LTC4075EDD(RevA) 查看數據表（PDF） - Linear Technology
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LTC4075EDD Datasheet PDF : 16 Pages
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LTC4075/LTC4075X
APPLICATIO S I FOR ATIO
makes them especially susceptible to this problem. To
bypass the USB pin and the wall adapter input, add a 1Ω
resistor in series with a ceramic capacitor to lower the
effective Q of the network and greatly reduce the ringing.
A tantalum, OS-CON, or electrolytic capacitor can be used
in place of the ceramic and resistor, as their higher ESR
reduces the Q, thus reducing the voltage ringing.
The oscilloscope photograph in Figure 4 shows how
serious the overvoltage transient can be for the USB
and wall adapter inputs. For both traces, a 5V supply is
hot-plugged using a three foot long cable. For the top
trace, only a 4.7µF capacitor (without the recommended
1Ω series resistor) is used to locally bypass the input.
This trace shows excessive ringing when the 5V cable
is inserted, with the overvoltage spike reaching 10V. For
the bottom trace, a 1Ω resistor is added in series with the
4.7µF capacitor to locally bypass the 5V input. This trace
shows the clean response resulting from the addition of
the 1Ω resistor.
Even with the additional 1Ω resistor, bad design techniques
and poor board layout can often make the overvoltage
4.7µF ONLY
2V/DIV
4.7µF + 1Ω
2V/DIV
20µs/DIV
3455 F04
Figure 4. Waveforms Resulting from Hot-Plugging a
5V Input Supply
problem even worse. System designers often add extra
inductance in series with input lines in an attempt to mini-
mize the noise fed back to those inputs by the application.
In reality, adding these extra inductances only makes the
overvoltage transients worse. Since cable inductance is
one of the fundamental causes of the excessive ringing,
adding a series ferrite bead or inductor increases the ef-
fective cable inductance, making the problem even worse.
For this reason, do not add additional inductance (ferrite
beads or inductors) in series with the USB or wall adapter
inputs. For the most robust solution, 6V transorbs or zener
diodes may also be added to further protect the USB and
wall adapter inputs. Two possible protection devices are
the SM2T from STMicroelectronics and the EDZ series
devices from ROHM.
Always use an oscilloscope to check the voltage wave-
forms at the USBIN and DCIN pins during USB and wall
adapter hot-plug events to ensure that overvoltage
transients have been adequately removed.
Reverse Polarity Input Voltage Protection
In some applications, protection from reverse polarity
voltage on the input supply pins is desired. If the sup-
ply voltage is high enough, a series blocking diode can
be used. In other cases where the voltage drop must be
kept low, a P-channel MOSFET can be used (as shown in
Figure 5).
DRAIN-BULK
DIODE OF FET
WALL
ADAPTER
LTC4075
DCIN
4075 F05
Figure 5. Low Loss Input Reverse Polarity Protection
4075Xfa
14
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