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RT9712 Datasheet(PDF) 11 Page - Richtek Technology Corporation |
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RT9712 Datasheet(HTML) 11 Page - Richtek Technology Corporation |
11 / 14 page RT9712 11 DS9712-04 November 2012 www.richtek.com © 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 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 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 transients. Ferrite beads in series with VBUS, the ground 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 ). 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 (90m Ω 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 30m 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. If the hub consumes the maximum current (II) of 500mA, the maximum resistance of the cable is 90m Ω. The resistance of the switch can be 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 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 °C when output short circuit occurs (VOUT < 1V) as shown in Figure 2. Figure 2. Short Circuit Thermal Folded Back Protection when Output Short Circuit Occurs (Patent) VOUT Short to GND 1V VOUT IOUT Thermal Shutdown OTP Trip Point 100 C ° 100 C ° 80 C ° IC Temperature 120 C ° |
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