Rainbow Electronics MAX1538 - Manual (Page 16)

MAX1538
Power-Source Selector for
Dual-Battery Systems
16 ______________________________________________________________________________________
Airline Mode and AC Adapter
The MAX1538 provides compatibility with airline
adapters. For airplane safety, the use of an airline
adapter requires that the battery charger or charge
path is disabled. The MAX1538 disables the charge
path when an airline adapter is detected. In airline
mode, ADPPWR and REVBLK drive P1 and P2 on, and
all other MOSFETs are off, regardless of the state of
RELRN, CHG, BATSEL, or the batteries. If the AC
threshold is above the airline threshold, select a resis-
tive voltage-divider (as shown in Figure 1) according to
the following equations:
where V
ACDET_Threshold
and V
AIRDET_Threshold
are typ-
ically 2.0V (see the Electrical Characteristics). An AC
adapter is detected when the adapter voltage is above
V
AC_Threshold
, and an airline adapter is detected when
the adapter voltage is between V AC_Threshold and
V AIR_Threshold .
To minimize error, use 1% accuracy or better divider
resistors, and ensure that the impedance of the divider
results in a current about 100 times the ACDET and
AIRDET input bias current. To optimize error due to 1µA
input bias current at ACDET/AIRDET and minimize cur-
rent consumption, typically choose R3 less than 20kΩ.
See the Adapter Removal Debouncing section for more
information regarding R1, R2, and R3. Short R2 to dis-
able airline-adapter mode.
Optionally, an external circuit can be implemented to
determine the presence of an AC/airline adapter. The
circuit in
Figure 5 provides fast detection of an airline
adapter, yet allows external circuitry to discriminate
between airline and AC adapters. If V
AC_Threshold <
V
AIR_Threshold
, this circuit must be used for airline-
adapter detection. Other permutations that directly
drive AIRDET instead do not work properly on the
MAX1538 because adapter removal is not detected
fast enough, causing the system load to crash.
OUT[2:0] = 011 if the MAX1538 is in airline-adapter
mode. If RELRN = 0 and CHG = 0, only OUT[1:0] are
necessary to indicate airline-adapter mode.
V V
R R R
R
V V
R R R
R R
AC Threshold ACDET Threshold
Air Threshold AIRDET Threshold
_ _
_ _
= Ч
+ +
= Ч
+ +
+
1 2 3
3
1 2 3
2 3
MAX1538
EXTERNAL AC/AIRLINE
DETECTION CIRCUIT
REVBLK
ADPIN
EXTLD
AIRDET
ACDET
ADAPTER
ADPPWR
R1 R2 + R3
P1
P2
OUT
ACDET
ADPIN
ADAPTER INSERTION
ACDET
ADPIN
ADAPTER REMOVAL
ACDET MUST WAIT
ACDET MAY OCCUR
BEFORE OR AFTER ADPIN
FOR AC ADAPTER
FOR AIRLINE ADAPTER
FOR AC ADAPTER
FOR AIRLINE ADAPTER
Figure 5. Using an External Adapter Detection Circuit