Rainbow-electronics MAX1538 Manual de usuario descargar pdf (Pagina 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

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

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:

BAT

BAT CHG MAX

BAT MIN

SOURCE

SOURCE SYS MAX

SOURCE

−

II I I

Board DS OFF P DS OFF P D leakage

D leakage BAT Sink V

++++

_() _() _

__@

671

ADPBLK/REVBLK

REVBLK/ADPBLK

OFF

TRANS

SYS_MIN

OTHER POWER SOURCE

EXTLD

5 x V

MINV

OR AC/AIR THRESHOLD

MINV

OR t

ADP

Figure 8. System Holdup Capacitor Timing

TO BATTERY

OR ADAPTER

PARASITIC

INDUCTANCE

SOURCE

)

SOURCE

MAX1538

Figure 9. Inductive Kick Upon Source Disconnect

1 2 ... 14 15 16 17 18 19 20 21 22

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Rainbow-electronics MAX1538 Manual de usuario Pagina 19

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