Rainbow Electronics MAX1538 - Manual (Page 19)

MAX1538
Power-Source Selector for
Dual-Battery Systems
______________________________________________________________________________________ 19
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:
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).
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:
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:
C
L I
V
BAT
BAT CHG MAX
BAT MIN
_
_ _
_ _
>
×
2
2
C
L I
V
SOURCE
SOURCE SYS MAX
SOURCE
_
>
×
−
2
2
2
30
I I I I
I I
Board DS OFF P DS OFF P D leakage
D leakage BAT Sink V
+ + + +
<
_ ( ) _ ( ) _
_ _ @
6 7 1
2 2
ADPBLK/REVBLK
REVBLK/ADPBLK
t OFF
t TRANS
t ON
V SYS_MIN
OTHER POWER SOURCE
EXTLD
5 x V MINV OR AC/AIR THRESHOLD
t MINV OR t ADP
Figure 8. System Holdup Capacitor Timing
TO BATTERY
OR ADAPTER
PARASITIC
INDUCTANCE
(L SOURCE )
I
SOURCE
C SOURCE
MAX1538
Figure 9. Inductive Kick Upon Source Disconnect