MAX8971
Industry's Smallest 1.55A
1-Cell Li+ DC-DC Charger
Timer Fault State
The timer fault state occurs when either the prequalifica-
tion or fast-charge timers expire. In this state, the charger
is off. The charger can exit this timer fault state by cycling
input power.
Overvoltage and Protection
The IC provides for a +22V absolute maximum positive
input voltage, and a -0.3V absolute maximum negative
input voltage. Excursions to the absolute maximum volt-
age levels should be on a transient basis only, but can
be withstood by the IC indefinitely.
Situations that typically require extended input voltage
ratings include, but are not limited to the following:
U Inductive kick
U Charge source failure
U Power surge
U Improperly wired wall adapter
U Improperly set universal wall adapter
U Wall adapter with the correct plug, but wrong voltage
U Home-built computer with USB wiring harness con-
nected backwards (negative voltage)
U USB connector failure
U Excessive ripple voltage on a switch-mode wall charger
U USB-powered hub that is powered by a wall charger
(typically through a barrel connector) that has any of
the aforementioned issues
U Unregulated charger
Automatic Input Current
Limit Protection
The IC includes an input-current-limiting feature. The
amplifiers required for sensing the currents and associ-
ated logic circuitry for making decisions and changing
the battery-charger current are fully integrated in the ICs.
This not only helps in reducing cost, but also improves
the speed of system response.
Maxim Integrated
The IC works by monitoring the current being drawn
from the AC adapter and comparing it to the pro-
grammed current limit. The current limit should be set
based on the current-handling capability of the AC
adapter. Generally, this limit is chosen to optimally
fulfill the system-power requirements while achieving
a satisfactory charging time for the batteries. If the
AC-adapter current exceeds its output capability, the
charger responds by cutting back on the charger
current, thereby keeping the current drawn from the AC
adapter within its capability. With such a battery charger,
the AC adapter doesn’t need to be oversized to meet
maximum system and battery-charging requirements
simultaneously, thereby reducing AC adapter cost.
The input current limit has two control inputs, one based
on voltage and one based on current. The voltage input
monitors the input voltage, and when it drops below the
desired input (4.5V), it generates a flag (AICL) to decre-
ment the fast-charge current.
When the voltage comparator initially trips at 4.5V, fast-
charge current decrements at a slow rate, allowing the
charger output to settle until the voltage on DC returns
above this voltage threshold. Once the DC voltage
resolves itself, the current delivery of the adapter is maxi-
mized. In the event of a limited input current source, an
example being a 500mA adaptor plugged into a 1A input
current limit setting, a second voltage comparator set at
4.4V triggers and throttles the fast-charge current to a
minimum of 75mA. Once the DC voltage corrects itself to
above 4.5V, the fast-charge level is checked every 16ms
to allow the system to recover if the available input power
increases.
The current-limit input monitors the current through the
input FET and generates a flag (DC_I) to decrement the
fast-charge current when the input limit is exceeded. The
fast-charge current is slowly decremented until the input-
limit condition is cleared. At this point, the fast-charge
current is maintained for 16ms and is then sampled
again.
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