MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
______________________________________________________________________________________ 15
Fault Management
The MAX5943 offers either latch or autoretry fault man-
agement configurable by the LATCH input. Connect
LATCH to IN for latch fault management or connect
LATCH to GND for autoretry fault management. In latch
fault management, FAULT latches low, GATE1 and
GATE2 latch off indefinitely. Cycle ON low and then
high to unlatch and restart the MAX5943. However, the
MAX5943 will not enter a startup cycle until t
OFF
has
expired. Figure 9 illustrates a way to reset the MAX5943
after a fault using a pushbutton switch.
In autoretry fault management, the MAX5943_ attempt
to restart after a t
OFF
of 128 x t
ILIM
(or 128 x t
CB
) limit-
ing the duty cycle of the MOSFETs to 1/129 under con-
tinuous fault conditions. FAULT deasserts every time a
restart attempt is made.
Applications Information
Startup Consideration
MAX5943A
During startup, a large capacitor at OUT may result in a
charging current equivalent to the current limit. Choose a
current-limit timeout that will allow a successful startup.
The timeout can be approximated using the following
equation:
\where I
LIMIT
is the programmed current limit, C
OUT
is
the capacitor at OUT, V
IN
is the supply voltage, and
I
LOAD
is the load current during startup. With IN = 12V,
C
OUT
= 330µF, I
LIMIT
= 1.5A, and I
LOAD
= 0, the
MAX5943 commences by charging the output capacitor
with 1.5A for approximately 2.7ms. Therefore, the
MAX5943A current-limit timeout period (t
ILIM
) should be
greater than 2.7ms for a successful startup. Otherwise,
the MAX5943A powers up in fault management mode
by exceeding the current-limit timeout period.
MAX5943B–MAX5943E
The MAX5943B–MAX5943E do not control the inrush
current during startup. Inrush current control can be
implemented by placing a resistor and capacitor at
GATE2 (Figure 10) to slowly ramp up the gate voltage,
thus limiting the inrush current. The inrush current can
be approximated using the following formula:
Where I
G2U
is GATE2’s 45µA sourcing current and
I
LOAD
is the load current at startup.
To prevent the MAX5943B–MAX5943E from starting up
in a fault condition set:
Optimizing for Short-Circuit Conditions
Choosing R
SENSE
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:
P
RSENSE
= I
2
LIMIT
x R
SENSE
where P
RSENSE
is the power dissipated across R
SENSE
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
TOTAL
).
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:
I
SC
= V
IN
/R
TOTAL
where:
R
TOTAL
= R
SENSE
+ 2 x (R
ON
)
= 30mΩ + 2 x (30mΩ) = 90mΩ
For example, an input voltage of 20V produces a current
at approximately 222A (or I
SAT
of Q2, whichever is less)
in the power path for the circuit-breaker timeout period.
Choose an R
SENSE
capable of handling the high power
dissipation during a short-circuit event.
