MAX1538
Power-Source Selector for
Dual-Battery Systems
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Battery Presence and
Undervoltage Detection
The MAX1538 determines battery absence and under-
voltage and does not allow discharge from an under-
voltage battery. A battery is considered undervoltage
when V
BAT_
< 5 x V
MINV_
, and remains classified as
undervoltage until V
BAT_
falls below 2V and again rises
above 5 x V
MINV
. The undervoltage latch is also
cleared when the charge path is enabled. Set the bat-
tery undervoltage threshold using resistive voltage-
dividers R10, R11, R12, and R13, as shown in Figure 1.
The corresponding undervoltage threshold is:
To minimize error, use 1% or better accuracy divider
resistors, and ensure that the impedance of the divider
results in a current about 100 times the MINV_ input
bias current at the MINV_ threshold voltage. To opti-
mize error due to 50nA input bias current at MINV_ and
minimize current consumption, typically choose resis-
tors (R10 + R11) or (R12 + R13) smaller than 600kΩ.
Since batteries often exhibit large changes in their ter-
minal voltage when a load current is removed, further
discharge after the undervoltage latch has been set is
not allowed until the battery is removed or the charge
path to the battery is selected. Battery removal is
detected when V
BAT_
falls below 2V. For correct detec-
tion of battery removal, ensure that the leakage current
into BAT_ is lower than the leakage current out of BAT_
so that BAT_ falls below 2V when the battery is
removed. The contributors to leakage current into BAT_
are D1, D2, P6, and P7.
Battery Relearn Mode
The MAX1538 implements a battery relearn mode,
which allows for host-device manufacturers to imple-
ment a mode for coulomb-counting fuel gauges (such
as the MAX1781) to measure battery capacity without
user intervention. In battery relearn mode, the AC
adapter is switched off and battery discharge is select-
ed. In this implementation, the host system could
prompt users when their battery capacity becomes
inaccurate, use the host system as a load to discharge
the battery, and then recharge the battery fully.
Coulomb-counting fuel-gauge accuracy is increased
after a relearning cycle.
Battery relearn mode requires the addition of MOSFET
P1, which blocks current from the adapter to the sys-
tem. To enable relearn mode, drive RELRN high and
drive BATSEL low to relearn battery A or high to relearn
battery B. Relearn mode overrides the functionality of
the CHG pin. Battery relearn mode does not occur
when the selected battery’s undervoltage latch has
been set, or when the selector is in airline mode (see
the Airline Mode and AC Adapter section.) The RELRN
pin only applies when an AC adapter is present. If the
AC adapter is absent and RELRN is ignored, OUT[2:1]
= 10 when the MAX1538 is in battery relearn mode. If
CHG = 0, only OUT2 is needed to indicate that the
MAX1538 was properly placed in relearn mode.
If the selected battery trips the undervoltage latch when
in relearn mode, the AC adapter is switched in without
causing a crash to the system. OUT2 can indicate that
the relearn cycle is terminated due to battery undervolt-
age. Typically, after the host system performs a battery
relearn cycle, it either charges the discharged battery
or begins a relearn cycle on the other battery. To switch
to charge mode, drive RELRN low and CHG high.
Since RELRN overrides CHG, in many applications it is
best to permanently keep CHG high and reduce the IO
needed to control the selector.
When the AC adapter is available, it is used as the
power source for EXTLD unless the RELRN pin is high.
In this state, the charger can be enabled and a
battery charged.
