Datasheet

MAX8677C
1.5A Dual-Input USB/AC Adapter Charger
and Smart Power Selector
14 ______________________________________________________________________________________
Input Limiter
The input voltage limiter is essentially an LDO regulator.
While in dropout, the regulator dissipates a small I
2
R loss
through the 0.2Ω MOSFET (Q1, Figure 4) between DC
and SYS. With an AC adapter or USB source connect-
ed, 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 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 MAX8677C
enters overvoltage lockout (OVLO). OVLO protects
the MAX8677C and downstream circuitry from high-
voltage stress up to 14V at DC and 8V at USB. In
OVLO, VL remains on, the input switch that sees
overvoltage (Q1, Q3, Figure 4) 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, Figure 4) open and the
charger turns off. The BAT-SYS switch (Q3, Figure
4) closes, allowing the battery to power SYS.
An input is also invalid if it is less than BAT, or less
than the DC undervoltage threshold of 3.5V (falling).
With an invalid input voltage, SYS connects to BAT
through a 40mΩ switch (Q3, Figure 4).
• Input Overcurrent Protection. The current at DC
and USB is limited to prevent input overload. This cur-
rent 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 68mV below BAT
and the battery supplies supplemental load current.
• Thermal Limiting. The MAX8677C reduces input lim-
iter current by 5%/°C when its die temperature
exceeds +100°C. The system load (SYS) has priority
over the charger current, so input current is first
reduced by lowering charge current. If the junction
temperature still reaches +120°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 68mV below BAT. 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 cur-
rent to prevent the SYS voltage from collapsing.
Maintaining a higher SYS voltage improves efficien-
cy and reduces power dissipation in the input limiter.
The total current through the switch (Q1 or Q2 in
Figure 4) is the sum of the load current at SYS and
the battery charging current. The limiter clamps at
4.35V, so input voltages greater than 4.35V can
increase power dissipation in the limiter. The limiter
power loss is (V
DC
- 4.35) x I, but not less than I
2
x
0.2Ω. Also note that the MAX8677C turns off any
input that exceeds 6.9V (nominal).
DC and USB Connections and
Current-Limit Options
Input Current Limit
The input and charger current limits are set as shown in
Table 2. It is often preferable to change the input cur-
rent limit as the input power source is changed. The
MAX8677C facilitates this by allowing different input
current limits for DC and USB as shown in Table 2.
POWER SOURCE DOK UOK PEN1
PEN2 USUS
DC INPUT
CURRENT
LIMIT
USB INPUT
CURRENT
LIMIT
MAXIMUM
CHARGE
CURRENT*
AC adapter at DC input L X H X X
3000/R
PSET
3000/R
ISET
L X L L L 100mA 100mA
L X L H L 500mA 500mAUSB power at DC input
LX L X H
USB suspend
USB input off;
DC input has
priority
0
H L X L L 100mA
H L X H L 500mA
3000/R
ISET
USB power at USB input;
DC unconnected
HL X X H
USB suspend
0
DC and USB unconnected H H X X X
No DC input
No USB input
0
Table 2. Input Limiter Control Logic
* Charge current cannot exceed the input current limit. Charge may be less than the maximum charge current if the total SYS load
exceeds the input current limit.