MAX5943
FireWire Current Limiter and Low-Drop
ORing Switch Controller
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The MAX5943 current-limit/circuit-breaker timeout is
programmable by connecting an external resistor from
TIM to GND. The MAX5943A has a default 2ms current-
limit timeout. The MAX5943B, MAX5943C, MAX5943D,
and MAX5943E have 0.5ms, 1ms, 2ms, and 4ms
default circuit-breaker timeouts, respectively.
GATE1 controls the MAX5943 ORing function. Initially,
GATE1 is off (ONQ1 = low) and the load current con-
ducts through the body diode of Q1. GATE1 rises to
5.5V above SENSE when V
IS
exceeds V
OR
, thereby
enhancing Q1 and reducing the voltage drop, power
dissipation, and heat generation in the power-supply
path. When a voltage greater than V
IN
is connected at
OUT, the higher voltage source commences, providing
current to the load. The MAX5943 rapidly turns off
GATE1 upon V
IS
falling below V
OR
- hysteresis, thus
blocking the higher voltage from back-driving V
IN
.
GATE1 also turns off when the load current drops,
causing V
IS
to fall below the V
OR
hysteresis.
Current Limiting/Circuit Breaker
Current Limiting (MAX5943A Only)
The MAX5943A limits the load current by monitoring the
voltage across R
SENSE
at all times and by regulating the
current to the load as necessary to keep V
IS
to its pro-
grammable current-limit threshold voltage (V
TH
). Set the
maximum current limit (I
LIMIT
) by placing the appropriate
sense resistor between IN and SENSE, where I
LIMIT
=
V
TH
/R
SENSE
. When the load current is less than I
LIMIT
,
GATE2 rises to V
G2
(5.5V) to fully enhance MOSFET Q2
(see the
Typical Operating Circuit)
.
When the load current attempts to draw more than
I
LIMIT
, the MAX5943A’s GATE2 pulldown current (I
G2D
)
regulates the current through Q2 causing OUT to act as
a constant-current source. The output current is limited
to the programmed I
LIMIT
. If the current-limit condition
persists after the current-limit timeout (t
ILIM
) period has
expired, GATE2’s strong pulldown current (I
G2 OFF
)
quickly turns off GATE2 to disconnect the load from the
power supply. FAULT then asserts low (Figure 2a). If the
current-limit duration is less than t
ILIM
, GATE2 pulls to
V
G2
(Figure 2b). During an output short circuit or a
gross overload, the load current overshoots and causes
V
IS
to exceed the fast pulldown current-limit threshold
voltage (V
THF
). The MAX5943A responds with much
stronger GATE2 pulldown currents (I
G2DP
and/or I
G2DF
)
to quickly bring the load current back down to the pro-
grammed current limit.
