19-3426; Rev 0; 10/04 Dual FireWire Current Limiter and Low-Drop ORing Switch Controller 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 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. 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 Applications FireWire Desktop/Notebook Port FireWire Peripheral Device Current Limit Ordering Information PART MAX5944ESE TEMP RANGE PIN-PACKAGE -40°C to +85°C 16 SO FireWire Hub Pin Configuration Hot Swap TOP VIEW INA 1 16 SENSEA ONA 2 15 GATE1A FAULTA 3 14 GATE2A ONQ1 4 MAX5944 OUTB 5 Typical Operating Circuit appears at end of data sheet. FireWire is a registered trademark of Apple Computer, Inc. 13 OUTA 12 GND GATE2B 6 11 FAULTB GATE1B 7 10 ONB SENSEB 8 9 INB SO ________________________________________________________________ Maxim Integrated Products 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. 1 MAX5944 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-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. MAX5944 Dual FireWire Current Limiter and Low-Drop ORing Switch Controller 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 = +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 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 = -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 37 V 1.3 2.3 mA 5 10 µA 6.5 6.8 V SUPPLIES Supply Voltage Range VIN_ 7.5 Supply Current IIN VON_ = 3V, no load, per channel Shutdown Current IN_ Default Undervoltage Lockout ISD VON_ = 0.4V, per channel VUVLO VIN_ rising, ON_ = IN_ 6.2 IN_ Default UndervoltageLockout Hysteresis ON_ Undervoltage-Lockout Threshold 0.35 VON_REF VON_ rising 1.20 ON_ Undervoltage-LockoutThreshold Hysteresis ON_ Input Bias Current 1.24 V 1.275 70 ION V mV 1 µA µA GATE2_ (Current-Limit Control) GATE2_ Pullup Current (Note 2) GATE2_ High Voltage VG2 VGATE2_ - VOUT_ Current-Limit Threshold Voltage VTH VIN_ - VSENSE_ 36 4.25 4.5 47.5 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 2 IG2U 7.5V ≤ VIN ≤ 8V 8V ≤ VIN ≤ 37V 45 5.5 5.5 50 54 6.50 6.5 52.5 mV _______________________________________________________________________________________ V Dual FireWire Current Limiter and Low-Drop ORing Switch Controller (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 1.0 UNITS GATE2_ Peak Pulldown Current IG2DP (VIN_ - VSENSE_) = 1V A GATE2_ Pulldown Current During Any GATE2_ Turn-Off Condition (Note 4) IG2OFF VGATE2_ = (VOUT_ + 4.5V) 80 125 200 mA (VIN_ - VSENSE_) rising 3 5 7 mV GATE1_ (ORing Control) ORing Switch Turn-On-Current Threshold Voltage VOR ORing Switch Turn-On-Current Threshold Voltage Hysteresis GATE1_ Pullup Current (Note 5) GATE1_ High Voltage GATE1_ Turn-Off Switch OnResistance (VIN_ - VSENSE_) falling 0.5 IG1U VG1 RG1PD VGATE_ - VSENSE_ mV 7.5V ≤ VIN ≤ 8V 36 4.25 45 5.5 54 6.50 8V ≤ VIN ≤ 37V 4.5 5.5 6.5 2 3 Measured between GATE1_ and SENSE_ when (VIN_ - VSENSE_) < VOR µA V Ω LOGIC INPUTS ONQ1 Input-Logic Low Voltage VIL VON_ > VON_REF ONQ1 Input-Logic High Voltage VIH VON_ > VON_REF ONQ1 Input Current IIH 0.7 1.8 V V 1 µA 0.25 V FAULT_ Output Low Voltage VOL Low-impedance state, IFAULT_ = 5mA FAULT_ Output High Leakage Current IOH High-impedance state, VFAULT_ = 37V 1 µA OUT_ Input Current IOUT VON_ = GND 1 µA 0.12 _______________________________________________________________________________________ 3 MAX5944 ELECTRICAL CHARACTERISTICS (continued) 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 1.78 2.02 2.26 ms 240 259 278 ms TIMING Current-Limit Timeout tILIM Automatic Restart Delay After Current-Limit Timeout (Note 7) tOFF GATE1_ Turn-Off Time (ORing Response Time) tOR Turn-Off Response Time (Note 7) tON_OFF Minimum Delay from ON_ Low to Low-Current Shutdown Mode Note 1: Note 2: Note 3: Note 4: Note 5: Note 6: Note 7: Current-limit mode From VOR trigger until (VGATE1_ - VIN_) < 1V, CGATE1_ = 10nF 100 Time from VON_ < (VON_OFF - Hysteresis) until (VGATE2_ - VOUT_) < 1V, for CGATE2_ = open 0.34 ON_ step from 1.3V to 0.4V tSD ns 0.6 µs 50 µs All min/max parameters are tested at +25°C and +85°C. Limits at -40°C 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 = +25°C, unless otherwise noted.) 1.7 TA = +85°C 1.5 TA = +25°C 1.4 1.3 1.2 1.1 TA = -40°C 10 15 20 25 30 SUPPLY VOLTAGE (V) 4 4.0 3.5 3.0 2.5 2.0 1.5 1.0 35 40 51.5 51.0 50.5 VIN = 32V 50.0 49.5 49.0 48.5 VIN = 7.5V VIN = 12V 48.0 47.5 47.0 46.5 46.0 0.5 0 1.0 5 4.5 52.0 THRESHOLD VOLTAGE (mV) 1.8 6.0 5.5 5.0 MAX5944 toc02 1.9 SHUTDOWN SUPPLY CURRENT (µA) MAX5944 toc01 2.0 1.6 CURRENT-LIMIT/CIRCUIT-BREAKER THRESHOLD VOLTAGE vs. TEMPERATURE SHUTDOWN SUPPLY CURRENT vs. SUPPLY VOLTAGE MAX5944 toc03 SUPPLY CURRENT vs. SUPPLY VOLTAGE SUPPLY CURRENT (mA) MAX5944 Dual FireWire Current Limiter and Low-Drop ORing Switch Controller 5 10 15 20 25 30 SUPPLY VOLTAGE (V) 35 40 -40 -15 10 35 TEMPERATURE (°C) _______________________________________________________________________________________ 60 85 Dual FireWire Current Limiter and Low-Drop ORing Switch Controller 2.15 2.10 2.05 5.4 5.3 46.5 GATE CURRENTS (µA) 2.20 47.0 MAX5944 toc05 2.25 GATE CURRENTS vs. TEMPERATURE 5.5 ORing THRESHOLD VOLTAGE (mV) MAX5944 toc04 CURRENT-LIMIT TIMEOUT (ms) 2.30 ORing THRESHOLD VOLTAGE vs. TEMPERATURE MAX5944 toc06 CURRENT-LIMIT TIMEOUT vs. TEMPERATURE 5.2 5.1 5.0 4.9 4.8 GATE1 46.0 45.5 45.0 GATE2 4.7 44.5 4.6 4.5 -15 10 35 60 44.0 -40 -15 10 35 TEMPERATURE (°C) GATE1 DRIVE VOLTAGE vs. TEMPERATURE GATE2 DRIVE VOLTAGE vs. TEMPERATURE 5.248 5.245 5.242 5.239 5.224 5.218 5.215 5.212 5.209 5.206 5.233 5.203 5.230 10 35 60 85 TEMPERATURE (°C) 10 35 1.254 60 85 -15 -40 TEMPERATURE (°C) 200 MAX5944 toc10 1.198 180 FAULT LOW VOLTAGE (mV) 1.196 10 35 60 85 TEMPERATURE (°C) FAULT LOW VOLTAGE vs. TEMPERATURE ON LOW VOLTAGE vs. TEMPERATURE ON LOW VOLTAGE (V) 1.256 1.250 -15 -40 1.200 85 1.194 1.192 1.190 1.188 1.186 MAX5944 toc11 -15 60 1.252 5.200 -40 35 1.258 5.221 5.236 10 1.260 ON HIGH VOLTAGE (V) 5.251 -15 ON HIGH VOLTAGE vs. TEMPERATURE 5.227 GATE2 DRIVE VOLTAGE (V) 5.254 -40 TEMPERATURE (°C) 5.230 MAX5944 toc07 5.257 85 60 TEMPERATURE (°C) 5.260 GATE1 DRIVE VOLTAGE (V) 85 MAX5944 toc08 -40 MAX5944 toc09 2.00 IFAULT = 5mA 160 140 120 100 80 60 1.184 40 1.182 20 0 1.180 -40 -15 10 35 TEMPERATURE (°C) 60 85 -40 -15 10 35 60 85 TEMPERATURE (°C) _______________________________________________________________________________________ 5 MAX5944 Typical Operating Characteristics (continued) (VIN_ = +12V, TA = +25°C, unless otherwise noted.) MAX5944 Dual FireWire Current Limiter and Low-Drop ORing Switch Controller Typical Operating Characteristics (continued) (VIN_ = +12V, TA = +25°C, unless otherwise noted.) TURN-OFF RESPONSE TIME ORing THRESHOLD RESPONSE TIME MAX5944 toc12 MAX5944 toc13 VIN 10V/div VON 2V/div IIN 500mA/div IIN 1A/div (VGATE - VSENSE) 5V/div VGATE2 10V/div VOUT 5V/div VOUT 10V/div 100µs/div 200ns/div SHORT-CIRCUIT EVENT (VIN = 37V) SHORT-CIRCUIT EVENT (VIN = 37V) MAX5944 toc14 VIN 50V/div VIN 50V/div IIN 1A/div IIN 10A/div VGATE2 50V/div VGATE2 50V/div VOUT 50V/div VFAULT 50V/div VOUT 50V/div VFAULT 50V/div 400µs/div 100ns/div SHORT-CIRCUIT RESPONSE (VIN = 8V) SHORT-CIRCUIT RESPONSE (VIN = 8V) MAX5944 toc16 400µs/div 6 MAX5944 toc15 MAX5944 toc17 VIN 10V/div VIN 10V/div IIN 1A/div IIN 10A/div VGATE2 10V/div VGATE2 10V/div VOUT 10V/div VOUT 10V/div VFAULT 10V/div VFAULT 10V/div 100ns/div _______________________________________________________________________________________ Dual FireWire Current Limiter and Low-Drop ORing Switch Controller ORing SWITCHOVER WAVEFORM (VINB RISING ABOVE VINA) ORing SWITCHOVER WAVEFORM MAX5944 toc19 MAX5944 toc18 VINA = 12V VINB = 12.8V VOUT 1V/div VOUT 1V/div IINA 500mA/div IINA 500mA/div IINB 500mA/div IINB 500mA/div 200µs/div VONA RISING = 12V COUT = 0 VINA = 12V VINB = 13V ORing SWITCHOVER WAVEFORM (VINB FALLING BELOW VINA) 10µs/div VONB RISING = 12V COUT = 0 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 VINA = 12V VINB = 13V 20µs/div VONB FALLING = 12V COUT = 0 VOUT 10V/div 400µs/div CLOAD = 220µF 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 400µs/div CLOAD = 470µF _______________________________________________________________________________________ 7 MAX5944 Typical Operating Characteristics (continued) (VIN_ = +12V, TA = +25°C, unless otherwise noted.) Dual FireWire Current Limiter and Low-Drop ORing Switch Controller MAX5944 Pin Description PIN FUNCTION INA 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). 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 currentlimit 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 currentsense 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 currentlimit fault has exceeded the 2ms (typ) current-limit timeout period (see the FAULT_ section). 1 8 NAME 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). _______________________________________________________________________________________ Dual FireWire Current Limiter and Low-Drop ORing Switch Controller Q1 Q2 SENSEA GATE1A INA MAX5944 RSENSEA BATTERY SOURCE GATE2A OUTA ONQ1 OR COMPARATOR SENSEA GATE1A INA - 5mV OUTA GATE2A DRIVE CURRENT-LIMIT COMPARATOR MAX5944 2.02ms FAULT TIMEOUT INA - 50mV CP1A FAST CURRENT-LIMIT COMPARATOR UVLO COMPARATOR GATE2A GATE1A DRIVE CP2A x128 GATE CHARGE PUMPS 259ms AUTORETRY PERIOD INA - 75mV 6.5V FAULTA ONA COMPARATOR ONA 1.24V SHUTDOWN MODE GND 0.4V SHUTDOWN MODE 1.24V ONB ONB COMPARATOR FAULTB 6.5V UVLO COMPARATOR INB - 5mV OR COMPARATOR GATE CHARGE PUMPS CP1B INB - 50mV 259ms AUTORETRY PERIOD CP2B x128 CURRENT-LIMIT COMPARATOR INB - 75mV GATE1B DRIVE FAST CURRENT-LIMIT COMPARATOR GATE2B DRIVE SENSEB GATE1B GATE2B SENSEB GATE1B INB POWER SOURCE RSENSEB Q3 2.02ms FAULT TIMEOUT OUTB GATE2B OUTB Q4 Figure 1. MAX5944 Functional Diagram _______________________________________________________________________________________ 9 MAX5944 Dual FireWire Current Limiter and Low-Drop ORing Switch Controller 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 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 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, 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. 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 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. ______________________________________________________________________________________ Dual FireWire Current Limiter and Low-Drop ORing Switch Controller VIN MAX5944 TO FireWire PORT RSENSE VFAULT Q1A Q2A ILIMIT tILIM ON_ IN_ SENSE_ IOUT GATE1_ GATE2_ OUT_ MAX5944 GND VIN ONQ1 VGATE1 Figure 3. Q1 Current Path During Inital Startup VGATE2 FROM BATTERY SOURCE VINA ILIMIT x RLOAD VOUT + VIS - Q1A Q2A VINB > VINA 30mΩ TO FireWire PORT ILOAD (a) INA SENSEA GATE1A GATE2A OUTA ONA ONQ1 MAX5944 GND VFAULT ONB INB < tILIM SENSEB GATE1B GATE2B OUTB ILIMIT VINB 30mΩ Q1B Q2B IOUT Figure 4. MAX5944 ORing Controller Rerouting Load Current from the higher supply input (INB) to OUTB, and then to the load (Figure 5). VGATE1 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 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. ______________________________________________________________________________________ 11 MAX5944 Dual FireWire Current Limiter and Low-Drop ORing Switch Controller VINA ONQ1 IINA ONB ONA INB INA 30mΩ 30mΩ MAX5944 IINB VOUT SENSEB SENSEA GATE1B GATE1A Q1 GATE2B GATE2A Q2 OUTB OUTA GND PORT 2 VINB PORT 1 VINB > VINA Figure 5. MAX5944 Power Switch Over ORing Timing Diagram 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 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 thresh12 Figure 6. MAX5944 Operating in Reverse-Current Mode (Routing Power from Port 1 to Port 2) old, the power-supply MOSFETs turn off immediately (less than 1µs). 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 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 ______________________________________________________________________________________ Dual FireWire Current Limiter and Low-Drop ORing Switch Controller IN_ R1 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. 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 × VIN + ILOAD tILIM 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 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. 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 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. VIN + VIS - ILIMIT + VQ1 - + VQ2 Q1 IN_ SENSE_ GATE1_ Q2 GATE2_ OUT_ MAX5944 Figure 8. Power Dissipated Across MOSFET Q2 During a Short-Circuit Fault ______________________________________________________________________________________ 13 MAX5944 MOSFET Selection VIN MAX5944 Dual FireWire Current Limiter and Low-Drop ORing Switch Controller FB VIN BST ON CPA SENSE MAX5035 PHY POWER OUT LX IN4148 IN4148 VD GND SGND IN4148 15mΩ CABLE POWER SOURCE INA IRF7478 30mΩ IRF7478 SENSEA GATE1A GATE2A OUTA INA ONA IRF7478 SENSEA GATE1A IRF7478 PORT 0 GATE2A OUTA ONA ONQ1 ONQ1 FAULTA GND MAX5944 FAULTA GND FAULTB ONB MAX5944 FAULTB ONB INB SENSEB GATE1B BATTERY-POWER SOURCE GATE2B OUTB IRF7478 15mΩ INB IRF7478 CABLE NOT ON SLEEP SENSEB GATE1B IRF7478 30mΩ GATE2B OUTB PORT 1 IRF7478 IN4148 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 INA SENSEA GATE1A GATE2A OUTA ONA TO FireWire PORT 0 POWER SOURCE ONQ1 CURRENT-LIMIT/ CIRCUIT-BREAKER AND INRUSH CONTROL MAX5944 GND TO FireWire PORT 1 ONB FAULTB INB SENSEB GATE1B RSENSEB Q1B GATE2B OUTB Q2B LOAD DISCONECT SWITCH VERY-LOW-DROP ORing DIODE TO FireWire PORT 1 Chip Information TRANSISTOR COUNT: 4,648 PROCESS: BiCMOS 14 ______________________________________________________________________________________ Dual FireWire Current Limiter and Low-Drop ORing Switch Controller N E H INCHES MILLIMETERS MAX MIN 0.053 0.069 0.010 0.004 0.014 0.019 0.007 0.010 0.050 BSC 0.150 0.157 0.228 0.244 0.016 0.050 MAX MIN 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 1.27 BSC 3.80 4.00 5.80 6.20 0.40 SOICN .EPS DIM A A1 B C e E H L 1.27 VARIATIONS: 1 INCHES TOP VIEW DIM D D D MIN 0.189 0.337 0.386 MAX 0.197 0.344 0.394 MILLIMETERS MIN 4.80 8.55 9.80 MAX 5.00 8.75 10.00 N MS012 8 AA 14 AB 16 AC D A B e C 0∞-8∞ A1 L FRONT VIEW SIDE VIEW PROPRIETARY INFORMATION TITLE: PACKAGE OUTLINE, .150" SOIC APPROVAL DOCUMENT CONTROL NO. 21-0041 REV. 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 © 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX5944 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.)