Rainbow Electronics MAX5943 User Manual
Page 12
MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
12
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MAX5943B–MAX5943E Circuit Breaker
The MAX5943B–MAX5943E provide a circuit-breaker
function that protects against excessive load currents
and short-circuit conditions. The load current is monitored
by sensing the voltage across an external sense resistor
(R
SENSE
) and disconnects the load from the power
source when the voltage across R
SENSE
, V
IS
, exceeds
the programmed circuit-breaker thresholds (V
TH
) for
longer than the circuit-breaker timeout period, t
CB
.
Unlike the MAX5943A, when an output short circuit or
overcurrent occurs, the MAX5943B–MAX5943E keep
GATE2 fully enhanced. The short-circuit current is limit-
ed only by the input source impedance, R
SENSE
, Q1’s
R
DS(ON),
Q2’s R
DS(ON)
, and any parasitic impedance in
the power path. If the excessive current condition per-
sists (as indicated by V
IS
> V
TH
) for longer than t
CB
,
GATE2 turns off with a strong pulldown (I
G2 OFF
) to
quickly disconnect the power source from the load and
FAULT asserts low (Figure 3).
Exercise caution when using the MAX5943B–MAX5943E.
Unlike the MAX5943A, the MAX5943B–MAX5943E do not
limit the load current during t
CB
. High current during t
CB
may cause damage to external devices connected in the
power path.
Current-Limit/Circuit-Breaker Thresholds
The MAX5943 allows three selectable current-limit/cir-
cuit-breaker thresholds (V
TH
). Choose the voltage
threshold by connecting ILIM to the appropriate level;
see Table 1.
Current-Limit/Circuit-Breaker
Timeout Period (t
ILIM
/t
CB
)
The MAX5943’s current-limit/circuit-breaker timeout
function disconnects power from the load if the current-
limit or circuit-breaker threshold voltage exceeds its
maximum limit for a programmable amount of time.
Connect TIM to IN for the MAX5943 default timeout
periods (see Table 2).
Adjust the fault timeout period (t
ILIM/
t
CB
) by connect-
ing a resistor (R
TIM
) from TIM to GND (see the Typical
Operating Characteristics). Calculate the fault-limit
timeout period using the following equation:
where R
TIM
is in kΩ, t
ILIM
/t
CB
is in ms, and K = 1 for
MAX5943A, K = 32 for MAX5943B, K = 16 for
MAX5943C, K = 8 for MAX5943D, and K = 4 for the
MAX5943E.
t
ms
t
ms
R
k
K
ILIM
CB
TIM
(
)
(
)
.
(
)
=
=
×
0 25
Ω
I
OUT
I
LIMIT
t
CB
V
IN
V
G2
V
G2
V
OUT
V
GATE1
V
GATE2
V
FAULT
Figure 3. MAX5943B–MAX5943E Circuit-Breaker Timing Diagram
Table 1. Current-Limit/Circuit-Breaker
Trip Threshold Level
ILIM CONNECTION
V
TH
(mV)
Open
40
GND
50
IN
60
PART
DEFAULT
TIMEOUT,
TIM = IN
(ms)
PROGRAMMABLE
TIMEOUT RANGE
(4k
Ω
< R
TIM
< 50k
Ω
)
MAXIMUM
TIMEOUT,
TIM =
FLOATING
(ms)
MAX5943A
2
1.04ms to 11.05ms
12.5
MAX5943B
0.5
32.5µs to 345µs
4.8
MAX5943C
1
65µs to 690µs
9.6
MAX5943D
2
130µs to 1.38ms
19.2
MAX5943E
4
260µs to 2.76ms
38.4
Table 2. MAX5943_ Timeout Periods