MAX5943 Maxim Integrated Products, MAX5943 Datasheet - Page 16

• Manufacturer Part Number: MAX5943
• Description: FireWire Current Limiter and Low-Drop ORing Switch Controller
• Manufacturer: Maxim Integrated Products

FireWire Current Limiter and Low-Drop
ORing Switch Controller
Select a sense resistor that causes the circuit-breaker
voltage drop at a current-limit/circuit-breaker level
above the maximum normal operating current.
Typically, set the overload current at 1.2 to 1.5 times
the full load current.
Choose the sense-resistor power rating to accommodate
an overcurrent condition:
where P
during a current-limit/circuit-breaker fault.
Under short-circuit conditions, it is imperative that the
appropriate sense resistor is utilized. Operating the
MAX5943B–MAX5943E at high input voltages can
cause very large currents during the circuit-breaker
timeout period. The peak current will be limited by the
saturation current of Q2 or the series resistance in the
power path (R
Using a 30mΩ on-resistance MOSFET at GATE1 and
GATE2 and a 30mΩ sense resistor results in a short-cir-
cuit current approximately equal to:
where:
For example, an input voltage of 20V produces a current
at approximately 222A (or I
in the power path for the circuit-breaker timeout period.
Choose an R
dissipation during a short-circuit event.
Select external MOSFETs according to the application
current level. The MOSFETs’ on-resistance (R
should be chosen low enough to have minimum voltage
drop at full load to limit the MOSFET power dissipation.
High R
a pulsating load. Determine the device power rating to
accommodate startup, a short-circuit condition, and
when the device is in autoretry mode.
16
______________________________________________________________________________________
Optimizing for Short-Circuit Conditions
DS(ON)
RSENSE
R
SENSE
P
TOTAL
TOTAL
also causes large output ripple if there is
RSENSE
is the power dissipated across R
I
SC
capable of handling the high power
).
= R
= 30mΩ + 2 x (30mΩ) = 90mΩ
= V
= I
SENSE
2
IN
LIMIT
SAT
/R
TOTAL
of Q2, whichever is less)
+ 2 x (R
x R
SENSE
MOSFET Selection
Choosing R
ON
)
DS(ON)
SENSE
SENSE
)
During normal operation, the external MOSFETs dissi-
pate little power. The power dissipated in normal oper-
ation is:
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:
For the MAX5943B–MAX5943E, a short-circuit event
results in high power dissipated in both Q1 and Q2 dur-
ing the timeout period (Figure 12) where the total power
dissipated in either MOSFET is:
where:
and
The programmable timeout of the MAX5943 allows the
use of MOSFETs with low power ratings. A MOSFET
typically withstands single-shot pulses with higher dis-
sipation than the specified package rating.
The MAX5943 serves to regulate and protect FireWire
power over a system interface. The MAX5943 program-
mable features make it suitable for both power provider
and power receiver applications. Figure 13 shows a
high-end two-port FireWire power management system
using two MAX5943As and a dual-channel MAX5944
FireWire current-limiting IC.
Figure 11. Power Dissipated Across MOSFETs During a Short-
Circuit Fault for MAX5943A
V
IN
R
EQ
IN
+
P
Q2
= R
V
= (V
P = I
SENSE
IS
P = I
FireWire Power Management
-
SENSE GATE1
I
IN
SC
LOAD
SC
MAX5943A
- V
+R
= V
+
2
IS
V
DS(ON1)
2
x R
Q1
IN
– V
x R
-
/R
DS(ON)
Q1
DS(ON)
EQ
+
) x I
GATE2
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+ R
V
Q2
LIMIT
-
DS(ON2)
OUT