Rainbow Electronics MAX1538 User Manual
Page 18
MAX1538
Power-Source Selector for
Dual-Battery Systems
18
______________________________________________________________________________________
MOSFETs, they should be paired as shown in
Figure
6
for optimal power dissipation.
The MAX1538 provides asymmetric MOSFET gate
drive, typically turning MOSFETs on faster than they are
turned off. The t
TRANS
timer ensures that the MOSFETs
that are turning on begin to turn on 10µs after those
MOSFETs that are turning off begin to turn off. Choose
MOSFETs with low enough gate charge that all off-tran-
sitioning MOSFETs turn off before any on-transitioning
MOSFET turns on. Use the following equations to esti-
mate the worst-case turn-on and turn-off times:
where t
ON
is the turn-on time, t
OFF
is the turn-off time,
Q
G
is the MOSFET’s total gate charge specified at volt-
age V
G
, I
OFF1
is the 18mA (min) gate current when dri-
ving the gate from 7.5V gate drive to 2V gate drive, ∆V
1
is the voltage change during the 18mA gate drive
(5.5V), I
OFF2
is 3mA gate current when driving the gate
from 2V to 0V, ∆V
2
is the 2V change, and I
ON
is the
turn-on current.
The MAX1538’s gate-drive current is nonlinear and is a
function of gate voltage. For example, the gate driver
slows down as the MOSFET approaches off. See the
Typical Operating Characteristics for a scope shot
showing MAX1538 turn-on and turn-off times when dri-
ving FDS6679 MOSFETs. The MAX1538 typically turns
the FDS6679 on in 0.7µs and off in 1µs.
Combining the MAX1538 with a Charger
To configure the MAX1538 for use with a step-down
charger, use the circuit of Figure 7. Connect the charg-
er’s power input to EXTLD. Do not connect the charg-
er’s power input to ADPIN. This ensures that the
charger does not bias ADPIN through its high-side
MOSFET.
System Holdup Capacitor
C
SYS
must be capable of sustaining the maximum sys-
tem load during the transition time between source
selection. Size the capacitor so that:
where t
MINV
is the battery undervoltage comparator
delay, t
TRANS
is the fixed time between switching
MOSFETs off and switching MOSFETs on, t
ON
is the
time to turn a MOSFET on (see the MOSFET Selection
section), V
MINV
is the lower of V
MINVA
and V
MINVB
,
I
SYS_MAX
is the maximum system load, V
SYS_MIN
is the
minimum allowable system voltage before system
5 Ч
−
+
+
(
)
Ч
>
V
t
t
t
I
C
V
MINV
MINV
TRANS
ON
SYS MAX
SYS
SYS MIN
_
_
t
Q
V
V
I
V
I
Q
V
k
t
Q
V
V
I
Q
V
k
ON
G
G
OFF
OFF
G
G
ON
G
G
ON
G
G
=
∆
+
∆
=
×
Ω
=
Ч
=
Ч
Ω
1
1
2
2
0 93
5
0 25
.
.
MAX1538
MAX1908
MAX1909 OR
MAX1535
REVBLK
CHGIN
ADPIN
DCIN
P2
C
SYS
C2
EXTLD
ADPBLK
P3
SYSTEM LOAD
CSSP
CSSN
1µF
BATT
ADAPTER
C
ADAPTER
Figure 7. Combining the MAX1538 with a Charger