Rainbow Electronics MAX1538 - Manual (Page 14)

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 1 0 0 On Off On Off 1 1 0 Charge A
AC X X 1 0 1 On Off Off On 1 1 1 Charge B
AC N X X 1 0 Off On On Off 1 0 0 Relearn A
AC X N X 1 1 Off On Off On 1 0 1 Relearn B
AC Otherwise On Off Off Off 0 1 0 AC adapter
AIR X X X X X On Off Off Off 0 1 1 Airline
Absent N X X X 0
Absent N U X X X
Off On On Off 0 0 0 Discharge A
Absent X N X X 1
Absent U N X X X
Off On Off On 0 0 1 Discharge B
Absent U U X X X Off Off Off Off 0 0 0 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 X X X X indicates don’t care. The output does not depend on any inputs labeled X.
Table 1. MAX1538 State Table
MAX1538