LTC3550 Linear Technology, LTC3550 Datasheet - Page 20

• Manufacturer Part Number: LTC3550
• Description: Dual Input USB/AC Adapter Li-Ion Battery Charger
• Manufacturer: Linear Technology

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LTC3550
APPLICATIO S I FOR ATIO
Protecting the USB Pin and Wall Adapter Input from
Overvoltage Transients
Caution must be exercised when using ceramic capacitors
to bypass the USBIN 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 could
exceed the maximum voltage ratings and damage the
LTC3550. 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 and the wall adapter inputs, 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 5 shows how seri-
ous 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 ceramic X5R 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
20
Figure 5. Waveforms Resulting from Hot-Plugging a 5V Input
Supply When Using Ceramic Bypass Capacitors
4.7μF ONLY
4.7μF + 1Ω
2V/DIV
2V/DIV
U
U
20μs/DIV
W
3550 F04
U
4.7µF ceramic 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 USBIN and DCIN pins during USB and wall
adapter hot-plug events to ensure that overvoltage
transients have been adequately removed.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3550. These items are also illustrated graphically
in Figures 6 and 7. Check the following in your layout:
V
CC
+
–
C
R1
R2
Figure 6. DC-DC Converter Layout Diagram
F
C
IN
6
7
8
V
V
GND
FB
CC
LTC3550
17
GND
SW
10
9
C
OUT
BOLD LINES INDICATE
HIGH CURRENT PATHS
–
+
V
OUT
3550 F06
3550fa
L1