MAX8677C
1.5A Dual-Input USB/AC Adapter Charger
and Smart Power Selector
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For a typical 10kΩ (at +25°C) thermistor and a 10kΩ
R
TB
resistor, the charger enters a temperature suspend
state when the thermistor resistance falls below 3.97kΩ
(too hot) or rises above 28.7kΩ (too cold). This corre-
sponds to a 0°C to +50°C range when using a 10kΩ
NTC thermistor with a beta of 3500. The general rela-
tion of thermistor resistance to temperature is defined
by the following equation:
where:
R
T
= The resistance in Ω of the thermistor at tempe-
rature T in Celsius
R
25
= The resistance in Ω of the thermistor at +25°C
β = The material constant of the thermistor, which
typically ranges from 3000K to 5000K
T = The temperature of the thermistor in °C
Table 3 shows the MAX8677C THM temperature limits
for different thermistor material constants.
Some designs might prefer other thermistor tempera-
ture limits. Threshold adjustment can be accommodat-
ed by changing R
TB
, connecting a resistor in series
and/or in parallel with the thermistor, or using a thermis-
tor with different β. For example, a +45°C hot threshold
and 0°C cold threshold can be realized by using a ther-
mistor with a β of 4250 and connecting 120kΩ in paral-
lel. Since the thermistor resistance near 0°C is much
higher than it is near +50°C, a large parallel resistance
lowers the cold threshold, while only slightly lowering
the hot threshold. Conversely, a small series resistance
raises the cold threshold, while only slightly raising the
hot threshold. Raising R
TB
lowers both the hot and cold
thresholds, while lowering R
TB
raises both thresholds.
Power Dissipation
It is important to ensure that the heat generated by the
MAX8677C is dissipated into the PCB. The package’s
exposed paddle must be soldered to the PCB with mul-
tiple vias tightly packed under the exposed paddle to
ensure optimum thermal contact to the ground plane.
This minimizes heat rise in the IC and ensures that
maximum charging current is maintained over the
widest range of external conditions. Table 4 shows the
thermal characteristics of the MAX8677C package.
PCB Layout and Routing
Good design minimizes ground bounce and voltage
gradients in the ground plane, which can result in insta-
bility or regulation errors. GND should connect to the
power-ground plane at only one point to minimize the
effects of power-ground currents. Battery ground should
connect directly to the power-ground plane. Connect
GND to the exposed paddle directly under the IC. Use
multiple tightly spaced vias to the ground plane under
the exposed paddle to help cool the IC. Position input
capacitors from DC, SYS, BAT, and USB to the power-
ground plane as close as possible to the IC. Keep high-
current traces, such as those to DC, SYS, and BAT, as
short and wide as possible. Refer to the MAX8677C
evaluation kit for a suitable PCB layout example.
