MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
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Detailed Description
The MAX5944 dual-channel controller IC performs hot-
swapping, power-supply ORing, and current limiting for
FireWire power-supply applications. The MAX5944
incorporates four MOSFET drivers (GATE1_ and
GATE2_) to control external n-channel power MOSFETs
to perform low-voltage-drop power-supply ORing
(GATE1_), hot-swapping, and current limiting (GATE2_)
from the input power supply to the load. A sense resis-
tor provides accurate current limiting. GATE1_ and
GATE2_ provide true load disconnect to prevent current
flowing from either direction between IN_ and OUT_.
The MAX5944 independent channels remain in low-cur-
rent shutdown mode when V
ON_
is below the 0.4V
threshold. Low-current shutdown mode disables the
MAX5944 channels resulting in less than 10µA supply
current per channel. Shutdown places all logic inputs in
a high-impedance state, which allows the inputs to be
connected to IN_ without drawing additional current
from the supply.
When either input supply voltage (V
IN_
) is above 7.5V
and V
ON_
is above the V
ON_REF
threshold, the
MAX5944 channel turns on, sourcing 50µA current from
GATE2_ (current regulation), to enhance Q2_ slowly. If
the voltage across the current-sense resistor, V
IS
, is
greater than the current-limit threshold, the MAX5944
regulates the GATE2_ voltage to limit the load current at
the current-limit level so that V
IS
is equal to V
TH
. In nor-
mal operation, V
IS
drops below V
TH
and GATE2_ rises
to approximately 5.5V above OUT.
GATE1_ controls the MAX5944 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 IN_ when V
IS
exceeds V
OR
, thereby
enhancing Q1_ and reducing the voltage drop, power
dissipation, and heat generation in the power-supply
pathway. When a voltage greater than V
IN_
is connect-
ed at OUT_, the higher voltage source commences pro-
viding current to the load. The MAX5944 turns off
GATE1_ rapidly upon V
IS
falling below the V
OR
hystere-
sis, thus blocking the higher voltage from backdriving
V
IN_
. GATE1_ also turns off when the load current
drops, causing V
IS
to fall below V
OR_
hysteresis.
Current Limiting
The MAX5944 limits the load current by monitoring the
voltage across R
SENSE_
at all times and regulating the
current to the load as necessary to keep V
IS
to its cur-
rent-limit threshold voltage (V
TH
). Set the maximum cur-
rent 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 attempts to draw more current than
I
LIMIT
, the MAX5944’s GATE2_ pulldown current (I
G2D
)
regulates the current through Q2_, causing OUT_ to act
as a constant current source. The output current is limit-
ed to I
LIMIT
. If the current-limit condition persists after
the 2ms current-limit timeout period (t
ILIM
) has expired,
the GATE2_ strong pulldown current (I
G2_OFF
) quickly
turns off GATE2_ to disconnect the load from the power
supply and FAULT_ asserts low (Figure 2a). If the cur-
rent-limit duration is less than t
ILIM
, GATE2_ pulls to
V
GS2
(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 MAX5944 responds with much
stronger GATE2_ pulldown currents (I
G2DP
and/or
I
G2DF
) to quickly bring the load current back down to
the programmed current limit.
Current-Limit Threshold
The MAX5944 current limit has internal 50mV trip
threshold voltages (V
TH
). Use the following formula to
calculate the maximum current:
I
LIMIT
= V
TH
/ R
SENSE_
where V
TH
is the 50mV (typ) threshold voltage across
R
SENSE_
, and R
SENSE_
is the sense resistor connected
between IN_ and SENSE_.
Power-Supply ORing, GATE1_
GATE1_ controls the MAX5944 ORing function. The
MAX5944 accomplishes a very low voltage-drop diode
ORing function using n-channel power MOSFETs.
Initially, GATE1_ is off (ONQ1 = low) and the load cur-
rent conducts through the body diode of Q1_, Figure 3.
GATE1_ rises to 5.5V above SENSE_ when V
IS
exceeds
the 5mV V
OR
threshold, enhancing Q1_. Q1_’s low
R
DS(ON)
provides a very low voltage drop across its
source to drain, which results in less power dissipation
and heat generation in the power-supply path than a
traditional diode.
The MAX5944 continuously monitors the load currents
through the voltage drops, V
IS
, across the current-sens-
ing resistors. Q1_ turns off rapidly when V
IS
decreases
below the V
OR
minus its hysteresis.
Connect ONQ1 to GND for a normal ORing function.
Drive ONQ1 high to force Q1 on, regardless of the
load-current condition, as long as voltages at IN_ and
ON_ exceed V
UVLO
and V
ON-REF
, repectively, and the
device is not in a current-fault shutdown.
