MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
16 ______________________________________________________________________________________
Optimizing for Short-Circuit Conditions
Choosing RSENSE
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:
PRSENSE = I2LIMIT x RSENSE
where PRSENSE is the power dissipated across RSENSE
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 (RTOTAL).
Using a 30mΩon-resistance MOSFET at GATE1 and
GATE2 and a 30mΩsense resistor results in a short-cir-
cuit current approximately equal to:
ISC = VIN/RTOTAL
where:
RTOTAL = RSENSE + 2 x (RON)
= 30mΩ+ 2 x (30mΩ) = 90mΩ
For example, an input voltage of 20V produces a current
at approximately 222A (or ISAT of Q2, whichever is less)
in the power path for the circuit-breaker timeout period.
Choose an RSENSE capable of handling the high power
dissipation during a short-circuit event.
MOSFET Selection
Select external MOSFETs according to the application
current level. The MOSFETs’ on-resistance (RDS(ON))
should be chosen low enough to have minimum voltage
drop at full load to limit the MOSFET power dissipation.
High RDS(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 dissi-
pate little power. The power dissipated in normal oper-
ation is:
P = ILOAD2x RDS(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:
PQ2 = (VIN - VIS – VQ1 ) x ILIMIT
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:
P = ISC2x RDS(ON)
where:
ISC = VIN/REQ
and
REQ = RSENSE +RDS(ON1) + RDS(ON2)
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.
FireWire Power Management
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.