MAX1538 Datasheet, PDF (19/22 Pages) Maxim Integrated Products – Power-Source Selector for Dual-Battery Systems

Power-Source Selector for
Dual-Battery Systems
5 x V_MINV OR AC/AIR THRESHOLD
OTHER POWER SOURCE
EXTLD
MAX1538
ADPBLK/REVBLK
REVBLK/ADPBLK
t_MINV OR t_ADP
t_OFF
t_TRANS
V_{SYS_MIN}
t_ON
I_SOURCE
PARASITIC
INDUCTANCE
(L_SOURCE)
C_SOURCE
TO BATTERY
OR ADAPTER
Figure 8. System Holdup Capacitor Timing
crash, and C_SYS is the total system holdup capaci-
tance, which does not need to be near the MAX1538.
The timing related to the system holdup capacitance is
shown in Figure 8.
Charger output capacitance contributes to C_SYS for the
step-down charger topology (Figure 1), but not for the
step-up/step-down charger topology (Figure 2).
Leakage Current into BAT_
Leakage current into BATA or BATB can interfere with
proper battery-removal detection. D1 and D2 must be
low leakage to ensure that battery removal is properly
detected. Choose MOSFETs P6 and P7 with low off-
leakage current. Board leakage current can also be a
problem. For example, neighbor pins BATA and
BATSUP should have greater than 50MΩ impedance
between each other. Proper battery-removal detection
requires that:
I_Board + I_{DS_ OFF(P6)} + I_{DS_ OFF(P7)} + I_{D1_leakage} +
I_{D2_leakage} < I_{BAT _ Sink@2V}
where I_Board is board leakage current, I_{DS_OFF} is the
off-leakage current of MOSFETs P6 and P7, I_{D_Leakage}
is the reverse leakage current of the diodes, and
I_{BAT_Sink@2V} is the BAT_ leakage current at 2V (0.4µA;
see the Typical Operating Characteristics).
Figure 9. Inductive Kick Upon Source Disconnect
Inductive “Kick”
When the adapter or a battery is delivering a significant
current to the system and that path is disabled (typical-
ly to enable another path), a voltage spike is generated
at the source. This is due to a parasitic inductance
shown in Figure 9. When the adapter is disconnected, a
positive voltage spike occurs at ADPIN. When a dis-
charging battery is disconnected, a positive voltage
spike occurs at BAT_. Connect a capacitor from BAT_
or ADPIN to GND to limit this inductive kick. Choose the
source capacitance according to the following equation:
2
C_SOURCE
>
L_SOURCE
30² −
× I_{SYS_MAX}
2
V_SOURCE
where V_SOURCE is the maximum DC voltage of the
source in question, I_{SYS_MAX} is the maximum system
load, and L_SOURCE (parasitic inductance) and
C_SOURCE are shown in Figure 9.
During battery charge, the voltage spike during battery
disconnect is negative. To ensure that this negative
voltage spike does not go below 0V, choose C_{BAT_}
according to the following equation:
2
C_{BAT _}
>
L_{BAT_} × I_{CHG _MAX}
2
V_{BAT_ _MIN}
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