MAX5944ESE+T Maxim Integrated Products, MAX5944ESE+T Datasheet - Page 10

• Manufacturer Part Number: MAX5944ESE+T
• Description: IC FW CURR LIMITER DUAL 16-SOIC
• Manufacturer: Maxim Integrated Products

Specifications of MAX5944ESE+T

• Applications: FireWire® Apps
• Fet Type: N-Channel
• Number Of Outputs: 2
• Internal Switch(s): No
• Delay Time - Off: 100ns
• Voltage - Supply: 7.5 V ~ 37 V
• Current - Supply: 1.3mA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Delay Time - On: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

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
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
and V
MAX5944 channel turns on, sourcing 50µA current from
GATE2_ (current regulation), to enhance Q2_ slowly. If
the voltage across the current-sense resistor, V
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
mal operation, V
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
enhancing Q1_ and reducing the voltage drop, power
dissipation, and heat generation in the power-supply
pathway. When a voltage greater than V
ed at OUT_, the higher voltage source commences pro-
viding current to the load. The MAX5944 turns off
GATE1_ rapidly upon V
sis, thus blocking the higher voltage from backdriving
V
drops, causing V
The MAX5944 limits the load current by monitoring the
voltage across R
current to the load as necessary to keep V
rent-limit threshold voltage (V
rent limit (I
resistor between IN_ and SENSE_, where I
R
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
10
IN_
SENSE_
. GATE1_ also turns off when the load current
______________________________________________________________________________________
ON_
. When the load current is less than I
LIMIT
is above the V
IS
IS
SENSE_
) by placing the appropriate sense
drops below V
to fall below V
Detailed Description
IS
at all times and regulating the
falling below the V
IS
TH
ON_REF
exceeds V
IS
ON_
). Set the maximum cur-
is equal to V
OR_
TH
Current Limiting
is below the 0.4V
IN_
and GATE2_ rises
hysteresis.
threshold, the
) is above 7.5V
IN_
LIMIT_
OR
IS
OR
is connect-
TH
, thereby
to its cur-
hystere-
. In nor-
= V
LIMIT
IS
TH
, is
/
,
GATE2_ rises to V
Q2_ (see the Typical Operating Circuit).
When the load attempts to draw more current than
I
regulates the current through Q2_, causing OUT_ to act
as a constant current source. The output current is limit-
ed to I
the 2ms current-limit timeout period (t
the GATE2_ strong pulldown current (I
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
V
gross overload, the load current overshoots and causes
V
voltage (V
stronger GATE2_ pulldown currents (I
I
the programmed current limit.
The MAX5944 current limit has internal 50mV trip
threshold voltages (V
calculate the maximum current:
where V
R
between IN_ and SENSE_.
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
the 5mV V
R
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
ing resistors. Q1_ turns off rapidly when V
below the V
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
device is not in a current-fault shutdown.
LIMIT
G2DF
IS
GS2
SENSE_
DS(ON)
to exceed the fast pulldown current-limit threshold
, the MAX5944’s GATE2_ pulldown current (I
) to quickly bring the load current back down to
(Figure 2b). During an output short circuit or a
LIMIT
TH
, and R
provides a very low voltage drop across its
THF
OR
OR
. If the current-limit condition persists after
is the 50mV (typ) threshold voltage across
). The MAX5944 responds with much
UVLO
minus its hysteresis.
threshold, enhancing Q1_. Q1_’s low
SENSE_
I
LIMIT
Power-Supply ORing, GATE1_
G2
and V
TH
(5.5V) to fully enhance MOSFET
= V
is the sense resistor connected
). Use the following formula to
Current-Limit Threshold
TH
ON-REF
IS
/ R
, across the current-sens-
SENSE_
ILIM
, repectively, and the
, GATE2_ pulls to
ILIM
G2_OFF
) has expired,
G2DP
IS
IS
decreases
) quickly
exceeds
and/or
G2D
)