243.3°C/W) = 0.41W for

SOT-23-3 package

The maximum power dissipation depends on the operating

ambient temperature for fixed TJ(MAX) and thermal

resistance,

θJA. The derating curve in Figure 2 allows the

designer to see the effect of rising ambient temperature

on the maximum power dissipation.

Figure 2. Derating Curve of Maximum Power Dissipation

0.0

0.1

0.2

0.3

0.4

0.5

100

125

Ambient Temperature (°C)

Four-Layer PCB

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