LTC3455_15 Datasheet, PDF (18/28 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455/LTC3455-1
APPLICATIONS INFORMATION
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/LTC3455-1. Refer to Linear Technology Applica-
tion 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
4.7μF ONLY
2V/DIV
4.7μF + 1Ω
2V/DIV
20μs/DIV
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Figure 6. Waveforms Resulting from Hot-Plugging a
5V Input Supply
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; more
than enough to damage the LTC3455/LTC3455-1. 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
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 USB and V_MAX pins during USB and wall
adapter hot-plug events to ensure that overvoltage
transients have been adequately removed.
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