General Description
The MAX5944 is a dual, current limiter/low-drop ORing,
switch controller power-management IC for FireWire®
applications. Each independent channel controls two
external power MOSFETs to regulate current from the
input power supply to the load and performs low-voltage-
drop power-supply ORing. The MAX5944 operates over
the 7.5V to 37V input range and provides inrush and out-
put current limit in addition to diode ORing functions suit-
able for FireWire applications.
The MAX5944 allows the safe insertion and removal of
FireWire peripherals into a live FireWire port by providing
inrush current control and output current-limiting func-
tions. The MAX5944 features a current-limit function that
actively limits the current drawn by the load with a 2ms
timeout. Its autoretry function then automatically restarts
after the 259ms autoretry delay time.
The ORing feature provides a very efficient way to pro-
tect the FireWire port when connected to another
FireWire peripheral that sources a higher voltage. By
turning off the external MOSFETS, the MAX5944 provides
a true bidirectional load disconnect. Each channel has
an independent ON/OFF control that results in less than
10µA supply current per channel.
The MAX5944 operates over the -40°C to +85°C
extended temperature range and is available in a
16-pin SO package.
Applications
FireWire Desktop/Notebook Port
FireWire Peripheral Device Current Limit
FireWire Hub
Hot Swap
Features
♦Provides Safe Hot-Swap for 7.5V to 37V Power
Supplies
♦Dual-Channel Hot-Swap
♦Low-Drop Power ORing with Ultra-Fast Turn-Off
♦Active Current Limit
♦±5% Current-Limit Sensing Accuracy
♦2ms Current-Limit Timeout
♦Fast Current-Limit Response to Output
Short-Circuit Conditions
♦Drives External n-Channel Power MOSFETs
♦Independent Overcurrent Fault Status Outputs
♦259ms Autoretry Period
♦ 10µA (Per Channel) Shutdown Current
♦Independent ON/OFF Control Allows
Undervoltage Lockout Programming
♦Fast Load Disconnect Control Through
ON/OFF Inputs
♦Bidirectional Load Disconnect
♦Power Routing from One Port to Another
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
________________________________________________________________ Maxim Integrated Products 1
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
INA SENSEA
GATE1A
GATE2A
OUTA
GND
FAULTB
ONB
INB
TOP VIEW
MAX5944
SO
ONA
FAULTA
GATE2B
ONQ1
OUTB
GATE1B
SENSEB
Pin Configuration
Ordering Information
19-3426; Rev 0; 10/04
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
EVALUATION KIT
AVAILABLE
Typical Operating Circuit appears at end of data sheet.
FireWire is a registered trademark of Apple Computer, Inc.
PART TEMP RANGE PIN-PACKAGE
MAX5944ESE -40°C to +85°C 16 SO
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN = 7.5V to 37V, GND = 0V, and TA= -40°C to +85°C, unless otherwise noted. Typical values are at VIN_ = 12V, TA= +25°C.)
(Note 1)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
(Voltages referenced to GND, unless otherwise noted.)
IN_ ..........................................................................-0.3V to +80V
OUT_ ......................................................................-0.3V to +80V
SENSE_ to IN_ ..........................................................-1V to +0.3V
GATE1_ to SENSE_ ..................................................-0.3V to +8V
GATE2_ to OUT_ ......................................................-0.3V to +8V
FAULT_...................................................................-0.3V to +85V
ON_ONQ1 ...................................................................-0.3V to +85V
Continuous Power Dissipation (TA= +70°C)
16-Pin SO (derate 9.1mW/°C above +70°C)................728mW
Operating Temperature Range ...........................-40°C to +85°C
Maximum Junction Temperature .....................................+150°C
Storage Temperature Range .............................-60°C to +150°C
ESD Rating (Human Body Model)......................................2000V
Lead Temperature (soldering, 10s) .................................+300°C
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
UNITS
SUPPLIES
Supply Voltage Range VIN_ 7.5 37 V
Supply Current IIN VON_ = 3V, no load, per channel 1.3 2.3 mA
Shutdown Current ISD VON_ = 0.4V, per channel 5 10 µA
IN_ Default Undervoltage
Lockout VUVLO VIN_ rising, ON_ = IN_ 6.2 6.5 6.8 V
IN_ Default Undervoltage-
Lockout Hysteresis
0.35
V
ON_ Undervoltage-Lockout
Threshold
VON_REF
VON_ rising
1.20 1.24 1.275
V
ON_ Undervoltage-Lockout-
Threshold Hysteresis 70 mV
ON_ Input Bias Current ION 1µA
GATE2_ (Current-Limit Control)
GATE2_ Pullup Current (Note 2) IG2U 36 45 54 µA
7.5V ≤ VIN ≤ 8V
4.25
5.5
6.50
GATE2_ High Voltage VG2 VGATE2_ - VOUT_ 8V ≤ VIN ≤ 37V 4.5 5.5 6.5 V
Current-Limit Threshold Voltage VTH VIN_ - VSENSE_
47.5
50
52.5
mV
Fast Pulldown Current-Limit
Threshold Voltage (Note 3) VTHF VIN_ - VSENSE_ 70 75 80 mV
GATE2_ Pulldown Current IG2D VTH < (VIN_ - VSENSE_) < VTHF
335 435 535
µA
GATE2_ Fast Pulldown Current
(Note 3) IG2DF (VIN_ - VSENSE_) > VTHF,
VGATE2_ = (VOUT_ + 4.5V) 80
125 200
mA
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 7.5V to 37V, GND = 0V, and TA= -40°C to +85°C, unless otherwise noted. Typical values are at VIN_ = 12V, TA= +25°C.)
(Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
UNITS
GATE2_ Peak Pulldown Current
IG2DP (VIN_ - VSENSE_) = 1V 1.0 A
GATE2_ Pulldown Current During
Any GATE2_ Turn-Off Condition
(Note 4)
IG2OFF VGATE2_ = (VOUT_ + 4.5V) 80
125 200
mA
GATE1_ (ORing Control)
ORing Switch Turn-On-Current
Threshold Voltage VOR (VIN_ - VSENSE_) rising 3 5 7 mV
ORing Switch Turn-On-Current
Threshold Voltage Hysteresis (VIN_ - VSENSE_) falling 0.5 mV
GATE1_ Pullup Current (Note 5) IG1U 36 45 54 µA
7.5V ≤ VIN ≤ 8V
4.25
5.5
6.50
GATE1_ High Voltage VG1 VGATE_ - VSENSE_ 8V ≤ VIN ≤ 37V 4.5 5.5 6.5 V
GATE1_ Turn-Off Switch On-
Resistance RG1PD Measured between GATE1_ and SENSE_
when (VIN_ - VSENSE_) < VOR 23Ω
LOGIC INPUTS
ONQ1 Input-Logic Low Voltage VIL VON_ > VON_REF 0.7 V
ONQ1 Input-Logic High Voltage VIH VON_ > VON_REF 1.8 V
ONQ1 Input Current IIH 1µA
FAULT_ Output Low Voltage VOL Low-impedance state, IFAULT_ = 5mA
0.12 0.25
V
FAULT_ Output High Leakage
Current IOH High-impedance state, VFAULT_ = 37V 1 µA
OUT_ Input Current IOUT VON_ = GND 1 µA
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 7.5V to 37V, GND = 0V, and TA= -40°C to +85°C, unless otherwise noted. Typical values are at VIN_ = 12V, TA= +25°C.)
(Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
UNITS
TIMING
Current-Limit Timeout tILIM Current-limit mode
1.78 2.02 2.26
ms
Automatic Restart Delay After
Current-Limit Timeout (Note 7) tOFF
240 259 278
ms
GATE1_ Turn-Off Time
(ORing Response Time) tOR From VOR trigger until
(VGATE1_ - VIN_) < 1V, CGATE1_ = 10nF
100
ns
Turn-Off Response Time
(Note 7)
tON_OFF
Time from VON_ < (VON_OFF - Hysteresis)
until (VGATE2_ - VOUT_) < 1V,
for CGATE2_ = open
0.34
0.6 µs
Minimum Delay from ON_ Low to
Low-Current Shutdown Mode tSD ON_ step from 1.3V to 0.4V 50 µs
Note 1: All min/max parameters are tested at +25°C and +85°C. Limits at -40°C are guaranteed by design.
Note 2: Measured at both VGATE2 = VOUT = 0V and VGATE2 = VOUT = VIN.
Note 3: The typical value of parameter VTHF is 1.5 x VTH.
Note 4: Current-Limit Timeout. IN_ or ON_ low.
Note 5: See the Typical Operating Circuit. Measured at VGATE_ = VSENSE_.
Note 6: The tristate (“floating”) logic condition will be entered if the pin current is within these limits.
Note 7: If the ON_ voltages go below VON_REF - Hysteresis, then GATE2_ goes low immediately to disconnect power from the load
quickly (within tON_OFF).
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX5944 toc01
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)
353020 251510
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
1.0
540
TA = +85°C
TA = +25°C
TA = -40°C
SHUTDOWN SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX5944 toc02
SUPPLY VOLTAGE (V)
SHUTDOWN SUPPLY CURRENT (µA)
353020 251510
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
0
540
CURRENT-LIMIT/CIRCUIT-BREAKER
THRESHOLD VOLTAGE vs. TEMPERATURE
MAX5944 toc03
TEMPERATURE (°C)
THRESHOLD VOLTAGE (mV)
603510-15
46.5
47.0
47.5
48.0
48.5
49.0
49.5
50.0
50.5
51.0
51.5
52.0
46.0
-40 85
VIN = 32V
VIN = 12V
VIN = 7.5V
Typical Operating Characteristics
(VIN_ = +12V, TA= +25°C, unless otherwise noted.)
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
_______________________________________________________________________________________ 5
CURRENT-LIMIT TIMEOUT
vs. TEMPERATURE
MAX5944 toc04
TEMPERATURE (°C)
CURRENT-LIMIT TIMEOUT (ms)
603510-15
2.05
2.10
2.15
2.20
2.25
2.30
2.00
-40 85
ORing THRESHOLD VOLTAGE
vs. TEMPERATURE
MAX5944 toc05
TEMPERATURE (°C)
ORing THRESHOLD VOLTAGE (mV)
603510-15
5.5
5.4
5.3
5.2
5.1
5.0
4.9
4.8
4.7
4.6
4.5
-40 85
GATE CURRENTS vs. TEMPERATURE
MAX5944 toc06
TEMPERATURE (°C)
GATE CURRENTS (µA)
603510-15-40 85
GATE2
44.5
45.0
45.5
46.0
46.5
47.0
44.0
GATE1
GATE1 DRIVE VOLTAGE
vs. TEMPERATURE
MAX5944 toc07
TEMPERATURE (°C)
GATE1 DRIVE VOLTAGE (V)
603510-15
5.233
5.236
5.239
5.242
5.245
5.248
5.251
5.254
5.257
5.260
5.230
-40 85
GATE2 DRIVE VOLTAGE
vs. TEMPERATURE
MAX5944 toc08
TEMPERATURE (°C)
GATE2 DRIVE VOLTAGE (V)
603510-15
5.203
5.206
5.209
5.212
5.215
5.218
5.221
5.224
5.227
5.230
5.200
-40 85
ON HIGH VOLTAGE vs. TEMPERATURE
MAX5944 toc09
TEMPERATURE (°C)
ON HIGH VOLTAGE (V)
603510-15
1.252
1.254
1.256
1.258
1.260
1.250
-40 85
ON LOW VOLTAGE vs. TEMPERATURE
MAX5944 toc10
TEMPERATURE (°C)
ON LOW VOLTAGE (V)
603510-15-40 85
1.182
1.184
1.186
1.188
1.190
1.192
1.194
1.196
1.198
1.200
1.180
FAULT LOW VOLTAGE
vs. TEMPERATURE
MAX5944 toc11
TEMPERATURE (°C)
FAULT LOW VOLTAGE (mV)
603510-15-40 85
20
40
60
80
100
120
140
160
180
200
0
IFAULT = 5mA
Typical Operating Characteristics (continued)
(VIN_ = +12V, TA= +25°C, unless otherwise noted.)
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(VIN_ = +12V, TA= +25°C, unless otherwise noted.)
TURN-OFF RESPONSE TIME
MAX5944 toc13
IIN
1A/div
VOUT
10V/div
VGATE2
10V/div
VON
2V/div
200ns/div
SHORT-CIRCUIT EVENT
(VIN = 37V)
MAX5944 toc14
IIN
1A/div
VOUT
50V/div
VGATE2
50V/div
VIN
50V/div
400µs/div
VFAULT
50V/div
SHORT-CIRCUIT EVENT
(VIN = 37V)
MAX5944 toc15
IIN
10A/div
VOUT
50V/div
VGATE2
50V/div
VIN
50V/div
100ns/div
VFAULT
50V/div
SHORT-CIRCUIT RESPONSE
(VIN = 8V)
MAX5944 toc16
IIN
1A/div
VOUT
10V/div
VGATE2
10V/div
VIN
10V/div
400µs/div
VFAULT
10V/div
SHORT-CIRCUIT RESPONSE
(VIN = 8V)
MAX5944 toc17
IIN
10A/div
VOUT
10V/div
VGATE2
10V/div
VIN
10V/div
100ns/div
VFAULT
10V/div
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
_______________________________________________________________________________________ 7
Typical Operating Characteristics (continued)
(VIN_ = +12V, TA= +25°C, unless otherwise noted.)
ORing SWITCHOVER WAVEFORM
MAX5944 toc18
IINA
500mA/div
VOUT
1V/div
200µs/div
IINB
500mA/div
VINA = 12V VONA RISING = 12V
VINB = 12.8V COUT = 0
ORing SWITCHOVER WAVEFORM
(VINB RISING ABOVE VINA)
MAX5944 toc19
IINA
500mA/div
VOUT
1V/div
10µs/div
IINB
500mA/div
VINA = 12V VONB RISING = 12V
VINB = 13V COUT = 0
ORing SWITCHOVER WAVEFORM
(VINB FALLING BELOW VINA)
MAX5944 toc20
IINA
500mA/div
VOUT
1V/div
20µs/div
IINB
500mA/div
VINA = 12V VONB FALLING = 12V
VINB = 13V COUT = 0
HOT PLUG-IN LOAD CAPACITANCE
INTO ACTIVE PORT (NO FAULT)
MAX5944 toc21
IIN
2A/div
VFAULT
10V/div
400µs/div
VOUT
10V/div
CLOAD = 220µF
VGATE2
10V/div
HOT PLUG-IN LOAD CAPACITANCE INTO
ACTIVE PORT (CURRENT-LIMIT TIMEOUT)
MAX5944 toc22
IIN
1A/div
VFAULT
10V/div
400µs/div
VOUT
10V/div
CLOAD = 470µF
VGATE2
10V/div
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
8 _______________________________________________________________________________________
Pin Description
PIN NAME FUNCTION
1 INA Channel A Input Power and Current-Sense Positive Input. Connect INA to the other end of the current-
sense resistor and to the power source (see the Typical Operating Circuit).
2 ONA
Channel A On/Off Control Input. ONA sets the undervoltage lockout threshold and resets the part after a
fault latch. Drive ONA high to enable the device. Drive ON_ low to disable the device. Connect ONA to INA
for the default undervoltage lockout threshold. Connect a resistor-divider from INA to ONA and to GND to
program the desired undervoltage lockout threshold.
3
FAULTA
Channel A Current-Fault Status Output. FAULTA is an open-drain output. FAULTA goes low after a current-
limit fault has exceeded the 2ms (typ) current-limit timeout period (see the
FAULT_
section).
4 ONQ1
ORing Switch Manual Turn-On Input. Drive ONQ1 low to enable power-supply ORing for channels A and B.
Drive ONQ1 high to set GATE1A to GATE1B high to allow reverse-current flow. ONQ1 is disabled for the
respective channel when VIN is below the UVLO threshold, VON_ is below the undervoltage lockout
threshold and after a current-limit fault.
5 OUTB Channel B Output-Voltage Sense. Connect to the output.
6
GATE2B
Channel B Current-Limiter Switch Gate-Drive Output. Referenced to OUTB. Connect GATE2B to the gate of
the 2nd n-channel MOSFET. GATE2B is a charge pump with a 50µA pullup current to 5.5V (typ) above
OUTB when active (see the Typical Operating Circuit).
7
GATE1B
Channel B ORing Switch Gate-Drive Output. Referenced to SENSEB. Connect GATE1B to the gate of the
1st n-channel MOSFET. GATE1B is a charge pump with a 50µA pullup current to 5.5V (typ) above SENSEB
when active (see the Typical Operating Circuit).
8
SENSEB
Channel B Current-Sense Negative Input. Connect current-sense resistor, RSENSE, from INB to SENSEB.
9 INB Channel B Input Power and Current-Sense Positive Input. Connect INB to the other end of the current-
sense resistor and to the power source.
10 ONB
Channel B On/Off Control Input. ONB sets the undervoltage lockout threshold and resets the part after a
fault latch. Drive ONB high to enable the device. Drive ON_ low to disable the device. Connect ONB to INB
for the default undervoltage lockout threshold. Connect a resistor-divider from INB to ONB and to GND to
program the desired undervoltage lockout threshold.
11
FAULTB
Channel B Current-Fault Status Output. FAULTB is an open-drain output. FAULTB goes low after a current-
limit fault has exceeded the 2ms (typ) current-limit timeout period (see the
FAULT_
section).
12 GND Ground
13 OUTA Channel A Output-Voltage Sense. Connect to the output.
14
GATE2A
Channel A Current-Limiter Switch Gate-Drive Output. Referenced to OUTA. Connect GATE2A to the gate of
the 2nd n-channel MOSFET. GATE2A is a charge pump with a 50µA pullup current to 5.5V (typ) above
OUTA when active.
15
GATE1A
Channel A ORing Switch Gate-Drive Output. Referenced to SENSEA. Connect GATE1A to the gate of the
1st n-channel MOSFET. GATE1A is a charge pump with a 50µA pullup current to 5.5V (typ) above SENSEA
when active.
16
SENSEA
Channel A Current-Sense Negative Input. Connect current-sense resistor, RSENSEA, from INA to SENSEA
(see the Typical Operating Circuit).
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
_______________________________________________________________________________________ 9
OR
COMPARATOR
CURRENT-LIMIT
COMPARATOR
FAST CURRENT-LIMIT
COMPARATOR
INA - 5mV
INA - 50mV
INA - 75mV
SENSEA GATE1A
GATE1A
DRIVE
CP1A CP2A
GATE CHARGE
PUMPS
GATE2A OUTA
GATE2A
DRIVE
UVLO
COMPARATOR
ONA
COMPARATOR
SHUTDOWN
MODE
1.24V
6.5V
FAST CURRENT-LIMIT
COMPARATOR
CURRENT-LIMIT
COMPARATOR
OR
COMPARATOR
INB - 75mV
INB - 50mV
INB - 5mV
GATE1B
DRIVE
SENSEB GATE1B
GATE CHARGE
PUMPS
CP1B CP2B
GATE2B
DRIVE
GATE2B OUTB
UVLO
COMPARATOR
ONB
COMPARATOR
SHUTDOWN
MODE
1.24V
6.5V
0.4V
x128
259ms
AUTORETRY
PERIOD
2.02ms FAULT
TIMEOUT
MAX5944
BATTERY SOURCE
ONQ1
ONA
INA SENSEA GATE1A
Q1 Q2
GATE2A OUTA
FAULTA
GND
FAULTB
INB SENSEB GATE1B GATE2B OUTB
POWER SOURCE
ONB
RSENSEA
Q3 Q4
RSENSEB
x128
2.02ms
FAULT
TIMEOUT
259ms
AUTORETRY
PERIOD
Figure 1. MAX5944 Functional Diagram
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
10 ______________________________________________________________________________________
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 VON_ 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 (VIN_) is above 7.5V
and VON_ is above the VON_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, VIS, 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 VIS is equal to VTH. In nor-
mal operation, VIS drops below VTH 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 VIS exceeds VOR, thereby
enhancing Q1_ and reducing the voltage drop, power
dissipation, and heat generation in the power-supply
pathway. When a voltage greater than VIN_ is connect-
ed at OUT_, the higher voltage source commences pro-
viding current to the load. The MAX5944 turns off
GATE1_ rapidly upon VIS falling below the VOR hystere-
sis, thus blocking the higher voltage from backdriving
VIN_. GATE1_ also turns off when the load current
drops, causing VIS to fall below VOR_ hysteresis.
Current Limiting
The MAX5944 limits the load current by monitoring the
voltage across RSENSE_ at all times and regulating the
current to the load as necessary to keep VIS to its cur-
rent-limit threshold voltage (VTH). Set the maximum cur-
rent limit (ILIMIT) by placing the appropriate sense
resistor between IN_ and SENSE_, where ILIMIT_ = VTH /
RSENSE_. When the load current is less than ILIMIT,
GATE2_ rises to VG2 (5.5V) to fully enhance MOSFET
Q2_ (see the Typical Operating Circuit).
When the load attempts to draw more current than
ILIMIT, the MAX5944’s GATE2_ pulldown current (IG2D)
regulates the current through Q2_, causing OUT_ to act
as a constant current source. The output current is limit-
ed to ILIMIT. If the current-limit condition persists after
the 2ms current-limit timeout period (tILIM) has expired,
the GATE2_ strong pulldown current (IG2_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 tILIM, GATE2_ pulls to
VGS2 (Figure 2b). During an output short circuit or a
gross overload, the load current overshoots and causes
VIS to exceed the fast pulldown current-limit threshold
voltage (VTHF). The MAX5944 responds with much
stronger GATE2_ pulldown currents (IG2DP and/or
IG2DF) 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 (VTH). Use the following formula to
calculate the maximum current:
ILIMIT = VTH / RSENSE_
where VTH is the 50mV (typ) threshold voltage across
RSENSE_, and RSENSE_ 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 VIS exceeds
the 5mV VOR threshold, enhancing Q1_. Q1_’s low
RDS(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, VIS, across the current-sens-
ing resistors. Q1_ turns off rapidly when VIS decreases
below the VOR 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 VUVLO and VON-REF, repectively, and the
device is not in a current-fault shutdown.
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
______________________________________________________________________________________ 11
When a higher voltage supply is connected at OUTA
(Figure 4), VIS drops below the ORing threshold. The
MAX5944 immediately turns off Q1A, blocking reverse-
current flow from OUTA to INA. Power is then routed
from the higher supply input (INB) to OUTB, and then to
the load (Figure 5).
Reverse-Current Override (ONQ1 = HIGH)
Drive ONQ1 high to disable the MAX5944 power-sup-
ply ORing. Disabling the MAX5944 ORing function
allows reverse-current flow through the power-supply
system. When ONQ1 is high, GATE1A and GATE1B
remain fully enhanced as long as their respective VIN_
is greater than 7.5V, VON_ is above the 1.24V threshold,
and the channels are not in a current-limit fault condi-
tion. The MAX5944 reverse-current flow feature permits
a higher voltage source connected at Port 1 to be rout-
ed to Port 2, as long as the voltage at both inputs (IN_)
is initially above the minimum 7.5V operating range,
see Figure 6.
IOUT
VIN
VOUT
VGATE1
VGATE2
VFAULT
IOUT
VOUT
VGATE1
VGATE2
VFAULT
tILIM
< tILIM ILIMIT
ILIMIT
ILIMIT x RLOAD
ILIMIT x RLOAD
(a)
(b)
Figure 2. MAX5944 Current-Limit Timing Diagram
Figure 4. MAX5944 ORing Controller Rerouting Load Current
SENSEA GATE1A GATE2A OUTAINA
ONA
ONQ1
GND
ONB
30mΩ
Q1A Q2A
30mΩQ1B Q2B
TO FireWire
PORT
SENSEB GATE1B GATE2B OUTBINB
MAX5944
FROM BATTERY
SOURCE
ILOAD
VINB > VINA
VINA
VINB
+ VIS -
Figure 3. Q1 Current Path During Inital Startup
SENSE_ GATE1_
GND ONQ1
GATE2_ OUT_
Q1A Q2A
IN_
TO
FireWire
PORT
MAX5944
ON_
VIN RSENSE
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
12 ______________________________________________________________________________________
Undervoltage Lockout
The MAX5944 dual-channel, independent current-limit
switches have independent ON/OFF control. Both
channels operate from 7.5V to 37V and have default
6.5V undervoltage lockout thresholds. The external
MOSFETs remain off as long as VIN_ < 6.5V and/or
VON_ < VON_REF. The UVLO thresholds are program-
mable by connecting a resistor-divider at ON_. When
VON_ > 1.24V and VIN_ > 7.5V, GATE2_ enhances to
5.5V, with respect to OUT_ and the MAX5944 switch
goes into normal operation.
Program the switches’ individual turn-on threshold volt-
ages using the following formula:
where VTURN-ON is the desired turn-on voltage of
channel A and/or B. Choose an R2 value in the 1kΩ
range to minimize error due to ON input bias current.
When operating the MAX5944, ON_ must remain above
its 1.24V threshold. If VON_ falls below its 1.24V thresh-
old, the power-supply MOSFETs turn off immediately
(less than 1µs). This allows the power supply to discon-
nect in the event of a fault condition detected by the
system, such as shorting of the output to a different
power-supply voltage or a FireWire signal pin. If possi-
ble transient conditions exist at the main power-supply
line (IN_), place an RC filter at ON_ to reject transient
voltage dips to prevent unnecessary power-supply
interruptions.
The MAX5944 remain in low-current shutdown mode
when VON_ is below the 0.4V threshold. Low-current
shutdown mode disables the channel resulting in less
than 10µA supply current.
Current-Fault Status Outputs
(FAULT_)
FAULT_ are high-voltage, open-drain outputs that pull
low when a current-limit fault shutdown has occurred.
FAULT_ remain low until the next startup cycle.
Autoretry Fault Management
The MAX5944 autoretry feature attempts to restart after
a tOFF of 259ms, limiting the duty cycle of the MOSFETs
RVxR
TURN ON
1124 12 . =−
⎛
⎝
⎜⎞
⎠
⎟
−
VINA
IINA
IINB
VINB
VINB > VINA
VOUT
Figure 5. MAX5944 Power Switch Over ORing Timing Diagram
MAX5944
PORT 2 PORT 1
ONB
INB
SENSEB
GATE1B
GATE2B
GND
ONQ1
OUTB
ONA
INA
SENSEA
GATE1A
GATE2A
OUTA
Q1
Q2
30mΩ30mΩ
Figure 6. MAX5944 Operating in Reverse-Current Mode
(Routing Power from Port 1 to Port 2)
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
______________________________________________________________________________________ 13
to 1/129 under continuous fault conditions. FAULT_
deasserts every time a restart attempt is made.
Applications Information
Startup Considerations
Set the appropriate current-limit threshold for a suc-
cessful MAX5944 startup. A successful startup is
dependent on the MAX5944 current-limit threshold and
timeout period. A large capacitor at OUT will result in a
charging current equivalent to the current-limit thresh-
old and may cause the MAX5944 to exceed its 2ms
timeout period, if the current-limit threshold is set too
low. Use the following formula to compute the minimum
current-limit setting:
where ILIMIT is the programmed current limit, COUT is
the capacitor at OUT, VIN is the supply voltage, tILIM is
the 2ms current-limit timeout period, and ILOAD is the
load current during startup. With VIN = 12V, COUT =
220µF, and ILOAD = 0, set the MAX5944 current limit
greater than 1.3A. This calculation does not include tol-
erances.
Choosing RSENSE
Select a sense resistor that causes the current-limit
voltage drop at a current-limit level above the maximum
normal operating current. Typically, set the current limit
at 1.2 to 1.5 times the nominal load current.
Choose the sense-resistor power rating to accommo-
date a current-limit condition:
PSENSE = (VTH)2/ RSENSE
where PSENSE is the power dissipated across RSENSE
during a current-limit fault.
MOSFET Selection
Select external MOSFETs according to the application
current level. The MOSFETs’ on-resistance (RDS(ON))
should be chosen low enough to have minimum voltage
drop at full load to limit the MOSFET power dissipation.
High RDS(ON) also causes large output ripple if there is
a pulsating load. Determine the device power rating to
accommodate a short-circuit condition on the board, at
startup, and when the device is in autoretry mode.
During normal operation, the external MOSFETs dissipate
little power. The power dissipated in normal operation is:
P = ILOAD2 x RDS(ON)
The most power dissipation will occur during a short-
circuit event, resulting in high power dissipated in Q2
(Figure 8) during the timeout period for the MAX5944,
where the power dissipated across Q2 is:
PQ2 = (VIN - VIS - VQ1) x ILIMIT
Transient Protection
If IN_ or OUT_ experiences a fast transient rise in volt-
age, the drain-to-gate overlap capacitance of GATE1_
and/or GATE2_ FETs may be sufficient to enhance one
of the transistors, allowing current to flow. If the circuit is
subjected to large transients, connect capacitors from
the gate to source across the appropriate MOSFET to
prevent the overlap capacitance from turning on the
device. This results in the turn-off time of the FETs to
increase due to the additional discharge of the capaci-
tor. Use the minimum capacitor value that will prevent
reverse currents from flowing in hot-plug situations.
FireWire Power Management
The MAX5944 serves to regulate and protect FireWire
power over a system interface. The MAX5944 program-
mable features that make it suitable for both power-
provider and power-receiver applications. Figure 9
shows a high-end two-port FireWire power-manage-
ment system using two MAX5944 dual-channel FireWire
current-limiting ICs.
ICV
tI
LIMIT
OUT IN
ILIM LOAD
>+
×
Figure 7. Setting the MAX5944 Turn-On Voltage
MAX5944
R2
R1
VIN
IN_
GND
ON_
VQ1 VQ2
VIS
SENSE_ GATE1_ GATE2_ OUT_IN_
+-
+- +-
VIN
MAX5944
ILIMIT
Q1 Q2
Figure 8. Power Dissipated Across MOSFET Q2 During a
Short-Circuit Fault
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
14 ______________________________________________________________________________________
Chip Information
TRANSISTOR COUNT: 4,648
PROCESS: BiCMOS
SENSEA GATE1A GATE2A OUTAINA
ONA
ONQ1
GND
ONB
RSENSEA Q1A Q2A
RSENSEB Q1B Q2B
TO FireWire
PORT 0
SENSEB GATE1B GATE2B OUTBINB
MAX5944
FAULTA
FAULTB
POWER SOURCE
TO FireWire
PORT 1
TO FireWire
PORT 0
TO FireWire
PORT 1
POWER SOURCE
EQUIVALENT CIRCUIT
VERY-LOW-DROP
ORing DIODE
CURRENT-LIMIT/
CIRCUIT-BREAKER
AND INRUSH CONTROL
LOAD DISCONECT
SWITCH
Typical Operating Circuit
15mΩIRF7478 IRF7478
SENSEA GATE1A GATE2A OUTAINA
ONA
ONQ1
GND
ONB
FAULTA
FAULTB
30mΩIRF7478 IRF7478
30mΩIRF7478 IRF7478
MAX5035
SGND
VIN
ON
FB
GND
VD
IN4148 IN4148
IN4148
BST
LX
PHY
POWER OUT
CABLE
ON
NOT
SLEEP
CURRENT-LIMIT
SENSE
PORT 1
SENSE
PORT 0
SENSE
CABLE
POWER
SOURCE
BATTERY-POWER
SOURCE
IN4148
CPA SENSE
PORT 0
PORT 1
SENSEB GATE1B GATE2B OUTBINB
SENSEA GATE1A GATE2A OUTAINA
ONA
ONQ1
GND
ONB
FAULTA
FAULTB
15mΩIRF7478 IRF7478
SENSEB GATE1B GATE2B OUTBINB
MAX5944 MAX5944
Figure 9. Two-Port FireWire Power Provider
MAX5944
Dual FireWire Current Limiter and Low-Drop
ORing Switch Controller
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 15
© 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages.)
SOICN .EPS
PACKAGE OUTLINE, .150" SOIC
1
1
21-0041 B
REV.DOCUMENT CONTROL NO.APPROVAL
PROPRIETARY INFORMATION
TITLE:
TOP VIEW
FRONT VIEW
MAX
0.010
0.069
0.019
0.157
0.010
INCHES
0.150
0.007
E
C
DIM
0.014
0.004
B
A1
MIN
0.053A
0.19
3.80 4.00
0.25
MILLIMETERS
0.10
0.35
1.35
MIN
0.49
0.25
MAX
1.75
0.050
0.016L0.40 1.27
0.3940.386D
D
MINDIM
D
INCHES
MAX
9.80 10.00
MILLIMETERS
MIN MAX
16 AC
0.337 0.344 AB8.758.55 14
0.189 0.197 AA5.004.80 8
N MS012
N
SIDE VIEW
H 0.2440.228 5.80 6.20
e 0.050 BSC 1.27 BSC
C
HE
eBA1
A
D
0∞-8∞
L
1
VARIATIONS: