19-2497; Rev 0; 7/02 Triple USB Switch with Autoreset and Fault Blanking The MAX1940 triple current-limited switch with autoreset supplies a guaranteed 500mA load per channel in accordance with USB specifications. The MAX1940 operates from a 4V to 5.5V input supply and consumes only 60µA of quiescent current when operating and only 3µA in standby. Selectable active-high/active-low control logic and independent shutdown controls for each channel provide additional flexibility. An autoreset feature latches the switch off in the event of a short circuit, saving system power. The switch reactivates upon removal of the shorted condition. The MAX1940 provides several safety features to protect the USB port. Built-in thermal-overload protection turns off the switch when the die temperature exceeds +160°C. Accurate internal current-limiting circuitry protects the input supply against both overload and shortcircuit conditions. Independent open-drain fault signals (FAULTA, FAULTB, and FAULTC) notify the microprocessor when a thermal-overload, current-limit, undervoltage lockout (UVLO), or short-circuit fault occurs. A 20ms fault-blanking feature enables the circuit to ignore momentary faults, such as those caused when hot-swapping a capacitive load, preventing false alarms to the host system. The fault-blanking feature also prevents fault signals from being issued when the device powers up the load. The MAX1940 is available in a space-saving 16-pin QSOP package and operates over the extended (-40°C to +85°C) temperature range. Features ♦ Triple USB Switch in Tiny 16-Pin QSOP Package ♦ Autoreset Feature Saves System Power ♦ Guaranteed 500mA Load Current per Channel ♦ Built-In 20ms Fault-Blanking Circuitry ♦ Active-High/Active-Low Control Logic ♦ Fully Compliant to USB Specifications ♦ 4V to 5.5V Input Voltage Range ♦ Independent Shutdown Control ♦ Independent Fault Indicator Outputs ♦ Thermal-Overload Protection ♦ 3µA Standby Current ♦ UL Certification Pending Ordering Information PART TEMP RANGE PIN-PACKAGE MAX1940EEE -40°C to +85°C 16 QSOP Applications USB Hubs PDAs and Palmtop Computers Notebook Computers Docking Stations USB Ports Typical Operating Circuit MAX1940 Desktop Computers Pin Configuration INPUT 4V TO 5.5V IN1 FAULTA IN2 FAULTB IN3 FAULTC TOP VIEW ONA 1 IN1 2 IN5 15 OUTA OUTA USB PORT A OUTB USB PORT B OUTC USB PORT C 14 GND IN2 3 IN3 4 IN4 16 FAULTA MAX1940 ONA 13 OUTB 12 FAULTB ONB 5 IN4 6 11 SEL IN5 7 10 OUTC 9 ONC 8 FAULTC ACTIVE-HIGH/ACTIVE-LOW SELECTABLE ONB ONC SEL GND QSOP ________________________________________________________________ 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 MAX1940 General Description MAX1940 Triple USB Switch with Autoreset and Fault Blanking ABSOLUTE MAXIMUM RATINGS IN_, ON_, OUT_, SEL, FAULT_, to GND...................-0.3V to +6V IN1, IN2 to OUTA......................................................-0.3V to +6V IN2, IN3 to OUTB......................................................-0.3V to +6V IN4, IN5 to OUTC .....................................................-0.3V to +6V OUT_ Continuous Switch Current (per channel, internally limited).........................................1.4A FAULT_ DC Current ............................................................20mA Continuous Power Dissipation (TA = +70°C) 16-Pin QSOP (derate 8.3mW/°C above +70°C)...........667mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C 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_ = 5V, CIN_ = 0.1µF, COUT_ = 1µF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL Supply Voltage Range VIN_ Switch On-Resistance RON Standby Supply Current Quiescent Supply Current OUT_ Off-Leakage Current Undervoltage Lockout Threshold IIN_ ILKG VULVO CONDITIONS Short-Circuit Current Limit TYP 4.0 TA = 0°C to +85°C, each switch 85 MAX V 135 mΩ µA All switches disabled 3 10 47 75 Two switches enabled, IOUT_ = 0 55 90 All switches enabled, IOUT_ = 0 63 100 0.02 10 3.4 3.8 All switches disabled, VOUT_ = 0, TA = 0°C to +85°C Rising edge, 3% hysteresis 3.0 500 ILIM ISC VIN_ - VOUT_ = 0.5V VOUT_ = 0 (IOUT_ pulsing) UNITS 5.5 One switch enabled, IOUT_ = 0 Continuous Load Current Continuous Current Limit MIN µA µA V mA 0.7 0.9 1.2 A 0.9 1.2 1.6 APK 0.35 ARMS 1 V Short-Circuit Detect Threshold (Note 1) Continuous Current-Limit Blanking Timeout Period From continuous current-limit condition to FAULT_ asserted 10 20 35 ms Short-Circuit Blanking Timeout Period From short-circuit current-limit condition to FAULT_ asserted 7.5 18 35.0 ms 0.5 1.2 4.0 ms Turn-On Delay tON ROUT_ = 10Ω, does not include rise time (from ON_ to 10% of VOUT_) Output Rise Time tRISE ROUT_ = 10Ω, from 10% to 90% of VOUT_ 2.5 Turn-Off Delay tOFF ROUT_ = 10Ω, does not include fall time (from ON_ to 90% of VOUT_) 0.8 Output Fall Time tFALL ROUT_ = 10Ω, from 90% to 10% of VOUT_ 2.5 ms 160 °C Thermal-Shutdown Threshold 15°C hysteresis ON_, SEL Input High Level VIH VIN_ = 4V to 5.5V ON_, SEL Input Low Level VIL VIN_ = 4V to 5.5V ON_, SEL Input Leakage Current 2 VON_ = 0 or VIN_ ms 3 2 -1 _______________________________________________________________________________________ ms V 0.8 V +1 µA Triple USB Switch with Autoreset and Fault Blanking (VIN_ = 5V, CIN_ = 0.1µF, COUT_ = 1µF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER FAULT_ Output Low Voltage SYMBOL VOL FAULT_ Output High Leakage Current OUT_ Autoreset Current CONDITIONS MIN TYP MAX UNITS ISINK = 1mA, VIN_ = 4V 0.4 V VIN_ = V FAULT_ = 5.5V 1 µA mA In latched off state, VOUT_ = 0 10 25 45 OUT_ Autoreset Threshold In latched off state, OUT_ rising 0.4 0.5 0.6 V OUT_ Autoreset Blanking Time In latched off state, VOUT_ > 0.5V 10 20 35 ms MIN MAX UNITS 4.0 5.5 V Each switch 135 mΩ All switches disabled 10 µA One switch enabled, IOUT_ = 0 75 ELECTRICAL CHARACTERISTICS (VIN_ = 5V, CIN_ = 0.1µF, COUT_ = 1µF, TA = -40°C to +85°C, unless otherwise noted.) (Note 2) PARAMETER SYMBOL Supply Voltage Range VIN_ Switch On-Resistance RON Standby Supply Current Quiescent Supply Current OUT_ Off-Leakage Current Undervoltage Lockout Threshold IIN_ ILKG VULVO CONDITIONS Two switches enabled, IOUT_ = 0 90 All switches enabled, IOUT_ = 0 100 All switches disabled, VOUT_ = 0 10 Rising edge, 3% hysteresis Continuous Load Current 3.0 3.8 500 Continuous Current Limit ILIM Short-Circuit Current Limit ISC µA µA V mA VIN_ - VOUT_ = 0.5V 0.7 1.2 A VOUT_ = 0 (IOUT_ pulsing) 0.9 1.6 APK Continuous Current-Limit Blanking Timeout Period From continuous current-limit condition to FAULT_ asserted 10 35 ms Short-Circuit Blanking Timeout Period From short-circuit current-limit condition to FAULT_ asserted 7.5 35.0 ms 0.5 4.0 ms 3 ms Turn-On Delay tON ROUT_ = 10Ω, does not include rise time (from ON_ to 10% of VOUT_) Turn-Off Delay tOFF ROUT_ = 10Ω, does not include fall time (from ON_ to 90% of VOUT_) ON_, SEL Input High Level VIH VIN_ = 4V to 5.5V ON_, SEL Input Low Level VIL VIN_ = 4V to 5.5V 2 V 0.8 V +1 µA ISINK = 1mA, VIN_ = 4V 0.4 V FAULT_ Output High Leakage Current VIN_ = V FAULT_ = 5.5V 1 µA OUT_ Autoreset Current In latched off state, VOUT_ = 0 10 50 mA OUT_ Autoreset Threshold In latched off state, OUT_ rising 0.4 0.6 V OUT_ Autoreset Blanking Time In latched off state, VOUT_ > 0.5V 10 35 ms ON_, SEL Input Leakage Current FAULT_ Output Low Voltage VON_ = 0 or VIN_ VOL -1 Note 1: Short-circuit detect threshold is the output voltage at which the device transitions from short-circuit current limit to continuous current limit. Note 2: Specifications to -40°C are guaranteed by design, not production tested. _______________________________________________________________________________________ 3 MAX1940 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (Circuit of Figure 2, VIN_ = 5V, CIN_ = 0.1µF, COUT_ = 1µF, ON_ = SEL, TA = +25°C, unless otherwise noted.) 40 30 20 10 68 VIN_ = 5.5V 67 66 65 VIN_ = 5V 64 63 62 VIN_ = 4.5V 3.0 ON_ = GND, VSEL = 5V 2.9 2.8 2.7 2.6 61 0 2.5 60 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 -40 -15 10 35 60 -40 85 -15 10 35 60 85 TEMPERATURE (°C) TEMPERATURE (°C) SWITCH OFF-LEAKAGE (ONE SWITCH) vs. TEMPERATURE NORMALIZED ON-RESISTANCE vs. TEMPERATURE CONTINUOUS CURRENT-LIMIT THRESHOLD vs. TEMPERATURE 10 1 0.1 1.2 1.1 1.0 0.9 0.8 0.01 0.7 -15 10 35 60 85 -15 10 35 VIN_ = 5.0V 912 908 904 VIN_ = 4.5V 900 -40 85 -15 10 35 60 TEMPERATURE (°C) TEMPERATURE (°C) TURN-ON TIME (tON + tRISE) vs. TEMPERATURE TURN-OFF TIME (tOFF + tFALL) vs. TEMPERATURE FAULT-BLANKING TIME vs. TEMPERATURE VIN_ = 5V 3.2 3.1 VIN_ = 5.5V VIN_ = 5.5V 3.2 TURN-OFF TIME (ms) 3.4 ROUT_ = 10Ω 3.1 VIN_ = 5V 3.0 2.9 2.8 VIN_ = 4.5V 2.6 -15 10 35 TEMPERATURE (°C) 60 85 21.5 VIN_ = 4.5V 21.0 20.5 20.0 VIN_ = 5.5V VIN_ = 5V 19.5 2.7 3.0 22.0 85 MAX1940 toc09 3.3 FAULT-BLANKING TIME (ms) VIN_ = 4.5V ROUT_ = 10Ω -40 60 VIN_ = 5.5V 916 TEMPERATURE (°C) 3.5 3.3 -40 MAX1940 toc07 -40 920 MAX1940 toc06 MAX1940 toc05 100 1.3 NORMALIZED ON-RESISTANCE MAX1940 toc04 ON_ = OUT_, VSEL = 5V CONTINUOUS CURRENT-LIMIT THRESHOLD (mA) INPUT VOLTAGE (V) 1000 SWITCH OFF-LEAKAGE (nA) ALL SWITCHES ACTIVE SHUTDOWN SUPPLY CURRENT (µA) 50 69 MAX1940 toc02 60 70 MAX1940 toc08 QUIESCENT SUPPLY CURRENT (µA) ALL SWITCHES ACTIVE QUIESCENT SUPPLY CURRENT (µA) MAX1940 toc01 70 4 SHUTDOWN SUPPLY CURRENT vs. TEMPERATURE QUIESCENT SUPPLY CURRENT vs. TEMPERATURE MAX1940 toc03 QUIESCENT SUPPLY CURRENT vs. INPUT VOLTAGE TURN-ON TIME (ms) MAX1940 Triple USB Switch with Autoreset and Fault Blanking -40 -15 10 35 TEMPERATURE (°C) 60 19.0 85 -40 -15 10 35 TEMPERATURE (°C) _______________________________________________________________________________________ 60 85 Triple USB Switch with Autoreset and Fault Blanking AUTORESET CURRENT (mA) VIN_ = 5V 36 VIN_ = 5.5V VIN_ = 4.5V VIN_ = 5.5V 34 32 VIN_ = 5V 30 28 26 24 22 18 -40 -15 10 35 60 85 35 30 TA = -40°C 25 20 15 TA = +85°C 10 0 -40 -15 TEMPERATURE (°C) 10 35 60 85 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 TEMPERATURE (°C) MAX1940 toc13 SHORT-CIRCUIT RESPONSE INTO 0Ω MAX1940 toc14 5V A INPUT VOLTAGE (V) OVERLOAD RESPONSE INTO 2.5Ω (EXPANDED TIME SCALE) OVERLOAD RESPONSE INTO 2.5Ω TA = +25°C 5 VIN_ = 4.5V 20 40 MAX1940 toc12 38 AUTORESET CURRENT (mA) RPU = 4.7kΩ MAX1940 toc11 250 240 230 220 210 200 190 180 170 160 150 140 130 120 110 100 AUTORESET CURRENT vs. INPUT VOLTAGE AUTORESET CURRENT vs. TEMPERATURE MAX1940 toc10 FAULT OUTPUT LOW VOLTAGE (mV) FAULT OUTPUT LOW VOLTAGE vs. TEMPERATURE MAX1940 toc15 5V A 5V A 5V B 0 B 0 C 5V B C 5V C 0 D D D 0 A: VIN_ 5V/div B: VOUT_ 5V/div 0 10ms/div C: VFAULT_ 5V/div D: IOUT_ 1A/div A: VIN_ 5V/div B: VOUT_ 5V/div SHORT-CIRCUIT RESPONSE INTO 0Ω (EXPANDED TIME SCALE) SWITCH TURN-ON TIME (tON + tFALL) VSEL = 5V ROUT_ = 10Ω COUT_ = 1µF VSEL = 5V ROUT_ = 10Ω COUT_ = 1µF MAX1940 toc17 5V MAX1940 toc18 5V A B A: VIN_ 5V/div B: VOUT_ 5V/div 10ms/div C: VFAULT_ 5V/div D: IOUT_ 1A/div SWITCH TURN-ON TIME (tON + tRISE) MAX1940 toc16 A 0 400µs/div C: VFAULT_ 5V/div D: IOUT_ 1A/div 5V A 0 5V C 5V B B 0 D 0 A: VIN_ 5V/div B: VOUT_ 5V/div 400µs/div C: VFAULT_ 5V/div D: IIN_ 2A/div 1ms/div A: VON_ 5V/div B: VOUT_ 2V/div 1ms/div A: VON_ 5V/div B: VOUT_ 2V/div _______________________________________________________________________________________ 5 MAX1940 Typical Operating Characteristics (continued) (Circuit of Figure 2, VIN_ = 5V, CIN_ = 0.1µF, COUT_ = 1µF, ON_ = SEL, TA = +25°C, unless otherwise noted.) MAX1940 Triple USB Switch with Autoreset and Fault Blanking Typical Operating Characteristics (continued) (Circuit of Figure 2, VIN_ = 5V, CIN_ = 0.1µF, COUT_ = 1µF, ON_ = SEL, TA = +25°C, unless otherwise noted.) STARTUP TIME COUT = 1µF STARTUP TIME COUT = 330µF MAX1940 toc19 VSEL = 5V, ROUT_ = 10Ω, COUT_ = 1µF MAX1940 toc20 VSEL = 5V, ROUT_ = 10Ω, COUT_ = 330µF 5V A 5V A B 5V 5V C B 5V C 0 500mA D 500mA D 400µs/div C: VOUT_ 5V/div A: VON_ 5V/div B: VFAULT_ 5V/div D: IOUT_ 500mA/div STARTUP TIME COUT = 100µF 1ms/div C: VOUT_ 5V/div A: VON_ 5V/div B: VFAULT_ 5V/div D: IOUT_ 500mA/div UNDERVOLTAGE LOCKOUT RESPONSE MAX1940 toc21 MAX1940 toc22 VSEL = 5V, ROUT_ = 10Ω, COUT_ = 100µF ROUT_ = 10Ω, COUT_ = 1µF 5V A A 0 B 5V 5V B 0 C C 0 500mA D 1ms/div C: VOUT_ 2V/div A: VON_ 5V/div B: VFAULT_ 5V/div D: IOUT_ 500mA/div 6 A: VIN_ 5V/div B: VOUT_ 5V/div 10ms/div C: VFAULT_ 5V/div _______________________________________________________________________________________ Triple USB Switch with Autoreset and Fault Blanking PIN NAME 1 ONA FUNCTION Control Input for Switch A. The active polarity of ONA is set by SEL (see Table 1). 2 IN1 Power Input. Connect all IN_ inputs together and bypass with a 0.1µF ceramic capacitor to GND. Load conditions might require additional bulk capacitance to prevent pulling IN_ down. 3 IN2 Power Input. Connect all IN_ inputs together and bypass with a 0.1µF ceramic capacitor to GND. Load conditions might require additional bulk capacitance to prevent pulling IN_ down. 4 IN3 Power Input. Connect all IN_ inputs together and bypass with a 0.1µF ceramic capacitor to GND. Load conditions might require additional bulk capacitance to prevent pulling IN_ down. 5 ONB 6 IN4 Power Input. Connect all IN_ inputs together and bypass with a 0.1µF ceramic capacitor to GND. Load conditions might require additional bulk capacitance to prevent pulling IN_ down. 7 IN5 Power Input. Connect all IN_ inputs together and bypass with a 0.1µF ceramic capacitor to GND. Load conditions might require additional bulk capacitance to prevent pulling IN_ down. 8 ONC Control Input for Switch B. The active polarity of ONB is set by SEL (see Table 1). Control Input for Switch C. The active polarity of ONC is set by SEL (see Table 1). Fault Indicator Output for Switch C. Open-drain output asserts low when switch C enters thermal shutdown, undervoltage lockout, or a sustained (>20ms) current-limit or short-circuit condition. 9 FAULTC 10 OUTC 11 SEL 12 FAULTB 13 OUTB Power Output for Switch B. Bypass OUTB to GND with a 1µF ceramic capacitor. Load conditions might require additional bulk capacitance. When disabled, OUTB goes into a high-impedance state. 14 GND Ground 15 OUTA Power Output for Switch A. Bypass OUTA to GND with a 1µF ceramic capacitor. Load conditions might require additional bulk capacitance. When disabled, OUTA goes into a high-impedance state. 16 FAULTA Power Output for Switch C. Bypass OUTC to GND with a 1µF ceramic capacitor. Load conditions might require additional bulk capacitance. When disabled, OUTC goes into a high-impedance state. Logic Input Polarity Select. SEL sets the active polarity of the ON_ inputs. Connect SEL high to set activehigh inputs. Connect SEL to GND to set active-low inputs. Fault Indicator Output for Switch B. Open-drain output asserts low when switch B enters thermal shutdown, undervoltage lockout, or enters a sustained (>20ms) current-limit or short-circuit condition. Fault Indicator Output for Switch A. Open-drain output asserts low when switch A enters thermal shutdown, undervoltage lockout, or a sustained (>20ms) current-limit or short-circuit condition. _______________________________________________________________________________________ 7 MAX1940 Pin Description MAX1940 Triple USB Switch with Autoreset and Fault Blanking IN_ ONA SWITCH A (EXPANDED) CHARGE PUMP IN2 IN1 FAULTA SEL ILIM OUTA THERMAL SHUTDOWN 25mA MAX1940 FAULT LOGIC 20ms TIMERS OUTB SWITCH B (SEE SWITCH A) UVLO FAULTB OSC 25kHz OUTC SWITCH C (SEE SWITCH A) BIAS FAULTC ONB REF ONC GND IN2 IN3 IN4 IN5 Figure 1. Functional Diagram Detailed Description The MAX1940 triple current-limited USB power switch provides three independent switches, each with its own enable-control input and fault indicator (see Figure 1). A logic input sets the active polarity of the enable control inputs. The fault indicators notify the system when the current-limit, short-circuit, undervoltage lockout, or thermal-shutdown threshold is exceeded. The MAX1940 operates from a 4V to 5.5V input supply and supplies a minimum output current of 700mA per channel. A built-in current limit of 0.9A (typ) limits the output current in the event of an overload condition. Built-in short-circuit detection pulses the output current if 8 the output voltage falls below 1V, resulting in lower RMS output current and reduced power dissipation in the device. Independent thermal shutdown allows normal operation to continue if one channel experiences a prolonged overload or short-circuit condition. Low-RON NMOS switches enable the MAX1940 to provide three switches in the space-saving 16-pin QSOP package. An internal micropower charge pump generates the high-side supply needed for driving the gates of these high-side switches. Separate current-limiting and thermal-shutdown circuitry permits each switch to operate independently, improving system robustness. _______________________________________________________________________________________ Triple USB Switch with Autoreset and Fault Blanking SEL sets the active polarity of the logic inputs of the MAX1940. Connect ON_ to the same voltage as SEL to enable the respective OUT_ switch. Connect ON_ to the opposite voltage as SEL to disable the respective output (see Table 1). The output of a disabled switch enters a high-impedance state. The MAX1940 includes a UVLO circuit to prevent erroneous switch operation when the input voltage goes low during startup and brownout conditions. Input voltages of less than 3.4V inhibit operation of the device. FAULT_ asserts low during an undervoltage lockout condition. Output Fault Protection and Autoreset The MAX1940 senses the switch output voltage and selects continuous current limiting for VOUT_ greater than 1V, or short-circuit current limiting for VOUT_ less than 1V. When VOUT_ is greater than 1V, the device operates in a continuous current-limit mode that limits output current to 0.9A. When VOUT_ is less than 1V, the device operates in short-circuit current-limit mode, sourcing 1.2A pulses to the load. When either fault condition persists for 20ms, the output turns off and its fault flag is asserted. The output automatically restarts 20ms after the short or overload is removed. The MAX1940 detects short-circuit removal by sourcing 25mA from the output and monitoring the output voltage. When the voltage at the output exceeds 0.5V for 20ms, the fault flag resets, the output turns back on, and the 25mA current source turns off. Active loads are not expected to have measurable current when supplied with less than 0.5V. Thermal Shutdown Independent thermal shutdown for each channel permits normal operation of two switches to continue while a third experiences a thermal fault. The switch turns off and the FAULT_ output asserts low immediately when the junction temperature exceeds +160°C. Thermal shutdown does not utilize the 20ms fault-blanking timeout period. The switch turns on again and FAULT_ returns high when the junction temperature cools by +15°C. The switch cycles on and off if the overload condition persists, resulting in a pulsed output that reduces system power. Table 1. On/Off Control SEL GND VIN ON_ OUT_ BEHAVIOR GND ON VIN OFF GND OFF VIN ON Fault Indicators The MAX1940 provides an independent open-drain fault output (FAULT_) for each switch. Connect FAULT_ to IN_ through a 100kΩ pullup resistor for most applications. FAULT_ asserts low when any of the following conditions occur: • The input voltage is below the UVLO threshold. • The switch junction temperature exceeds the +160°C thermal-shutdown temperature limit. • The switch is in current-limit or short-circuit currentlimit mode after the fault-blanking period (20ms) expires. The FAULT_ output deasserts after a 20ms delay once the fault condition is removed. Ensure that the MAX1940 input bypass capacitance prevents glitches from triggering the FAULT_ outputs. Limit the input voltage slew rate to 0.2V/µs to prevent erroneous FAULT_ indications. To differentiate large capacitive loads from short circuits or sustained overloads, the MAX1940 has an independent fault-blanking circuit for each switch. When a load transient causes the device to enter current limit, an internal counter monitors the duration of the fault. For load faults exceeding the 20ms faultblanking time, the switch turns off, FAULT_ asserts low, and the device enters autoreset mode (see the Output Fault Protection and Autoreset Mode section). Only current-limit and short-circuit faults are blanked. Thermal overload faults and input voltage drops below the UVLO threshold immediately turn the switch off and assert FAULT_ low. Fault blanking allows the MAX1940 to handle USB loads that might not be fully compliant with USB specifications. The MAX1940 successfully powers USB loads with additional bypass capacitance and/or large startup currents while protecting the upstream power source. No fault is reported if the switch brings up the load within the 20ms blanking period. See Table 2 for a summary of current-limit and fault behavior. _______________________________________________________________________________________ 9 MAX1940 On/Off Control and Undervoltage Lockout MAX1940 Triple USB Switch with Autoreset and Fault Blanking Table 2. Current-Limiting and Fault Behavior CONDITION Output Short Circuit (VOUT_ < 1V) MAX1940 BEHAVIOR If a short is detected at the output, the channel turns off, and the blanking timer begins. FAULT_ remains high during the blanking timeout period. If the short persists during the fault-blanking period, the output pulses at 0.35ARMS. If the short is removed before the 18ms short-circuit blanking timeout period, the next ramped current pulse soft-starts the output. FAULT_ remains high. If the short circuit persists after the fault-blanking period, FAULT_ goes low, autoreset mode begins, and the output sources 25mA. If the output voltage rises above 0.5V for 20ms, the output turns on and FAULT_ goes high (see ShortCircuit Response in the Typical Operating Characteristics.) Output current regulates at ILIM and the blanking timer turns on. FAULT_ remains high during the blanking timeout period. Continuous current at ILIM persists until either the 20ms blanking period expires or a thermal fault occurs. Output Overload Current If overcurrent persists after 20ms, FAULT_ goes low, autoreset mode is enabled, and the output sources (VOUT_ > 1V) 25mA. If the output voltage rises above 0.5V for 20ms, the output turns on and FAULT_ goes high (see Overload Response into 2.5Ω in the Typical Operating Characteristics.) Thermal Fault (TJ > +160°C) A junction temperature of +160°C immediately asserts FAULT_ low (the blanking timeout period does not apply for thermal faults) and turns off the switch. When the junction cools by 15°C, the thermal fault is cleared and FAULT_ goes high. Note that if other fault conditions are present when a thermal fault clears, those fault states take effect. Applications Information Input Power Supply and Capacitance Connect all IN_ inputs together externally. IN_ powers the internal control circuitry and charge pump for each switch. Bypass IN_ to GND with a 0.1µF ceramic capacitor. When driving inductive loads or operating from inductive sources, which may occur when the MAX1940 is powered by long leads or PC traces, larger input bypass capacitance is required to prevent voltage spikes from exceeding the MAX1940’s absolute maximum ratings during short-circuit events. Output Capacitor Bypass OUT_ to GND with a 1µF ceramic capacitor for local decoupling. Additional bulk capacitance (up to 470µF) reduces output-voltage transients under dynamic load conditions. Using output capacitors greater than 470µF might assert FAULT_ if the current limit cannot charge the output capacitor within the 20ms fault-blanking period. In addition to bulk capacitance, small-value (0.1µF or greater) ceramic capacitors improve the output’s resilience to electrostatic discharge (ESD). Driving Inductive Loads A wide variety of devices (mice, keyboards, cameras, and printers) typically connect to the USB port with 10 cables, which might add an inductive component to the load. This inductance causes the output voltage at the USB port to oscillate during a load step. The MAX1940 drives inductive loads, but avoid exceeding the device’s absolute maximum ratings. Usually, the load inductance is relatively small, and the MAX1940’s input includes a substantial bulk capacitance from an upstream regulator as well as local bypass capacitors, limiting overshoot. If severe ringing occurs because of large load inductance, clamp the MAX1940 outputs below +6V and above -0.3V. Turn-On and Turn-Off Behavior The MAX1940’s slow turn-on and turn-off minimizes load transients on the upstream power source. Under fault conditions, the outputs of the MAX1940 turn off rapidly to provide maximum safety for the upstream power source and downstream devices. Internal blocks shut down to minimize supply current when all three channels are off. Layout and Thermal Dissipation Keep all traces as short as possible to reduce the effect of undesirable parasitic inductance and optimize the switch response time to output short-circuit conditions. Place input and output capacitors no more than 5mm from device leads. Connect IN_ and OUT_ to the ______________________________________________________________________________________ Triple USB Switch with Autoreset and Fault Blanking 100kΩ MAX1940 INPUT 4V TO 5.5V 100kΩ IN1 FAULTA IN2 FAULTB IN3 FAULTC 100kΩ O.1µF IN4 OUTA IN5 USB PORT A 1µF* ONA OFF OUTB ONB ON USB PORT B 1µF* ONC SEL OUTC GND USB PORT C 1µF* *USB APPLICATIONS MAY REQUIRE ADDITIONAL BULK CAPACITANCE Figure 2. Typical Application Circuit power bus with short traces. Wide power bus planes at IN_ and OUT provide superior heat dissipation as well. P = (IOUT_)2 x RON At the normal operating current (IOUT_ = 0.5A) and the maximum on-resistance of the switch (135mΩ), the power dissipation is: P = (0.5A)2 x 0.135Ω = 34mW per switch. The worst-case power dissipation occurs when the output current is just below the current-limit threshold (1.2A max) with an output voltage greater than 1V. In this case, the power dissipated in each switch is the voltage drop across the switch multiplied by the current limit: P = ILIM x (VIN - VOUT) For a 5V input and 1V output, the maximum power dissipation per switch is: P = 1.2A x (5V - 1V) = 4.8W Because the package power dissipation is 667mW, the MAX1940 die temperature exceeds the +160°C thermal shutdown threshold, and the switch output shuts down until the junction temperature cools by +15°C. The duty cycle and period are strong functions of the ambient temperature and the PC board layout (see the Thermal Shutdown section). If the output current exceeds the current-limit threshold, or the output voltage is pulled below the short-circuit detect threshold, the MAX1940 enters a fault state after 20ms, at which point autoreset mode is enabled and 25mA is sourced by the output. For a 5V input, OUT_ short-circuited to GND, and autoreset mode active, the power dissipation is as follows: P = 0.025A x 5V = 0.125W Chip Information TRANSISTOR COUNT: 4259 PROCESS: BiCMOS ______________________________________________________________________________________ 11 MAX1940 An active switch dissipates little power with minimal change in package temperature. Calculate the power dissipation for this condition as follows: 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.) QSOP.EPS MAX1940 Triple USB Switch with Autoreset and Fault Blanking 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. 12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.