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19-3426; Rev 0; 10/04
Dual FireWire Current Limiter and Low-Drop ORing Switch Controller
General Description
The MAX5944 is a dual, current limiter/low-drop ORing, switch controller power-management IC for FireWire (R) applications. Each independent channel controls two external power MOSFETs to regulate current from the input power supply to the load and performs low-voltagedrop power-supply ORing. The MAX5944 operates over the 7.5V to 37V input range and provides inrush and output current limit in addition to diode ORing functions suitable 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 functions. 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 protect 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 10A supply current per channel. The MAX5944 operates over the -40C to +85C extended temperature range and is available in a 16-pin SO package.
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 10A (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
Applications
FireWire Desktop/Notebook Port FireWire Peripheral Device Current Limit FireWire Hub Hot Swap
TOP VIEW
PART MAX5944ESE
Ordering Information
TEMP RANGE -40C to +85C PIN-PACKAGE 16 SO
Pin Configuration
INA 1 ONA 2 FAULTA 3 ONQ1 4 OUTB 5 GATE2B 6 GATE1B 7 16 SENSEA 15 GATE1A 14 GATE2A
MAX5944
13 OUTA 12 GND 11 FAULTB 10 ONB 9 INB
Typical Operating Circuit appears at end of data sheet. FireWire is a registered trademark of Apple Computer, Inc.
SENSEB 8
SO 1
________________________________________________________________ Maxim Integrated Products
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Dual FireWire Current Limiter and Low-Drop ORing Switch Controller MAX5944
ABSOLUTE MAXIMUM RATINGS
(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 = +70C) 16-Pin SO (derate 9.1mW/C above +70C)................728mW Operating Temperature Range ...........................-40C to +85C Maximum Junction Temperature .....................................+150C Storage Temperature Range .............................-60C to +150C ESD Rating (Human Body Model)......................................2000V Lead Temperature (soldering, 10s) .................................+300C
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.
ELECTRICAL CHARACTERISTICS
(VIN = 7.5V to 37V, GND = 0V, and TA = -40C to +85C, unless otherwise noted. Typical values are at VIN_ = 12V, TA = +25C.) (Note 1)
PARAMETER SUPPLIES Supply Voltage Range Supply Current Shutdown Current IN_ Default Undervoltage Lockout IN_ Default UndervoltageLockout Hysteresis ON_ Undervoltage-Lockout Threshold ON_ Undervoltage-LockoutThreshold Hysteresis ON_ Input Bias Current GATE2_ (Current-Limit Control) GATE2_ Pullup Current (Note 2) GATE2_ High Voltage Current-Limit Threshold Voltage Fast Pulldown Current-Limit Threshold Voltage (Note 3) GATE2_ Pulldown Current GATE2_ Fast Pulldown Current (Note 3) IG2U VG2 VTH VTHF IG2D IG2DF VGATE2_ - VOUT_ VIN_ - VSENSE_ VIN_ - VSENSE_ VTH < (VIN_ - VSENSE_) < VTHF (VIN_ - VSENSE_) > VTHF, VGATE2_ = (VOUT_ + 4.5V) 7.5V VIN 8V 8V VIN 37V 36 4.25 4.5 47.5 70 335 80 45 5.5 5.5 50 75 435 125 54 6.50 6.5 52.5 80 535 200 A V mV mV A mA ION VON_REF VON_ rising 1.20 VIN_ IIN ISD VUVLO VON_ = 3V, no load, per channel VON_ = 0.4V, per channel VIN_ rising, ON_ = IN_ 6.2 7.5 1.3 5 6.5 0.35 1.24 70 1 1.275 37 2.3 10 6.8 V mA A V V V mV A SYMBOL CONDITIONS MIN TYP MAX UNITS
2
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Dual FireWire Current Limiter and Low-Drop ORing Switch Controller
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 7.5V to 37V, GND = 0V, and TA = -40C to +85C, unless otherwise noted. Typical values are at VIN_ = 12V, TA = +25C.) (Note 1)
PARAMETER GATE2_ Peak Pulldown Current GATE2_ Pulldown Current During Any GATE2_ Turn-Off Condition (Note 4) GATE1_ (ORing Control) ORing Switch Turn-On-Current Threshold Voltage ORing Switch Turn-On-Current Threshold Voltage Hysteresis GATE1_ Pullup Current (Note 5) GATE1_ High Voltage GATE1_ Turn-Off Switch OnResistance LOGIC INPUTS ONQ1 Input-Logic Low Voltage ONQ1 Input-Logic High Voltage ONQ1 Input Current FAULT_ Output Low Voltage FAULT_ Output High Leakage Current OUT_ Input Current VIL VIH IIH VOL IOH IOUT Low-impedance state, IFAULT_ = 5mA High-impedance state, VFAULT_ = 37V VON_ = GND 0.12 VON_ > VON_REF VON_ > VON_REF 1.8 1 0.25 1 1 0.7 V V A V A A IG1U VG1 RG1PD VGATE_ - VSENSE_ 7.5V VIN 8V 8V VIN 37V VOR (VIN_ - VSENSE_) rising (VIN_ - VSENSE_) falling 36 4.25 4.5 3 5 0.5 45 5.5 5.5 2 54 6.50 6.5 3 7 mV mV A V SYMBOL IG2DP IG2OFF CONDITIONS (VIN_ - VSENSE_) = 1V VGATE2_ = (VOUT_ + 4.5V) 80 MIN TYP 1.0 125 200 MAX UNITS A mA
MAX5944
Measured between GATE1_ and SENSE_ when (VIN_ - VSENSE_) < VOR
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3
Dual FireWire Current Limiter and Low-Drop ORing Switch Controller MAX5944
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 7.5V to 37V, GND = 0V, and TA = -40C to +85C, unless otherwise noted. Typical values are at VIN_ = 12V, TA = +25C.) (Note 1)
PARAMETER TIMING Current-Limit Timeout Automatic Restart Delay After Current-Limit Timeout (Note 7) GATE1_ Turn-Off Time (ORing Response Time) Turn-Off Response Time (Note 7) Minimum Delay from ON_ Low to Low-Current Shutdown Mode tILIM tOFF tOR From VOR trigger until (VGATE1_ - VIN_) < 1V, CGATE1_ = 10nF Time from VON_ < (VON_OFF - Hysteresis) until (VGATE2_ - VOUT_) < 1V, for CGATE2_ = open ON_ step from 1.3V to 0.4V Current-limit mode 1.78 240 2.02 259 100 2.26 278 ms ms ns SYMBOL CONDITIONS MIN TYP MAX UNITS
tON_OFF
0.34
0.6
s
tSD
50
s
Note 1: Note 2: Note 3: Note 4: Note 5: Note 6: Note 7:
All min/max parameters are tested at +25C and +85C. Limits at -40C are guaranteed by design. Measured at both VGATE2 = VOUT = 0V and VGATE2 = VOUT = VIN. The typical value of parameter VTHF is 1.5 x VTH. Current-Limit Timeout. IN_ or ON_ low. See the Typical Operating Circuit. Measured at VGATE_ = VSENSE_. The tristate ("floating") logic condition will be entered if the pin current is within these limits. If the ON_ voltages go below VON_REF - Hysteresis, then GATE2_ goes low immediately to disconnect power from the load quickly (within tON_OFF).
Typical Operating Characteristics
(VIN_ = +12V, TA = +25C, unless otherwise noted.)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX5944 toc01
SHUTDOWN SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX5944 toc02
CURRENT-LIMIT/CIRCUIT-BREAKER THRESHOLD VOLTAGE vs. TEMPERATURE
51.5 51.0 50.5 50.0 49.5 49.0 48.5 48.0 47.5 47.0 46.5 46.0 VIN = 7.5V VIN = 12V
MAX5944 toc03
2.0 1.9 1.8 SUPPLY CURRENT (mA) 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 5 10 15 20 25 30 35 TA = -40C TA = +85C TA = +25C
SHUTDOWN SUPPLY CURRENT (A)
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0
THRESHOLD VOLTAGE (mV)
6.0 5.5 5.0
52.0 VIN = 32V
40
5
10
15
20
25
30
35
40
-40
-15
10
35
60
85
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
TEMPERATURE (C)
4
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Dual FireWire Current Limiter and Low-Drop ORing Switch Controller
Typical Operating Characteristics (continued)
(VIN_ = +12V, TA = +25C, unless otherwise noted.)
MAX5944
CURRENT-LIMIT TIMEOUT vs. TEMPERATURE
MAX5944 toc04
ORing THRESHOLD VOLTAGE vs. TEMPERATURE
MAX5944 toc05
GATE CURRENTS vs. TEMPERATURE
MAX5944 toc06
2.30 2.25 2.20 2.15 2.10 2.05 2.00 -40 -15 10 35 60
5.5 5.4 ORing THRESHOLD VOLTAGE (mV) 5.3 5.2 5.1 5.0 4.9 4.8 4.7 4.6 4.5
47.0 46.5 GATE CURRENTS (A) 46.0 45.5 45.0 GATE2 44.5 44.0 GATE1
CURRENT-LIMIT TIMEOUT (ms)
85
-40
-15
10
35
60
85
-40
-15
10
35
60
85
TEMPERATURE (C)
TEMPERATURE (C)
TEMPERATURE (C)
GATE1 DRIVE VOLTAGE vs. TEMPERATURE
MAX5944 toc07
GATE2 DRIVE VOLTAGE vs. TEMPERATURE
MAX5944 toc08
ON HIGH VOLTAGE vs. TEMPERATURE
MAX5944 toc09
5.260 5.257 GATE1 DRIVE VOLTAGE (V) 5.254 5.251 5.248 5.245 5.242 5.239 5.236 5.233 5.230 -40 -15 10 35 60
5.230 5.227 GATE2 DRIVE VOLTAGE (V) 5.224 5.221 5.218 5.215 5.212 5.209 5.206 5.203 5.200
1.260
1.258 ON HIGH VOLTAGE (V)
1.256
1.254
1.252
1.250 -40 -15 10 35 60 85 -40 -15 10 35 60 85 TEMPERATURE (C) TEMPERATURE (C)
85
TEMPERATURE (C)
ON LOW VOLTAGE vs. TEMPERATURE
MAX5944 toc10
FAULT LOW VOLTAGE vs. TEMPERATURE
180 FAULT LOW VOLTAGE (mV) 160 140 120 100 80 60 40 20 0 IFAULT = 5mA
MAX5944 toc11
1.200 1.198 1.196 ON LOW VOLTAGE (V) 1.194 1.192 1.190 1.188 1.186 1.184 1.182 1.180 -40 -15 10 35 60
200
85
-40
-15
10
35
60
85
TEMPERATURE (C)
TEMPERATURE (C)
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5
Dual FireWire Current Limiter and Low-Drop ORing Switch Controller MAX5944
Typical Operating Characteristics (continued)
(VIN_ = +12V, TA = +25C, unless otherwise noted.)
ORing THRESHOLD RESPONSE TIME
MAX5944 toc12
TURN-OFF RESPONSE TIME
MAX5944 toc13
VIN 10V/div IIN 500mA/div (VGATE - VSENSE) 5V/div VOUT 5V/div
VON 2V/div IIN 1A/div VGATE2 10V/div
VOUT 10V/div 100s/div 200ns/div
SHORT-CIRCUIT EVENT (VIN = 37V)
MAX5944 toc14
SHORT-CIRCUIT EVENT (VIN = 37V)
VIN 50V/div IIN 1A/div
MAX5944 toc15
VIN 50V/div IIN 10A/div
VGATE2 50V/div VOUT 50V/div VFAULT 50V/div 400s/div 100ns/div
VGATE2 50V/div VOUT 50V/div VFAULT 50V/div
SHORT-CIRCUIT RESPONSE (VIN = 8V)
MAX5944 toc16
SHORT-CIRCUIT RESPONSE (VIN = 8V)
VIN 10V/div IIN 1A/div
MAX5944 toc17
VIN 10V/div IIN 10A/div
VGATE2 10V/div VOUT 10V/div VFAULT 10V/div 400s/div 100ns/div
VGATE2 10V/div VOUT 10V/div VFAULT 10V/div
6
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Dual FireWire Current Limiter and Low-Drop ORing Switch Controller MAX5944
Typical Operating Characteristics (continued)
(VIN_ = +12V, TA = +25C, unless otherwise noted.)
ORing SWITCHOVER WAVEFORM (VINB RISING ABOVE VINA)
VOUT 1V/div
ORing SWITCHOVER WAVEFORM
MAX5944 toc18
MAX5944 toc19
VOUT 1V/div
IINA 500mA/div
IINA 500mA/div
IINB 500mA/div 200s/div VONA RISING = 12V COUT = 0 10s/div VONB RISING = 12V COUT = 0
IINB 500mA/div
VINA = 12V VINB = 12.8V
VINA = 12V VINB = 13V
ORing SWITCHOVER WAVEFORM (VINB FALLING BELOW VINA)
HOT PLUG-IN LOAD CAPACITANCE INTO ACTIVE PORT (NO FAULT)
MAX5944 toc21
MAX5944 toc20
VOUT 1V/div
VFAULT 10V/div
IIN 2A/div IINA 500mA/div VGATE2 10V/div IINB 500mA/div 20s/div VONB FALLING = 12V COUT = 0 400s/div CLOAD = 220F VOUT 10V/div
VINA = 12V VINB = 13V
HOT PLUG-IN LOAD CAPACITANCE INTO ACTIVE PORT (CURRENT-LIMIT TIMEOUT)
MAX5944 toc22
VFAULT 10V/div IIN 1A/div VGATE2 10V/div
VOUT 10V/div 400s/div CLOAD = 470F
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7
Dual FireWire Current Limiter and Low-Drop ORing Switch Controller MAX5944
Pin Description
PIN 1 NAME INA FUNCTION Channel A Input Power and Current-Sense Positive Input. Connect INA to the other end of the currentsense resistor and to the power source (see the Typical Operating Circuit). 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. Channel A Current-Fault Status Output. FAULTA is an open-drain output. FAULTA goes low after a currentlimit fault has exceeded the 2ms (typ) current-limit timeout period (see the FAULT_ section). 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. Channel B Output-Voltage Sense. Connect to the output. 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 50A pullup current to 5.5V (typ) above OUTB when active (see the Typical Operating Circuit). 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 50A pullup current to 5.5V (typ) above SENSEB when active (see the Typical Operating Circuit). Channel B Current-Sense Negative Input. Connect current-sense resistor, RSENSE, from INB to SENSEB. Channel B Input Power and Current-Sense Positive Input. Connect INB to the other end of the currentsense resistor and to the power source. 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. Channel B Current-Fault Status Output. FAULTB is an open-drain output. FAULTB goes low after a currentlimit fault has exceeded the 2ms (typ) current-limit timeout period (see the FAULT_ section). Ground Channel A Output-Voltage Sense. Connect to the output. 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 50A pullup current to 5.5V (typ) above OUTA when active. 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 50A pullup current to 5.5V (typ) above SENSEA when active. Channel A Current-Sense Negative Input. Connect current-sense resistor, RSENSEA, from INA to SENSEA (see the Typical Operating Circuit).
2
ONA
3
FAULTA
4
ONQ1
5 6
OUTB GATE2B
7 8 9
GATE1B SENSEB INB
10
ONB
11 12 13 14
FAULTB GND OUTA GATE2A
15
GATE1A
16
SENSEA
8
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Dual FireWire Current Limiter and Low-Drop ORing Switch Controller MAX5944
RSENSEA BATTERY SOURCE Q1 Q2
INA ONQ1 OR COMPARATOR INA - 5mV
SENSEA GATE1A
GATE2A OUTA
SENSEA GATE1A GATE1A DRIVE
GATE2A
OUTA
GATE2A DRIVE
MAX5944
CURRENT-LIMIT COMPARATOR
2.02ms FAULT TIMEOUT
INA - 50mV FAST CURRENT-LIMIT COMPARATOR UVLO COMPARATOR
CP1A
CP2A x128
GATE CHARGE PUMPS
INA - 75mV
259ms AUTORETRY PERIOD
6.5V ONA COMPARATOR ONA 1.24V SHUTDOWN MODE 0.4V SHUTDOWN MODE 1.24V ONB ONB COMPARATOR 6.5V FAULTB GND FAULTA
UVLO COMPARATOR
INB - 5mV
OR COMPARATOR INB - 50mV
GATE CHARGE PUMPS CP1B CP2B
259ms AUTORETRY PERIOD
x128 CURRENT-LIMIT COMPARATOR INB - 75mV GATE1B DRIVE FAST CURRENT-LIMIT COMPARATOR SENSEB GATE1B GATE2B DRIVE GATE2B OUTB 2.02ms FAULT TIMEOUT
INB POWER SOURCE RSENSEB
SENSEB GATE1B
GATE2B OUTB
Q3
Q4
Figure 1. MAX5944 Functional Diagram _______________________________________________________________________________________ 9
Dual FireWire Current Limiter and Low-Drop ORing Switch Controller MAX5944
Detailed Description
The MAX5944 dual-channel controller IC performs hotswapping, 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 resistor 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-current shutdown mode when V ON_ is below the 0.4V threshold. Low-current shutdown mode disables the MAX5944 channels resulting in less than 10A 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 V ON_ is above the V ON_REF threshold, the MAX5944 channel turns on, sourcing 50A 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 normal 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 conducts 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 VIN_ is connected at OUT_, the higher voltage source commences providing current to the load. The MAX5944 turns off GATE1_ rapidly upon VIS falling below the VOR hysteresis, thus blocking the higher voltage from backdriving V IN_ . GATE1_ also turns off when the load current drops, causing VIS to fall below VOR_ hysteresis. 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 limited 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 current-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 (V THF ). The MAX5944 responds with much stronger GATE2_ pulldown currents (I G2DP 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 current conducts through the body diode of Q1_, Figure 3. GATE1_ rises to 5.5V above SENSE_ when VIS exceeds the 5mV V OR 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-sensing 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.
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 current-limit threshold voltage (VTH). Set the maximum current limit (I LIMIT ) by placing the appropriate sense resistor between IN_ and SENSE_, where ILIMIT_ = VTH / R SENSE_. When the load current is less than I LIMIT,
10
______________________________________________________________________________________
Dual FireWire Current Limiter and Low-Drop ORing Switch Controller MAX5944
VIN
VFAULT tILIM ILIMIT
RSENSE Q1A Q2A
TO FireWire PORT
ON_ IN_
SENSE_
GATE1_
GATE2_
OUT_
IOUT
MAX5944
VIN
GND
ONQ1
VGATE1
Figure 3. Q1 Current Path During Inital Startup
VGATE2
FROM BATTERY SOURCE VINA + VIS 30m Q1A Q2A VINB > VINA ILOAD INA ONA ONQ1 SENSEA GATE1A GATE2A OUTA TO FireWire PORT
VOUT
ILIMIT x RLOAD
(a)
MAX5944
GND
VFAULT < tILIM ILIMIT
ONB INB SENSEB GATE1B GATE2B OUTB
VINB 30m
Q1B
Q2B
IOUT
Figure 4. MAX5944 ORing Controller Rerouting Load Current
VGATE1
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-supply 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 condition. The MAX5944 reverse-current flow feature permits a higher voltage source connected at Port 1 to be routed to Port 2, as long as the voltage at both inputs (IN_) is initially above the minimum 7.5V operating range, see Figure 6.
VGATE2 ILIMIT x RLOAD VOUT
(b)
Figure 2. MAX5944 Current-Limit Timing Diagram
When a higher voltage supply is connected at OUTA (Figure 4), VIS drops below the ORing threshold. The MAX5944 immediately turns off Q1A, blocking reversecurrent flow from OUTA to INA. Power is then routed
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11
Dual FireWire Current Limiter and Low-Drop ORing Switch Controller MAX5944
VINA
ONQ1 ONB
IINA
ONA INA 30m
INB 30m
MAX5944
IINB
SENSEB GATE1B
SENSEA GATE1A Q1
GATE2B
VOUT
GATE2A OUTA GND
Q2
OUTB
VINB VINB > VINA
PORT 2
PORT 1
Figure 5. MAX5944 Power Switch Over ORing Timing Diagram
Figure 6. MAX5944 Operating in Reverse-Current Mode (Routing Power from Port 1 to Port 2)
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 programmable 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 voltages using the following formula: V R1 = TURN-ON - 1 x R2 1.24 where V TURN-ON is the desired turn-on voltage of channel A and/or B. Choose an R2 value in the 1k 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 thresh12
old, the power-supply MOSFETs turn off immediately (less than 1s). This allows the power supply to disconnect 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 possible 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 10A 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
______________________________________________________________________________________
Dual FireWire Current Limiter and Low-Drop ORing Switch Controller
MOSFET Selection
VIN IN_ R1
MAX5944
MAX5944
ON_ R2 GND
Figure 7. Setting the MAX5944 Turn-On Voltage
to 1/129 under continuous fault conditions. FAULT_ deasserts every time a restart attempt is made.
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 shortcircuit 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
Applications Information
Startup Considerations
Set the appropriate current-limit threshold for a successful 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 threshold 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: ILIMIT > COUT x VIN + ILOAD tILIM
Transient Protection
If IN_ or OUT_ experiences a fast transient rise in voltage, 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 capacitor. 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 programmable features that make it suitable for both powerprovider and power-receiver applications. Figure 9 shows a high-end two-port FireWire power-management system using two MAX5944 dual-channel FireWire current-limiting ICs.
ILIMIT + VQ1 Q1 IN_ SENSE_ GATE1_ + VQ2 Q2 GATE2_ OUT_
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 = 220F, and ILOAD = 0, set the MAX5944 current limit greater than 1.3A. This calculation does not include tolerances.
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 accommodate a current-limit condition: PSENSE = (VTH)2 / RSENSE where PSENSE is the power dissipated across RSENSE during a current-limit fault.
VIN
+
VIS
-
MAX5944
Figure 8. Power Dissipated Across MOSFET Q2 During a Short-Circuit Fault 13
______________________________________________________________________________________
Dual FireWire Current Limiter and Low-Drop ORing Switch Controller MAX5944
FB BST VIN ON
CPA SENSE
MAX5035
PHY POWER OUT LX IN4148 VD GND SGND IN4148
IN4148 CABLE POWER SOURCE INA ONA ONQ1 GND ONB INB SENSEB GATE1B GATE2B OUTB FAULTA 15m IRF7478 IRF7478 30m IRF7478 IRF7478
PORT 0
SENSEA GATE1A
GATE2A OUTA ONA
INA ONQ1 GND ONB INB
SENSEA GATE1A
GATE2A OUTA
FAULTA
MAX5944
FAULTB
MAX5944
FAULTB
SENSEB GATE1B
GATE2B OUTB
BATTERY-POWER SOURCE
15m
IRF7478
IRF7478
30m
IRF7478
IRF7478 IN4148
PORT 1
CABLE NOT ON SLEEP
CURRENT-LIMIT SENSE
PORT 1 PORT 0 SENSE SENSE
Figure 9. Two-Port FireWire Power Provider
Typical Operating Circuit
RSENSEA POWER SOURCE Q1A Q2A TO FireWire PORT 0
EQUIVALENT CIRCUIT FAULTA ONA ONQ1 POWER SOURCE CURRENT-LIMIT/ CIRCUIT-BREAKER AND INRUSH CONTROL INA SENSEA GATE1A GATE2A OUTA TO FireWire PORT 0 LOAD DISCONECT SWITCH TO FireWire PORT 1 VERY-LOW-DROP ORing DIODE
MAX5944
GND ONB FAULTB INB SENSEB GATE1B GATE2B OUTB
RSENSEB
Q1B
Q2B
TO FireWire PORT 1
Chip Information
TRANSISTOR COUNT: 4,648 PROCESS: BiCMOS
14 ______________________________________________________________________________________
Dual FireWire Current Limiter and Low-Drop ORing Switch Controller
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
MAX5944
INCHES DIM A A1 B C e E H L MAX MIN 0.053 0.069 0.010 0.004 0.014 0.019 0.007 0.010 0.050BSC 0.150 0.157 0.228 0.244 0.016 0.050
MILLIMETERS MAX MIN 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 1.27BSC 3.80 4.00 5.80 6.20 0.40 1.27
N
E
H
VARIATIONS:
1
INCHES
MILLIMETERS MIN 4.80 8.55 9.80 MAX 5.00 8.75 10.00 N MS012 8 AA 14 AB 16 AC
TOPVIEW
DIM D D D
MIN 0.189 0.337 0.386
MAX 0.197 0.344 0.394
D A e B A1 L C
0-8
FRONTVIEW
SIDEVIEW
PROPRIETARYINFORMATION TITLE:
PACKAGEOUTLINE,.150"SOIC
APPROVAL DOCUMENTCONTROLNO. REV.
21-0041
B
1 1
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 (c) 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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