19-2550; Rev 3; 2/10 Single USB Switch with Autoreset and Fault Blanking in Tiny TDFN The MAX1946 single current-limited switch with autoreset supplies a guaranteed 500mA load in accordance with USB specifications. The MAX1946 operates from a 2.7V to 5.5V input supply and consumes only 40µA of quiescent current when operating and only 3µA in shutdown. Selectable active-high/active-low control logic and shutdown control 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 MAX1946 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. An open-drain fault signal, (FAULT), notifies the microprocessor (µP) 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 faultblanking feature prevents fault signals from being issued when the device powers up the load. The MAX1946 is available in a space-saving 8-pin (3mm x 3mm) TDFN package and operates over the extended (-40°C to +85°C) temperature range. Applications Features Single USB Switch in 3mm x 3mm Package Autoreset Feature Saves System Power Guaranteed 500mA Load Current Built-In 20ms Fault-Blanking Circuitry Active-High or Active-Low Control Logic Fully Compliant to USB Specifications 2.7V to 5.5V Input Voltage Range Fault-Indicator Output Thermal-Overload Protection Ordering Information PART TEMP RANGE PINPACKAGE TOP MARK MAX1946ETA+ -40°C to +85°C 8 TDFN-EP* ACQ +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed paddle. USB Ports and USB Hubs Notebook and Desktop Computers PDAs and Palmtop Computers Digital Cameras Docking Stations Typical Operating Circuit Pin Configuration INPUT 2.7V TO 5.5V TOP VIEW + GND 2 IN 8 SEL FAULT 1 MAX1946 FAULT IN 7 ON OUT 3 6 IN OUT 4 5 IN ACTIVE HIGH ACTIVE LOW SELECTABLE ON MAX1946 OUT OUT SEL GND TDFN ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX1946 General Description MAX1946 Single USB Switch with Autoreset and Fault Blanking in Tiny TDFN ABSOLUTE MAXIMUM RATINGS IN, ON, OUT, SEL to GND........................................-0.3V to +6V FAULT to GND .............................................-0.3V to (VIN + 0.3V) IN to OUT..................................................................-0.3V to +6V OUT Continuous Switch Current (internally limited)..............1.3A FAULT DC Current ..............................................................10mA Continuous Power Dissipation (TA = +70°C) Exposed Pad Must be Soldered to PC Board 8-Pin TDFN (derate 23.8mW/°C above +70°C) .........1905mW 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.) (Note1) PARAMETER Supply Voltage Range SYMBOL CONDITIONS VIN MIN VIN = 5, TA = +25°C Switch On-Resistance RON Standby Supply Current Quiescent Supply Current OUT Off-Leakage Current Undervoltage Lockout Threshold 75 ILKG VULVO MAX UNITS 5.5 V 90 VIN = 5V 110 VIN = 3.3V 125 Switch disabled IIN TYP 2.7 Switch enabled, IOUT = 0 3 10 µA µA 40 60 Switch disabled, VOUT = 0, TA = +25°C 0.01 1 Switch disabled, VOUT = 0, TA = +85°C 0.2 Rising edge, 3% hysteresis Continuous Load Current 2.3 2.5 2.7 500 Continuous Current Limit ILIM VIN_ - VOUT = 0.5V Short-Circuit Current Limit ISC VOUT = 0 (IOUT pulsing) 0.74 0.8 mΩ µA V mA 1.1 1.20 1.2 1.6 A APK 0.35 ARMS 1 V Short-Circuit Detect Threshold (Note 2) 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 0.25 0.6 1.50 ms Turn-On Delay tON ROUT = 10Ω, time from ON to 10% of VOUT, does not include rise time Output Rise Time tRISE ROUT = 10Ω, from 10% to 90% of VOUT 1.2 Turn-Off Delay tOFF ROUT = 10Ω, time from ON to 90% of VOUT, does not include fall time 0.8 Output Fall Time tFALL Thermal-Shutdown Threshold 2 ROUT = 10Ω, from 90% to 10% of VOUT 15oC hysteresis ms 2 ms 3 ms 160 °C _______________________________________________________________________________________ Single USB Switch with Autoreset and Fault Blanking in Tiny TDFN (VIN = 5V, CIN = 0.1µF, COUT = 1µF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note1) PARAMETER SYMBOL ON, SEL Input High Level VIH ON, SEL Input Low Level VIL ON, SEL Input Leakage Current FAULT Output Low Voltage CONDITIONS 1.6 VIN = 4V to 5.5V 2 TYP MAX UNITS V VIN = 2.7V to 4V 0.6 VIN = 4V to 5.5V 0.8 VON, VSEL = 0 or VIN, TA = +25°C VOL MIN VIN = 2.7V to 4V -1 ISINK = 1mA, VIN = 2.7V V +1 µA 0.5 V FAULT Output High Leakage Current VIN = V FAULT = 5.5V OUT Output Current Autoreset Mode In latched off state, VOUT = 0 10 25 45 mA OUT Autoreset Threshold In latched off state, VOUT rising 0.4 0.5 0.6 V OUT Autoreset Blanking Time In latched off state, VOUT > 0.5V 10 20 35 ms MIN TYP MAX UNITS 5.5 V 1 µA ELECTRICAL CHARACTERISTICS (VIN = 5V, CIN = 0.1µF, COUT = 1µF, TA = -40°C to +85°C, unless otherwise noted.) (Note1) PARAMETER Supply Voltage Range Switch On-Resistance SYMBOL VIN RON Standby Supply Current Quiescent Supply Current OUT Off-Leakage Current Undervoltage Lockout Threshold CONDITIONS IIN ILKG VULVO 2.7 VIN = 5, TA = +25°C 90 VIN = 5V 110 VIN = 3.3V 125 mΩ Switch disabled 10 µA Switch enabled, IOUT = 0 60 µA 1 µA Switch disabled, VOUT = 0, TA = -40°C to +25°C Rising edge, 3% hysteresis Continuous Load Current 2.3 2.7 500 V mA Continuous Current Limit ILIM VIN_ - VOUT = 0.5V 0.6 1.3 A Short-Circuit Current Limit ISC VOUT = 0 (IOUT pulsing) 0.8 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 ROUT = 10Ω. Time from ON to 10% of VOUT. Does not include rise time. 0.25 1.50 ms Turn-On Delay tON _______________________________________________________________________________________ 3 MAX1946 ELECTRICAL CHARACTERISTICS (continued) ELECTRICAL CHARACTERISTICS (continued) (VIN = 5V, CIN = 0.1µF, COUT = 1µF, TA = -40°C to +85°C, unless otherwise noted.) (Note1) PARAMETER SYMBOL Turn-Off Delay tOFF ON, SEL Input High Level VIH ON, SEL Input Low Level FAULT Output Low Voltage CONDITIONS MIN TYP ROUT = 10Ω, time from ON to 90% of VOUT, does not include fall time VIN = 2.7V to 4V 1.6 VIN = 4V to 5.5V 2 VIL UNITS 2 ms V 0.6 VIN = 4V to 5.5V VON, VSEL = 0 or VIN VOL MAX V VIN = 2.7V to 4V ON, SEL Input Leakage Current -1 ISINK = 1mA, VIN = 2.7V V 0.8 V +1 µA 0.5 V OUT Output Current Autoreset Mode In latched off state, VOUT = 0 10 45 mA OUT Autoreset Threshold In latched off state, VOUT rising 0.4 0.6 V OUT Autoreset Blanking Time In latched off state, VOUT > 0.5V 10 35 ms Note 1: All parts are 100% tested at TA = +25°C. Limits over the operating temperature range are guaranteed by design. Note 2: Short-circuit detect threshold is the output voltage at which the device transitions from short-circuit current limit to continuous current limit. Typical Operating Characteristics (Circuit of Figure 2, VIN = 5V, CIN = 0.1µF, COUT = 1µF, ON = SEL, TA = +25°C, unless otherwise noted.) 20 10 0 42 VIN = 5.5V 39 VIN = 5V 36 33 VIN = 2.7V INPUT VOLTAGE (V) 3.4 ON = GND, VSEL = 5V 3.3 3.2 3.1 3.0 2.9 2.8 2.7 2.6 30 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 4 VSEL = VON = GND SHUTDOWN SUPPLY CURRENT (µA) 30 MAX1946 toc02 40 45 QUIESCENT SUPPLY CURRENT (µA) VSEL = VON = GND MAX1946 toc01 50 SHUTDOWN SUPPLY CURRENT vs. TEMPERATURE QUIESCENT SUPPLY CURRENT vs. TEMPERATURE MAX1946 toc03 QUIESCENT SUPPLY CURRENT vs. INPUT VOLTAGE QUIESCENT SUPPLY CURRENT (µA) MAX1946 Single USB Switch with Autoreset and Fault Blanking in Tiny TDFN -40 -15 10 35 TEMPERATURE (°C) 60 85 -40 -15 10 35 TEMPERATURE (°C) _______________________________________________________________________________________ 60 85 Single USB Switch with Autoreset and Fault Blanking in Tiny TDFN NORMALIZED ON-RESISTANCE vs. TEMPERATURE 10 1 0.1 1.0 0.9 0.8 0.7 0.01 10 35 60 1080 VIN = 5.5V VIN = 5.0V 1070 1060 1050 VIN = 2.7V 1040 1030 -40 85 -15 10 35 60 85 -40 -15 10 35 60 TEMPERATURE (°C) TEMPERATURE (°C) TEMPERATURE (°C) TURN-ON TIME vs. TEMPERATURE (tON + tRISE) TURN-OFF TIME vs. TEMPERATURE (tOFF + tFALL) FAULT-BLANKING TIME vs. TEMPERATURE 3.0 VIN = 2.7V 2.5 VIN = 5V 2.0 3.5 VIN = 5.5V 1.5 VIN = 5.5V VIN = 5V 3.2 2.9 2.6 VIN = 2.7V 21.0 20.8 20.6 VIN = 2.7V 20.4 20.2 VIN = 5V 20.0 19.8 19.6 VIN = 5.5V 19.4 2.3 85 MAX1946 toc09 3.5 ROUT = 10Ω FAULT-BLANKING TIME (ms) 3.8 MAX1946 toc08 ROUT = 10Ω TURN-OFF TIME (ms) 4.0 -15 MAX1946 toc07 -40 TURN-ON TIME (ms) 1.1 1090 MAX1946 toc06 1.2 NORMALIZED RON 100 MAX1946 toc05 ON = OUT = GND, VSEL = 5V SWITCH OFF-LEAKAGE (nA) 1.3 MAX1946 toc04 1000 CONTINUOUS CURRENT-LIMIT THRESHOLD vs. TEMPERATURE CONTINUOUS CURRENT THRESHOLD (mA) SWITCH OFF-LEAKAGE vs. TEMPERATURE 19.2 1.0 10 35 60 -15 10 35 60 -15 10 35 60 TEMPERATURE (°C) FAULT OUTPUT LOW VOLTAGE vs. TEMPERATURE AUTORESET CURRENT vs. TEMPERATURE AUTORESET CURRENT vs. INPUT VOLTAGE 245 225 VIN = 5V VIN = 5.5V 30 25 VIN = 5V 20 15 185 VIN = 2.7V 10 VIN = 5.5V 165 -15 10 35 TEMPERATURE (°C) 60 85 31 27 23 TA = -40°C 19 TA = +25°C 15 11 TA = +85°C 7 5 145 85 MAX1946 toc12 35 35 AUTORESET CURRENT (mA) VIN = 2.7V 205 40 AUTORESET CURRENT (mA) 285 -40 -40 85 TEMPERATURE (°C) 305 265 -40 TEMPERATURE (°C) ISINK = 1mA 325 85 MAX1946 toc11 345 -15 MAX1946 toc10 -40 FAULT OUTPUT LOW VOLTAGE (mV) 19.0 2.0 -40 -15 10 35 TEMPERATURE (°C) 60 85 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 INPUT VOLTAGE (V) _______________________________________________________________________________________ 5 MAX1946 Typical Operating Characteristics (continued) (Circuit of Figure 2, VIN = 5V, CIN = 0.1µF, COUT = 1µF, ON = SEL, TA = +25°C, unless otherwise noted.) MAX1946 Single USB Switch with Autoreset and Fault Blanking in Tiny TDFN Typical Operating Characteristics (continued) (Circuit of Figure 2, VIN = 5V, CIN = 0.1µF, COUT = 1µF, ON = SEL, TA = +25°C, unless otherwise noted.) OVERLOAD RESPONSE INTO 2.5Ω (EXPANDED TIME SCALE) OVERLOAD RESPONSE INTO 2.5Ω MAX1946 toc13 MAX1946 toc14 A 0 A 0 B 0 B 0 C 0 C 0 0 D D 0 10ms/div 400µs/div A: VOUT, 5V/div B: VFAULT, 5V/div C: IIN, 1A/div D: IOUT, 1A/div A: VOUT, 5V/div B: VFAULT, 5V/div C: IIN, 1A/div D: IOUT, 1A/div SHORT-CIRCUIT RESPONSE INTO 0Ω (EXPANDED TIME SCALE) SHORT-CIRCUIT RESPONSE INTO 0Ω MAX1946 toc15 MAX1946 toc16 A 0 A 0 B 0 B 0 C 0 C CURRENT TRANSIENT FROM SHORTING OUTPUT CAPACITANCE 0 D 0 D 0 10ms/div A: VOUT, 5V/div B: VFAULT, 5V/div C: IIN, 1A/div D: IOUT, 1A/div 6 400µs/div A: VOUT, 5V/div B: VFAULT, 5V/div C: IIN, 2A/div D: IOUT, 10A/div _______________________________________________________________________________________ Single USB Switch with Autoreset and Fault Blanking in Tiny TDFN SWITCH TURN-ON TIME (tON + tRISE) SWITCH TURN-OFF TIME (tOFF + tFALL) MAX1946 toc17 MAX1946 toc18 COUT = 1µF ROUT = 10Ω COUT = 1µF ROUT = 10Ω A 0 A 0 B 0 B 0 1ms/div 1ms/div A: VON, 2V/div B: VOUT, 2V/div A: VON, 2V/div B: VOUT, 2V/div STARTUP WAVEFORMS COUT = 1µF COUT = 1µF ROUT = 10Ω STARTUP WAVEFORMS COUT = 330µF MAX1946 toc19 COUT = 330µF ROUT = 10Ω MAX1946 toc20 0 A B 0 B 0 C 0 C 0 A D 0 D 0 1ms/div 400µs/div A: VOUT, 5V/div B: VFAULT, 5V/div C: VOUT, 2V/div D: IOUT, 500mA/div 0 A: VOUT, 5V/div B: VFAULT, 5V/div C: VOUT, 2V/div D: IOUT, 500mA/div _______________________________________________________________________________________ 7 MAX1946 Typical Operating Characteristics (continued) (Circuit of Figure 2, VIN = 5V, CIN = 0.1µF, COUT = 1µF, ON = SEL, TA = +25°C, unless otherwise noted.) MAX1946 Single USB Switch with Autoreset and Fault Blanking in Tiny TDFN Typical Operating Characteristics (continued) (Circuit of Figure 2, VIN = 5V, CIN = 0.1µF, COUT = 1µF, ON = SEL, TA = +25°C, unless otherwise noted.) STARTUP WAVEFORMS COUT = 100µF UVLO RESPONSE MAX1946 toc21 A COUT = 100µF ROUT = 10Ω MAX1946 toc22 0 A 0 0 B C B 0 C 0 0 D 0 1ms/div A: VOUT, 5V/div B: VFAULT, 5V/div C: VOUT, 2V/div D: IOUT, 500mA/div 10ms/div A: VIN, 2V/div B: VOUT, 2V/div C: VFAULT, 2V/div Pin Description PIN NAME FUNCTION 1 FAULT Fault-Indicator Output Switch. Open-drain output asserts low when switch enters thermal shutdown, undervoltage lockout, or a sustained (>20ms) current-limit or short-circuit condition. 2 GND Ground 3, 4 OUT Switch Power Output. Connect OUT pins together at the device and bypass with a 1µF ceramic capacitor. Load conditions may require additional bulk capacitance. When disabled, OUT goes into a high-impedance state. 5, 6 IN Switch Power Input. Connect IN pins together at the device and bypass with a 0.1µF ceramic capacitor to GND. Input conditions may require additional bulk capacitance to prevent pulling the input supply down. 7 ON ON/OFF Control Input. The active polarity of ON is set by SEL. Connect SEL high to make ON active high. Connect SEL to GND to make ON active low. 8 SEL Logic Input Polarity Select. SEL sets the active polarity of the ON input. Connect SEL high to make ON active high. Connect SEL to GND to make ON active low. — EP Exposed Metal Paddle. This paddle is internally connected to GND through a soft connect. For enhanced thermal dissipation, connect EP to a large ground plane. Do not use EP as a sole ground connection. Detailed Description The MAX1946 includes output current limiting, short-circuit protection, thermal shutdown, an enable input, and fault indicator (see the Functional Diagram). A logic input at SEL sets the active polarity of the enable input. The fault indicator notifies the system when the currentlimit, short-circuit, undervoltage lockout, or thermalshutdown threshold is exceeded. 8 The MAX1946 operates from a 2.7V to 5.5V input supply and supplies a minimum output current of 740mA. A built-in current limit of 1.1A limits the output current in the event of an overload condition. A built-in short-circuit detection circuit pulses the output current if the output voltage falls below 1V. This lowers RMS output current and reduces power dissipation during continuous short conditions. _______________________________________________________________________________________ Single USB Switch with Autoreset and Fault Blanking in Tiny TDFN MAX1946 IN ON CHARGE PUMP SEL IN FAULT MAX1946 ILIM OUT THERMAL SHUTDOWN 25mA FAULT LOGIC 20ms TIMERS OSC 25kHz UVLO BIAS REF GND Figure 1. Functional Diagram An internal micropower charge pump generates a highside supply that drives the gate of an internal 75mΩ NMOS switch. On/Off Control and Undervoltage Lockout (UVLO) SEL sets the active polarity of the logic input of the MAX1946. Connect ON to the same voltage as SEL to enable OUT. Connect ON to the opposite voltage as SEL to disable OUT (see Table 1). The output enters a high-impedance state when disabled. The MAX1946 includes a UVLO circuit to prevent erroneous switch operation when the input voltage goes low during startup and brownout conditions. Input voltages less than 2.5V inhibit operation of the device. FAULT asserts low during a UVLO condition. Output Fault Protection and Autoreset The MAX1946 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 1.1A. 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 the Table 1. On/Off Control SEL GND VIN ON OUT BEHAVIOR GND ON VIN OFF GND OFF VIN ON fault flag asserts. The output automatically restarts 20ms after the short or overload is removed. The MAX1946 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. Thermal Shutdown When the MAX1946 die temperature exceeds +160°C the switch turns off and FAULT goes low. Thermal shutdown does not utilize the 20ms fault-blanking timeout period. When the junction temperature cools by 15°C the switch turns on again and FAULT returns high.The switch cycles on and off if the overload condition persists, resulting in a pulsed output that reduces the average system load. _______________________________________________________________________________________ 9 MAX1946 Single USB Switch with Autoreset and Fault Blanking in Tiny TDFN Fault Indicator The MAX1946 provides an open-drain fault output, FAULT. 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 for more than 20ms. The FAULT output deasserts after a 20ms delay once the fault condition is removed. Ensure that the MAX1946 input bypass capacitance is sufficiently large to prevent load glitches from triggering the FAULT output. 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 MAX1946 has a faultblanking circuit. 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 fault-blanking time, the switch turns off, FAULT asserts low, and the device enters autoreset mode (see the Output Fault Protection and Autoreset 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 MAX1946 to handle USB loads that may not be fully compliant with the USB specification. The MAX1946 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. Applications Information Typical Application Circuit Input Power Supply and Capacitance Connect both IN inputs together. IN powers the internal control circuitry and charge pump for the 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 MAX1946 is powered by long leads or PC traces, larger input bypass capacitance is required to prevent voltage spikes from exceeding the MAX1946’s absolute maximum ratings during short-circuit events. Table 2. Current Limiting and Fault Behavior CONDITION MAX1946 BEHAVIOR Output Short Circuit (VOUT < 1V) • If a short is detected at the output, the switch 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 switch resets, the output turns on, and FAULT goes high (see Short-Circuit Response in the Typical Operating Characteristics.) Output Overload Current (VOUT > 1V) • 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. • If overcurrent persists after 20ms, FAULT goes low, autoreset mode is enabled, and the output sources 25mA. • If the output voltage rises above 0.5V for 20ms, the switch resets, the output turns on, and FAULT goes high (see Short Overload Response in the Typical Operating Characteristics.) Thermal Fault (TJ > +160oC) • 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 then take effect. 10 ______________________________________________________________________________________ Single USB Switch with Autoreset and Fault Blanking in Tiny TDFN IN 100kΩ FAULT IN 0.1µF OFF ON MAX1946 OUT ON OUT SEL 1µF GND *USB APPLICATIONS MAY REQUIRE ADDITIONAL BULK CAPACITANCE Figure 2. Typical Application Circuit 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 can assert FAULT if the current limit cannot charge the output capacitor within the 20ms faultblanking period. In addition to bulk capacitance, smallvalue (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 cables, which can add an inductive component to the load. This inductance causes the output voltage at the USB port to oscillate during a load step. The MAX1946 drives inductive loads, but avoid exceeding the device’s absolute maximum ratings. The load inductance is usually relatively small, and the MAX1946 input typically includes substantial bulk capacitance from an upstream regulator as well as local bypass capacitors, limiting overshoot. If severe ringing occurs due to large load inductance, clamp the MAX1946 output below +6V and above -0.3V. Turn-On and Turn-Off Behavior When turned on, the MAX1946 output ramps up over 1.2ms to eliminate load transients on the upstream power source. When turned off, the output ramps down for 3ms. Under fault conditions, the output of the MAX1946 turns off rapidly to provide maximum safety for the upstream power source and downstream devices. Internal blocks shut down to minimize supply current when the switch is off. P = (IOUT)2 RON At the normal operating current (IOUT = 0.5A) and the maximum on-resistance of the switch (90mΩ), the power dissipation is: P = (0.5A)2 0.09Ω = 22.5mW The worst-case power dissipation occurs when the output current is just below the current-limit threshold (1.2A max) with an output voltage just greater than 1V. In this case, the power dissipated is the voltage drop across the switch multiplied by the current limit: P = ILIM (VIN - VOUT) For a 5V input and 1V output, the maximum power dissipation per switch is: P = 1.2A (5V - 1V) = 4.8W Since the package power dissipation is 1951mW for the 8-pin TDFN, the MAX1946 die temperature exceeds the +160°C thermal-shutdown threshold. 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 MAX1946 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 5V = 0.125W ______________________________________________________________________________________ 11 MAX1946 INPUT 2.7V TO 5.5V Layout and Thermal Dissipation Keep all traces as short as possible to reduce the effect of undesirable parasitic inductance, and to 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 power bus with short traces. The exposed pad on the TDFN package underside must be soldered to the PC board in order to realize the rated package dissipation. An active switch dissipates little power with minimal change in package temperature. Calculate the power dissipation for this condition as follows: MAX1946 Single USB Switch with Autoreset and Fault Blanking in Tiny TDFN Package Information Chip Information PROCESS: BiCMOS 12 For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE TYPE PACKAGE CODE DOCUMENT NO. 8 TDFN-EP T833+1 21-0137 ______________________________________________________________________________________ Single USB Switch with Autoreset and Fault Blanking in Tiny TDFN REVISION NUMBER REVISION DATE 3 2/10 DESCRIPTION Removed UL Certification Pending bullet from Features section PAGES CHANGED 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 ____________________ 13 © 2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc. MAX1946 Revision History