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Typical Application Waveforms (Continued)
Typical USB3.0 conditions.
─ V IN , 2V/div. The voltage applied on the load circuit
─ i IN , 2A/div. The input current
Time: 5 μ s/div
Figure 19. Startup Waveform without FR014H5JZ, DC Power Source=5V, C 1 =100μF, C 2 =10uF,
R 1 =R 2 =10k ? , R 3 =27 ?
Application Information
Figure 17 shows the voltage and current waveforms
when a virtual USB3.0 device is connected to a 5V
source. A USB application allows a maximum source
output capacitance of C 1 = 120μF and a maximum
device-side input capacitance of C 2 = 10μF plus a
maximum load (minimum resistance) of R 3 = 27 ? . C 1 =
100μF, C 2 = 10μF and R 3 = 27 ? were used for testing.
When the DC power source is connected to the circuit
(refer to Figure 13) , the built-in startup diode initially
conducts the current such that the USB device powers
up. Due to the initial diode voltage drop, the FR014H5JZ
effectively reduces the peak inrush current of a hot plug
event. Under these test conditions, the input inrush
current reaches about 6A peak. While the current flows,
the input voltage increases. The speed of this input
voltage increase depends on the time constant formed
by the load resistance R 3 and load capacitance C 2 . The
larger the time constant, the slower the input voltage
increase. As the input voltage approaches a level equal
to the protector’s turn-on voltage, V ON , the protector
turns on and operates in Low-Resistance Mode as
defined by V IN and operating current I IN .
In the event of a negative transient, or when the DC
power source is reversely connected to the circuit, the
device blocks the flow of current and holds off the
voltage, thereby protecting the USB device. Figure 18
shows the voltage and current waveforms when a virtual
? 2012 Fairchild Semiconductor Corporation
FR014H5JZ ? Rev. C2
10
USB3.0 device is reversely biased; the output voltage is
near 0 and response time is less than 50ns.
Figure 19 shows the voltage and current waveforms
when no reverse bias protection is implemented. In
Figure 17, while the reverse bias protector is present,
the input voltage, V IN , and the output voltage, V O , are
separated and look different. When this reverse bias
protector is removed, V IN and V O merge, as shown in
Figure 19 as V IN . This V IN is also the voltage applied to
the load circuit. It can be seen that, with reverse bias
protection, the voltage applied to the load and the
current flowing into the load look very much the same as
without reverse bias protection.
Benefits of Reverse Bias Protection
The most important benefit is to prevent accidently
reverse-biased voltage from damaging the USB load.
Another benefit is that the peak startup inrush current
can be reduced. How fast the input voltage rises, the
input/output capacitance, the input voltage, and how
heavy the load is determine how much the inrush
current can be reduced. In a 5V USB application, for
example, the inrush current can be 5% - 20% less with
different input voltage rising rate and other factors. This
can offer a system designer the option of increasing C 2
while keeping “effective” USB device capacitance down.
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