Owner manual

Power-Source Selector for
Dual-Battery Systems
14 ______________________________________________________________________________________
Detailed Description
The MAX1538 performs power path selection between
an adapter input and two batteries, relieving the host
system from the burden of real-time response to power-
source changes. The integrated selector implements a
fixed break-before-make timer to ensure that power
sources are not connected together and yet the load is
not left unserviced. The MAX1538 monitors battery and
adapter state and presence to determine which source
to select and whether to charge the battery. Logic
inputs CHRG, BATSEL, and RELRN allow the host to
enable/disable charging, select which battery to use,
and impose battery discharge even with adapter pres-
ence. The MAX1538 automatically detects airline
adapters and prevents charging when an airline
adapter is detected. Open-drain logic outputs OUT2,
OUT1, and OUT0 indicate the state of the selector so
the host can properly respond.
The MAX1538 can be configured for use with a step-
down battery charger, as shown in Figure 1, or with a
step-up/step-down battery charger, as shown in Figure
2. The minimum MAX1538 system requires only six
MOSFETs. The MAX1538 provides relearn-mode sup-
port with the addition of P1. Relearn mode allows the
system to relearn the battery’s capacity without user
intervention.
Table 1 summarizes the possible states and configura-
tions of the MAX1538.
SOURCE STATE
LOGIC INPUTS
MOSFET STATE (See Figure 4)
Battery
Adapter
A
B
CHG
RELRN
BATSEL
System
(ADPPWR
and REVBLK)
Battery
(ADPBLK
and DISBAT)
BATT A
(CHGA and
DISA)
BATT B
(CHGB and
DISB)
OUT2 OUT1 OUT0
STATE
AC
X
X
10
0 On Off On Off
110
Charge A
AC
X
X
10
1 On Off Off On
111
Charge B
AC
N
X
X 1
0 Off On On Off
100
Relearn A
AC
X N X 1
1 Off On Off On
101
Relearn B
AC Otherwise On Off Off Off
010
AC adapter
AIR
X
X
XX X
On Off Off Off
011
Airline
Absent
N
X
XX
0
Absent
NUXX X
Off On On Off
000Discharge A
Absent
X N XX
1
Absent
UNXX X
Off On Off On
001Discharge B
Absent
UUXX X
Off Off Off Off
000
Idle
Legend
AC AC adapter is present. V
ACDET
and V
AIRDET
are both above 2V.
AIR Airline adapter is present. V
ACDET
is below 2V and V
AIRDET
is above 2V.
Absent
External adapter is absent. V
ACDET
and V
AIRDET
are both below 2V.
N
N indicates the battery is normal. The battery is normal when it has not tripped the undervoltage latch (5 x
V
MINV_
). See the Battery Presence and Undervoltage Detection section.
U
U indicates the battery has tripped the undervoltage comparator. An undervoltage battery is detected
when V
BAT_
goes below 5 x V
MINV_
. See the Battery Presence and Undervoltage Detection section.
Otherwise Otherwise covers all cases not explicitly shown elsewhere in the table.
X
X
XX X
X indicates don’t care. The output does not depend on any inputs labeled X.
Table 1. MAX1538 State Table
MAX1538