MAX1538 Datasheet, PDF (14/22 Pages) Maxim Integrated Products – Power-Source Selector for Dual-Battery Systems

Power-Source Selector for
Dual-Battery Systems
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.
Table 1. MAX1538 State Table
SOURCE STATE
Battery
Adapter
AB
LOGIC INPUTS
MOSFET STATE (See Figure 4)
System
(ADPPWR
and REVBLK)
Battery
(ADPBLK
and DISBAT)
BATT A
BATT B
(CHGA and (CHGB and
DISA)
DISB)
STATE
AC
X X 10 0
On
AC
X X 10 1
On
AC
N X X1 0
Off
AC
X N X1 1
Off
AC
Otherwise
On
Off
On
Off
1 1 0 Charge A
Off
Off
On
1 1 1 Charge B
On
On
Off
1 0 0 Relearn A
On
Off
On
1 0 1 Relearn B
Off
Off
Off
0 1 0 AC adapter
AIR
Absent
Absent
Absent
Absent
Absent
Legend
AC
AIR
Absent
X X XX X
On
N X XX 0
Off
N U XX X
X N XX 1
Off
U N XX X
U U XX X
Off
Off
Off
Off
0 1 1 Airline
On
On
Off
0 0 0 Discharge A
On
Off
On
0 0 1 Discharge B
Off
Off
Off
0 0 0 Idle
AC adapter is present. V_ACDET and V_AIRDET are both above 2V.
Airline adapter is present. V_ACDET is below 2V and V_AIRDET is above 2V.
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 indicates the battery has tripped the undervoltage comparator. An undervoltage battery is detected
U
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 X X X X indicates don’t care. The output does not depend on any inputs labeled X.
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