US203_15 Datasheet, PDF (5/7 Pages) Unisonic Technologies – HIGH-SIDE POWER SWITCHES WITH FLAG

US203
Preliminary
CMOS IC
 APPLICATION INFORMATION(Cont.)
Power Dissipation
The UTC US203’s junction temperature varies depending the several factors such as the load, PCB layout,
ambient temperature and package type. The output pin of UTC US203 can deliver the current of up to
2.5A(US203AH/AL) , 2A(US203BH/BL), 1.5A(US203CH/CL) and 0.8A(US203DH/DL) over the full operating junction
temperature range. However, at higher ambient temperature the maximum output current must be derated to ensure
the junction temperature does not exceed 100°C. With all possible conditions, the junction temperature must be
within the range specified under operating conditions. Power dissipation is determined by the output current and the
R_DS(ON) of switch, the relationship between them is as seen is the following:
P_D = R_DS(ON) x I_OUT2
Although the devices are rated for 2.5A, 2A, 1.5A and 0.8A of output current, but the application may limit the
amount of output current based on the total power dissipation and the ambient temperature. The final operating
junction temperature for any set of conditions is calculated as follows:
P_{D (MAX)} = ( T_{J (MAX)} - T_A ) / θ_JA
Universal Serial Bus (USB) & Power Distribution
The USB’s goal is to be enabled device from different vendors to interoperate in an open architecture. The USB
is characterized incorporating ease of use for the end user, a wide range of workloads and applications, robustness,
synergy with the PC industry, and low-cost implementation. In addition, the benefits of the USB contain
self-identifying peripherals, dynamically attachable and reconfigurable peripherals, multiple connections (support for
concurrent operation of many devices), support physical devices up to 127, and compatibility with PC Plug-and-Play
architecture.
Each USB system has one USB host, and the USB connects USB devices with a USB host. USB devices can be
classified either as hubs, which provide additional attachment points to the USB, or as functions, which provide
capabilities to the system (for example, a digital joystick). Then the hub devices are classified as either bus-power
hubs or self-powered Hubs.
Self-powered hub power for the internal functions and downstream ports does not come from the USB, although
the USB interface may draw up to 100mA from its upstream connect, to allow the interface to function when the
remainder of the hub is powered down. The hub must be able to supply up to 500mA on all of its external
downstream ports. Over-current protection devices such as fuses and PTC resistors (also called poly fuse or
polyswitch) have slow trip times, high on-resistance, and lack the necessary circuitry for USB-required fault reporting.
A bus-powered hub draws all of the power to any internal functions and downstream ports from the USB connector
power pins. The hub may draw current as high as 500mA from the upstream device. External ports in a bus-powered
hub can supply up to 100mA per port, with a maximum of four external ports.
In order to protect the hubs to operating on the faults conditions, the faster trip time of the UTC US203 power
distribution can make it. For meeting voltage regulation and fault notification requirements, low on-resistance and
internal fault-reporting circuitry are required.
Furthermore, because the devices are power switches, they provide the designer of self-powered hubs flexibility
to turn off power to output ports. The devices have controlled rise and fall times to provide the needed inrush current
limiting required for the bus-powered hub power switch compared to a normal MOSFFT.
Supply Filter/Bypass Capacitor
To prevent input voltage droop occurs during hot-plug condition, a 1uF low-ESR ceramic capacitor located
between V_IN and GND is strongly desired. However, higher capacitor values will further reduce the voltage droop on
the input. Furthermore, without the bypass capacitor, an output short may cause sufficient ringing on the input (from
source lead inductance) to destroy the internal control circuitry. The input transient’s value must be not higher than
the absolute maximum supply voltage ( 6.5V) even for a short duration.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
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