ltc3455euf-trpbf Linear Technology Corporation, ltc3455euf-trpbf Datasheet - Page 18

• Manufacturer Part Number: ltc3455euf-trpbf
• Description: Dual Dc/dc Converter With Usb Power Manager And Li-ion Battery Charger
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3455
USB Pin and Wall Adapter Capacitor Selection
The USB and wall adapter inputs should be bypassed with
a 4.7µF to 10µF capacitor. For some applications, the wall
input can be bypassed locally with a lower value (down to
1µF), but only if other bulk capacitance is present. The USB
pin should always have at least 4.7µF. Ceramic capacitors
(only type X5R or X7R) are typically the best choice due to
their small size and good surge current ratings, but care
must be taken when they are used. When ceramic capaci-
tors are used for input bypassing, a 1Ω series resistor
must be added to prevent overvoltage ringing that often
occurs when these inputs are hot-plugged. 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.
Protecting the USB Pin and Wall Adapter Input from
Overvoltage Transients
Caution must be exercised when using ceramic capacitors
to bypass the USB pin or the wall adapter inputs. High
voltage transients can be generated when the USB or wall
adapter is hot plugged. When power is supplied via the
USB bus or wall adapter, the cable inductance along with
the self resonant and high Q characteristics of ceramic
capacitors can cause substantial ringing which can easily
exceed the maximum voltage pin ratings and damage the
LTC3455. Refer to Linear Technology Application Note 88,
entitled “Ceramic Input Capacitors Can Cause Overvoltage
Transients” for a detailed discussion of this problem. The
long cable lengths of most wall adapters and USB cables
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 6 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
18
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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; more
than enough to damage the LTC3455. 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 tech-
niques and poor board layout can often make the overvolt-
age problem even worse. System designers often add
extra inductance in series with input lines in an attempt to
minimize the noise fed back to those inputs by the appli-
cation. In reality, adding these extra inductances only
makes the overvoltage transients worse. Since cable in-
ductance is one of the fundamental causes of the exces-
sive ringing, adding a series ferrite bead or inductor
increases the effective cable inductance, making the prob-
lem 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 USB and V
adapter hot-plug events to ensure that overvoltage tran-
sients have been adequately removed.
4.7µF ONLY
4.7µF + 1Ω
Figure 6. Waveforms Resulting from Hot-Plugging a
5V Input Supply
2V/DIV
2V/DIV
MAX
20µs/DIV
pins during USB and wall
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