LTC3550-1_15 Datasheet, PDF (20/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter

LTC3550-1
APPLICATIO S I FOR ATIO
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.
4.7μF ONLY
2V/DIV
4.7μF + 1Ω
2V/DIV
20μs/DIV
3550-1 F04
Figure 4. Waveforms Resulting from
Hot-Plugging a 5V Input Supply When
Using Ceramic Bypass Capacitors
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.
20
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3550-1. These items are also illustrated graph-
ically in Figures 5 and 6. Check the following in your
layout:
1. The power traces, consisting of the GND trace, the SW
trace and the V_CC trace should be kept short, direct
and wide.
2. Does the V_OUT pin connect directly to the output?
3. Does the (+) plate of C_IN connect to V_CC as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
4. Keep the (–) plates of C_IN and C_OUT as close as
possible.
5. Solder the exposed pad on the backside of the package
to PC board ground for optimum thermal performance.
The thermal resistance of the package can be further
enhanced by increasing the area of the copper used for
PC board ground.
Design Example
As a design example, assume the LTC3550-1 is used
in a single lithium-ion battery-powered cellular phone
application. The battery is charged by either plugging
a wall adapter into the phone or putting the phone in a
USB cradle. The optimum charge current for this parti-
cular lithium-ion battery is determined to be 800mA.
Starting with the charger, choosing R_IDC to be 1.24k
programs the charger for 806mA. Choosing R_IUSB to
be 2.1k programs the charger for 475mA when charging
from the USB cradle, ensuring that the charger never
exceeds the 500mA maximum current supplied by the
USB port. A good rule of thumb for I_TERMINATE is one-
tenth the full charge current, so R_ITERM is picked to be
1.24k (I_TERMINATE = 80mA).
Moving on to the step-down converter, V_CC will be pow-
ered from the battery which can range from a maximum
of 4.2V down to about 2.7V. The load current requirement
is a maximum of 600mA but most of the time it will be in
standby mode, requiring only 2mA. Efficiency at both low
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