MAX8934A–MAX8934E
Dual-Input Linear Chargers, Smart Power Selector
with Advanced Battery Temperature Monitoring
Smart Power Selector
The MAX8934_ Smart Power Selector seamlessly distrib-
utes power among the external inputs, the battery, and
the system load (see the Typical Operating Circuit ). The
basic functions performed are:
U With both an external power supply (USB or adapter)
and battery connected:
U When the system load requirements are less than
the input current limit, the battery is charged with
residual power from the input.
U When the system load requirements exceed the
input current limit, the battery supplies supplemen-
tal current to the load.
U When the battery is connected and there is no external
power input, the system is powered from the battery.
U When an external power input is connected and there
is no battery, the system is powered from the external
power input.
A thermal-limiting circuit reduces the battery charge
rate and external power-source current to prevent the
MAX8934_ from overheating.
System Load Switch
An internal 40m I MOSFET connects SYS to BATT (Q3
in the Typical Operating Circuit ) when no voltage source
is available at DC or USB. When an external source is
detected at DC or USB, this switch is opened and SYS
is powered from the valid input source through the input
limiter.
The SYS-BATT switch also holds up SYS when the system
load exceeds the input current limit. If that should happen,
the SYS-BATT switch turns on so that the battery supplies
additional SYS load current. If the system load continu-
ously exceeds the input current limit, the battery does not
charge, even though external power is connected. This is
not expected to occur in most cases, since high loads usu-
ally occur only in short peaks. During these peaks, battery
energy is used, but at all other times the battery charges.
Input Limiter
The input voltage limiter is essentially an LDO regula-
tor. While in dropout, the regulator dissipates a small
I 2 R loss through the 0.2 I MOSFET (Q1 in the Typical
Operating Circuit ) between DC and SYS. With an AC
adapter or USB source connected, the input limiter
distributes power from the external power source to the
system load and battery charger. In addition to the input
limiter’s primary function of passing power to the system
20
and charger loads at SYS, it performs several additional
functions to optimize use of available power.
Input Voltage Limiting
If an input voltage is above the overvoltage threshold
(6.9V typ), the MAX8934_ enters overvoltage lockout
(OVLO). OVLO protects the MAX8934_ and downstream
circuitry from high-voltage stress up to 14V at DC and
8V at USB. In OVLO, V L remains on, the input switch that
sees overvoltage (Q1, Q2, Typical Operating Circuit )
opens, and the appropriate power-monitor output ( DOK ,
UOK ) is high impedance, and CHG is high impedance.
If both DC and USB see overvoltage, both input switches
(Q1 and Q2, Typical Operating Circuit ) open and the
charger turns off. The BATT-to-SYS switch (Q3, Typical
Operating Circuit ) closes, allowing the battery to power
SYS. An input is also invalid if it is less than BATT, or less
than the DC undervoltage threshold of 3.5V (falling). With
an invalid input voltage, SYS connects to BATT through a
40m I switch (Q3, Typical Operating Circuit ).
Input Overcurrent Protection
The current at DC and USB is limited to prevent input
overload. This current limit can be selected to match the
capabilities of the source, whether it is a 100mA or 500mA
USB source, or an AC adapter. When the load exceeds
the input current limit, SYS drops to 75mV below BATT
and the battery supplies supplemental load current.
Thermal Limiting
The MAX8934_ reduces input limiter current by 5%/ N C
when its die temperature exceeds +100 N C. The system
load (SYS) has priority over the charger current, so low-
ering the charge current first reduces the input current. If
the junction temperature still reaches +120 N C in spite of
charge current reduction, no input (DC or USB) current
is drawn, the battery supplies the entire system load,
and SYS is regulated at 75mV below BATT. Note that
this on-chip thermal-limiting circuitry is not related to and
operates independently from the thermistor input.
Adaptive Battery Charging
While the system is powered from DC, the charger
draws power from SYS to charge the battery. If the
charger load plus system load exceeds the input current
limit, an adaptive charger control loop reduces charge
current to prevent the SYS voltage from collapsing.
Maintaining a higher SYS voltage improves efficiency
and reduces power dissipation in the input limiter. The
total current through the switch (Q1 or Q2 in the Typical
Operating Circuit ) is the sum of the load current at SYS
and the battery charging current. The MAX8934A limiter
clamps at 5.3V (4.35V for the MAX8934B/MAX8934C/
Maxim Integrated