Datasheet
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
16 ______________________________________________________________________________________
MOSFET Selection
Select external MOSFETs according to the application
current level. The MOSFETs’ on-resistance (R
DS(ON)
)
should be chosen low enough to have minimum voltage
drop at full load to limit the MOSFET power dissipation.
High R
DS(ON)
also causes large output ripple if there is
a pulsating load. Determine the device power rating to
accommodate startup, a short-circuit condition, and
when the device is in autoretry mode.
During normal operation, the external MOSFETs dissipate
little power. The power dissipated in normal operation is:
P = I
LOAD
2
x R
DS(ON)
The most power dissipation occurs during a short-circuit
event, resulting in high power dissipated in Q2 (Figure
11) during the timeout period for the MAX5943A, where
the power dissipated across Q2 is:
P
Q2
= (V
IN
- V
IS
– V
Q1
) x I
LIMIT
For the MAX5943B–MAX5943E, a short-circuit event
results in high power dissipated in both Q1 and Q2 dur-
ing the timeout period (Figure 13) where the total power
dissipated in either MOSFET is:
P = I
SC
2
x R
DS(ON)
where:
I
SC
= V
IN
/R
EQ
and
R
EQ
= R
SENSE
+R
DS(ON1)
+ R
DS(ON2)
V
Q1
V
Q2
V
IS
SENSE GATE1 GATE2 OUTIN
+-
+- +-
V
IN
MAX5943A
Figure 11. Power Dissipated Across MOSFETs During a Short-
Circuit Fault for MAX5943A
R2
0.020Ω
1%
0.25W
C3
1μF
C4
0.1μF
L1
FERRITE
250Ω
R3
1kΩ
R4
1kΩ
R5
100kΩ
R6
100kΩ
DMMT3906
Q1B
Q1A
GND
NC
OUT
GATE2
N.C.
GATE1
SENSE
IN
R1
200kΩ
C2
0.01μF
C1
1μF
J1
V
IN
J2
GND
R7
100kΩ
TP1
V
IN
TP2
GND
TP3
PORT 0
TP4
FAULT
R8
100kΩ
TP5
ON
TP6
V
PULLUP
135
J3
N1
Si2318DS
N2A
Si7222DN
N2B
246
PORT 0
ON
FAULT
TIM
ILIM
LATCH
OR_ADJ
N.C.
ONQ1
1
2
3
4
5
6
7
89
10
11
12
13
14
15
16
MAX5943AEEE
U1
Figure 12. UL-Recognized Single-Port FireWire Protective Circuit