Datasheet

MAX8677C
1.5A Dual-Input USB/AC Adapter Charger
and Smart Power Selector
18 ______________________________________________________________________________________
When the MAX8677C is used in conjunction with a µP,
connect a pullup resistor between PREQ and the logic I/O
voltage to indicate charge status to the µP. Alternatively,
PREQ can sink up to 20mA for an LED indicator.
Fault Output (
FFLLTT
) and Charge Timer
FLT is an open-drain, active-low output that goes low
during a battery fault. The fault state occurs when either
the prequal or fast-charge timer expires. The prequal
and fast-charge fault timers are set by C
CT
:
While in fast-charge mode, a large system load or
device self-heating can cause the MAX8677C to
reduce charge current. Under these circumstances, the
fast-charge timer adjusts to ensure that adequate
charge time is still allowed. Consequently, the fast-
charge timer is slowed by 2x if charge current is
reduced below 50% of the programmed fast-charge
level. If charge current is reduced to below 20% of the
programmed level, the fast-charge timer is paused. The
fast-charge timer is not adjusted if the charger is in volt-
age mode where charge current reduces due to cur-
rent tapering under normal charging.
To exit a fault state, toggle CEN or remove and recon-
nect the input source(s). Note also that thermistor out-
of-range or on-chip thermal-limit conditions are not
considered faults.
When the MAX8677C is used in conjunction with a µP,
connect a pullup resistor between FLT and the logic I/O
voltage to indicate fault status to the µP. Alternatively,
FLT can sink up to 20mA for an LED indicator.
Thermistor Input (THM)
The THM input connects to an external negative tem-
perature coefficient (NTC) thermistor to monitor battery
or system temperature. Charging is suspended when
the thermistor temperature is out of range. The charge
timers are suspended and hold their state but no fault
is indicated. When the thermistor comes back into
range, charging resumes and the charge timer contin-
ues from where it left off. Connecting THM to GND dis-
ables the thermistor monitoring function. Table 3 lists
fault temperatures for different thermistors.
Since the thermistor monitoring circuit employs an exter-
nal bias resistor from THM to VL (R
TB
, Figure 7), the
thermistor is not limited only to 10kΩ (at +25°C).
Any resistance thermistor can be used as long as the
value of R
TB
is equivalent to the thermistor’s +25°C
resistance. For example, with a 10kΩ at +25°C thermis-
tor, use 10kΩ at R
TB
, and with a 100kΩ at +25°C ther-
mistor, use 100kΩ.
t
C
F
FSTCHG
CT
=×300
0 068
min
.
μ
t
C
F
PREQUAL
CT
=×30
0 068
min
.
μ
R
T
THM
GND
THM OK
DISABLE
CHARGER
VL
VL
0.74 VL
0.28 VL
0.03 VL
ALL COMPARATORS 60mV HYSTERESIS
COLD
HOT
R
TB
THERMISTOR CIRCUITRY
BYPASS THM
R
TP
R
TS
ALTERNATE
THERMISTOR
CONNECTION
MAX8677C
CEN
R
T
Figure 7. Thermistor Monitor Circuitry