Datasheet
MAX8677C
1.5A Dual-Input USB/AC Adapter Charger
and Smart Power Selector
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Table 1. External Components List for Figures 2 and 3
COMPONENT
(FIGURES 2, 3)
FUNCTION PART
C
IN
Input filter capacitor 4.7µF ceramic capacitor
CL VL filter capacitor 0.1µF ceramic capacitor
C
SYS
SYS output bypass capacitors 10µF ceramic capacitor
C
BAT
Battery bypass capacitor 4.7µF ceramic capacitor
CT Charger timing capacitor 0.068µF low TC ceramic capacitor
R
PU
(x 4) Logic output pullup resistors 560kΩ
THM Negative TC thermistor
Phillips NTC thermistor, P/N 2322-640-63103, 10kΩ ±5% at +25°C
RT THM pullup resistor 10kΩ ±1%
R
PSET
Input current-limit programming resistor 1.5kΩ ±1% for 2A limit
R
ISET
Fast-charge current programming resistor
3kΩ ±1% for 1A charging
The MAX8677C features OVP. Part of this protection is
a 4.35V voltage limiter at SYS. If the DC or USB input
exceeds 4.35V, SYS still limits at 4.35V.
The MAX8677C has numerous other charging and
power-management features, which are detailed in the
following sections.
Smart Power Selector
The MAX8677C Smart Power Selector seamlessly dis-
tributes power between the external inputs, the battery,
and the system load (Figure 4). The basic functions
performed are:
• With both an external power supply (USB or
adapter) and battery connected:
When the system load requirements are less
than the input current limit, the battery is
charged with residual power from the input.
When the system load requirements exceed the
input current limit, the battery supplies supple-
mental current to the load.
• When the battery is connected and there is no
external power input, the system is powered from
the battery.
• 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
MAX8677C from overheating.
System Load Switch
An internal 40mΩ MOSFET connects SYS to BAT (Q3,
Figure 4) 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-BAT switch also holds up SYS when the sys-
tem load exceeds the input current limit. If that should
happen, the SYS-BAT switch turns on so that the bat-
tery supplies additional SYS load current. If the system
load continuously 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 usually occur only in short
peaks. During these peaks, battery energy is used, but
at all other times the battery charges.
Q1
AC
ADAPTER
CHARGE AND
SYS LOAD
SWITCH
CHARGE
CURRENT
LOAD
CURRENT
USB
USB
BAT
SYS
GND
DC
Q2
Q3
BATTE
R
SYSTEM
LOAD
MAX8677C
Figure 4. Smart Power Selector Block Diagram