Rainbow Electronics MAX1538 - Manual (Page 17)

MAX1538
Power-Source Selector for
Dual-Battery Systems
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CHG Control
Toggle CHG to enable the charge path to the battery.
Charge control is overridden by RELRN (see the Battery
Relearn Mode section) or airline mode (see the Airline
Mode and AC Adapter section). When CHG is enabled,
the MAX1538 connects the selected battery (BATSEL = 0
for battery A and BATSEL = 1 for battery B) to the charg-
er. OUT[2:1] = 11 if the MAX1538 is in charge mode.
When the charge path is enabled, the corresponding
battery undervoltage latch is cleared. This allows charg-
ing of protected battery packs. In typical applications,
connect CHRG to VDD to reduce the system I/O.
Single Transition Break-Before-Make
Selection
The MAX1538 guarantees that no supplies are connect-
ed to each other during any transition by implementing
a fixed delay time (t
TRANS , the break-before-make tran-
sition timer). This is necessary as the batteries have very
low impedances, and momentarily shorting batteries
together can cause hundreds of amps to flow. For
example, when adapter removal is detected, ADPPWR
and REVBLK begin to turn off less than 10µs before
ADPBLK and DISBAT begin to turn on, connecting the
appropriate battery. For example, upon switching from
one battery to another, DISA and CHGA begin turning
off less than 10µs before DISB and CHGB begin to turn
on. To guarantee a break-before-make time, ensure that
the turn-off time of the MOSFETs is smaller than t
TRANS
(see the MOSFET Selection section).
The MAX1538 also guarantees that any change does
not cause unnecessary power-source transitions. When
switching from battery to battery; battery to adapter; or
adapter to battery because of adapter or battery inser-
tion or removal, or due to a change at BATSEL, a single
set of MOSFETs are turned off followed by another set
of MOSFETs turned on. No additional transitions are
necessary. The only exception occurs when RELRN is
high and the adapter is inserted because it is first
detected as an airline adapter and later detected as an
AC adapter. This results in a transition from discharge
mode to AC mode, followed by a transition from AC
mode to relearn mode. Although this extra transition is
generally harmless, it can be avoided by disabling
relearn mode when the adapter is absent.
Blanking
The MAX1538 implements sophisticated blanking at the
adapter and the batteries to correctly determine bat-
tery/adapter insertion and removal. Logic inputs CHRG,
RELRN, and BATSEL should be debounced to ensure
that fast repetitive transitions do not occur, in which
case the system holdup capacitor is not large enough
to sustain the system load.
Battery insertion is automatically debounced using the
battery-insertion blanking time (t
BBLANK ). A battery is
not discharged unless the battery has been above the
5 x V
MINV threshold for 21ms (typ). After t BBLANK is
expired, V BAT_ must exceed 5 x V MINV_ or the battery
is detected as undervoltage.
Applications Information
MOSFET Selection
Select P-channel MOSFETs P1–P8 according to their
power dissipation, R
DSON , and gate charge. Each
MOSFET must be rated for the full system load current.
Additionally, the battery discharge MOSFETs (P3, P5,
P6, P7, and P8) should be selected with low on-resis-
tance for high discharge efficiency. Since for any given
switch configuration at least half of the MOSFETs are
off, dual MOSFETs can be used without reducing the
effective MOSFET power dissipation. When using dual
MAX1538
DISA
BATA
DISB
CHGB
CHGA
CHGIN
ADPIN
ADPBLK
BATTERY A BATTERY B
ADPPWR P1
P2
P5
P6 P7
P8
REVBLK
EXTLD
P3
SYSTEM LOAD
FOR RELEARN
MODE ONLY
DUAL
FDS4935A
DUAL
FDS4935A
DUAL
FDS4935A
STEP-DOWN
BATTERY CHARGER
IN
OUT
BATB
ADAPTER
Figure 6. Optimal Use of Power Dissipation Using Dual
MOSFETs