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19-2550; Rev 1; 1/03
Single USB Switch with Autoreset and Fault Blanking in Tiny QFN
General Description
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 40A of quiescent current when operating and only 3A 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 +160C. Accurate internal current-limiting circuitry protects the input supply against both overload and shortcircuit conditions. An open-drain fault signal, (FAULT), notifies the microprocessor (P) 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 thin QFN package and operates over the extended (-40C to +85C) temperature range.
Features
o Single USB Switch in 3mm x 3mm Package o Autoreset Feature Saves System Power o Guaranteed 500mA Load Current o Built-In 20ms Fault-Blanking Circuitry o Active-High or Active-Low Control Logic o Fully Compliant to USB Specifications o 2.7V to 5.5V Input Voltage Range o Fault-Indicator Output o Thermal-Overload Protection o UL Certification Pending
MAX1946
Ordering Information
PART MAX1946ETA TEMP RANGE -40C to +85C PIN-PACKAGE 8 Thin QFN 3mm x 3mm
Applications
USB Ports and USB Hubs Notebook and Desktop Computers PDAs and Palmtop Computers Digital Cameras Docking Stations
Pin Configuration
TOP VIEW
FAULT 1 GND 2 OUT 3 OUT 4 8 SEL
INPUT 2.7V TO 5.5V
Typical Operating Circuit
IN IN FAULT
MAX1946
7 ON 6 IN 5 IN
ACTIVE HIGH ACTIVE LOW SELECTABLE ON SEL GND
MAX1946
OUT OUT
QFN
________________________________________________________________ Maxim Integrated Products
1
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Single USB Switch with Autoreset and Fault Blanking in Tiny QFN MAX1946
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 = +70C) Exposed Pad Must be Soldered to PC Board 8-Pin QFN (derate 24.4mW/C above +70C) ...........1951mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C 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 = 5V, CIN = 0.1F, COUT = 1F, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note1)
PARAMETER Supply Voltage Range Switch On-Resistance Standby Supply Current Quiescent Supply Current OUT Off-Leakage Current Undervoltage Lockout Threshold Continuous Load Current Continuous Current Limit Short-Circuit Current Limit Short-Circuit Detect Threshold Continuous Current-Limit Blanking Timeout Period Short-Circuit Blanking Timeout Period Turn-On Delay Output Rise Time Turn-Off Delay Output Fall Time Thermal-Shutdown Threshold tON tRISE tOFF tFALL ILIM ISC VIN_ - VOUT = 0.5V VOUT = 0 (IOUT pulsing) (Note 2) From continuous current-limit condition to FAULT asserted From short-circuit current-limit condition to FAULT asserted ROUT = 10, time from ON to 10% of VOUT, does not include rise time ROUT = 10, from 10% to 90% of VOUT ROUT = 10, time from ON to 90% of VOUT, does not include fall time ROUT = 10, from 90% to 10% of VOUT 15 C hysteresis
o
SYMBOL VIN
CONDITIONS VIN = 5, TA = +25C
MIN 2.7
TYP 75
MAX 5.5 90 110 125
UNITS V m A A A V mA A APK ARMS V
RON
VIN = 5V VIN = 3.3V Switch disabled 3 40 0.01 0.2 2.3 500 0.74 0.8 1.1 1.2 0.35 1 10 7.5 0.25 20 18 0.6 1.2 0.8 3 160 2.5
10 60 1 2.7 1.20 1.6
IIN ILKG VULVO
Switch enabled, IOUT = 0 Switch disabled, VOUT = 0, TA = +25C Switch disabled, VOUT = 0, TA = +85C Rising edge, 3% hysteresis
35 35.0 1.50
ms ms ms ms
2
ms ms C
2
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Single USB Switch with Autoreset and Fault Blanking in Tiny QFN
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 5V, CIN = 0.1F, COUT = 1F, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note1)
PARAMETER ON, SEL Input High Level ON, SEL Input Low Level ON, SEL Input Leakage Current FAULT Output Low Voltage FAULT Output High Leakage Current OUT Output Current Autoreset Mode OUT Autoreset Threshold OUT Autoreset Blanking Time VOL SYMBOL VIH VIL CONDITIONS VIN = 2.7V to 4V VIN = 4V to 5.5V VIN = 2.7V to 4V VIN = 4V to 5.5V VON, VSEL = 0 or VIN, TA = +25C ISINK = 1mA, VIN = 2.7V VIN = V FAULT = 5.5V In latched off state, VOUT = 0 In latched off state, VOUT rising In latched off state, VOUT > 0.5V 10 0.4 10 1 25 0.5 20 45 0.6 35 -1 MIN 1.6 2 0.6 0.8 +1 0.5 TYP MAX UNITS V V A V A mA V ms
MAX1946
ELECTRICAL CHARACTERISTICS
(VIN = 5V, CIN = 0.1F, COUT = 1F, TA = -40C to +85C, unless otherwise noted.) (Note1)
PARAMETER Supply Voltage Range Switch On-Resistance Standby Supply Current Quiescent Supply Current OUT Off-Leakage Current Undervoltage Lockout Threshold Continuous Load Current Continuous Current Limit Short-Circuit Current Limit Continuous Current-Limit Blanking Timeout Period Short-Circuit Blanking Timeout Period Turn-On Delay tON ILIM ISC VIN_ - VOUT = 0.5V VOUT = 0 (IOUT pulsing) From continuous current-limit condition to FAULT asserted From short-circuit current-limit condition to FAULT asserted ROUT = 10. Time from ON to 10% of VOUT. Does not include rise time. IIN ILKG VULVO SYMBOL VIN VIN = 5, TA = +25C RON VIN = 5V VIN = 3.3V Switch disabled Switch enabled, IOUT = 0 Switch disabled, VOUT = 0, TA = -40C to +25C Rising edge, 3% hysteresis 2.3 500 0.6 0.8 10 7.5 0.25 1.3 1.6 35 35.0 1.50 CONDITIONS MIN 2.7 TYP MAX 5.5 90 110 125 10 60 1 2.7 A A A V mA A APK ms ms ms m UNITS V
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Single USB Switch with Autoreset and Fault Blanking in Tiny QFN MAX1946
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 5V, CIN = 0.1F, COUT = 1F, TA = -40C to +85C, unless otherwise noted.) (Note1)
PARAMETER Turn-Off Delay ON, SEL Input High Level ON, SEL Input Low Level ON, SEL Input Leakage Current FAULT Output Low Voltage OUT Output Current Autoreset Mode OUT Autoreset Threshold OUT Autoreset Blanking Time VOL SYMBOL tOFF VIH VIL CONDITIONS ROUT = 10, time from ON to 90% of VOUT, does not include fall time VIN = 2.7V to 4V VIN = 4V to 5.5V VIN = 2.7V to 4V VIN = 4V to 5.5V VON, VSEL = 0 or VIN ISINK = 1mA, VIN = 2.7V In latched off state, VOUT = 0 In latched off state, VOUT rising In latched off state, VOUT > 0.5V 10 0.4 10 -1 1.6 2 0.6 0.8 +1 0.5 45 0.6 35 MIN TYP MAX 2 UNITS ms V V V V A V mA V ms
Note 1: All parts are 100% tested at TA = +25C. 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 = 0.1F, COUT = 1F, ON = SEL, TA = +25C, unless otherwise noted.)
QUIESCENT SUPPLY CURRENT vs. INPUT VOLTAGE
MAX1946 toc01
QUIESCENT SUPPLY CURRENT vs. TEMPERATURE
VSEL = VON = GND QUIESCENT SUPPLY CURRENT (A) 42 VIN = 5.5V 39 VIN = 5V
MAX1946 toc02
SHUTDOWN SUPPLY CURRENT vs. TEMPERATURE
SHUTDOWN SUPPLY CURRENT (A) 3.3 3.2 3.1 3.0 2.9 2.8 2.7 2.6 -40 -15 10 35 60 85 ON = GND, VSEL = 5V
MAX1946 toc03
50 QUIESCENT SUPPLY CURRENT (A)
VSEL = VON = GND
45
3.4
40
30
20
36
10
33
VIN = 2.7V
0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 INPUT VOLTAGE (V)
30 -40 -15 10 35 60 85 TEMPERATURE (C)
TEMPERATURE (C)
4
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Single USB Switch with Autoreset and Fault Blanking in Tiny QFN
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN = 5V, CIN = 0.1F, COUT = 1F, ON = SEL, TA = +25C, unless otherwise noted.)
NORMALIZED ON-RESISTANCE vs. TEMPERATURE
MAX1946 toc05
MAX1946 toc04
MAX1946
SWITCH OFF-LEAKAGE vs. TEMPERATURE
1000 ON = OUT = GND, VSEL = 5V SWITCH OFF-LEAKAGE (nA) 100
CONTINUOUS CURRENT-LIMIT THRESHOLD vs. TEMPERATURE
CONTINUOUS CURRENT THRESHOLD (mA)
MAX1946 toc06
1.3 1.2 NORMALIZED RON 1.1 1.0 0.9 0.8
1090 1080 1070 1060 1050 VIN = 2.7V 1040 1030 VIN = 5.5V VIN = 5.0V
10
1
0.1
0.01 -40 -15 10 35 60 85 TEMPERATURE (C)
0.7 -40 -15 10 35 60 85 TEMPERATURE (C)
-40
-15
10
35
60
85
TEMPERATURE (C)
TURN-ON TIME vs. TEMPERATURE (tON + tRISE)
ROUT = 10
MAX1946 toc07
TURN-OFF TIME vs. TEMPERATURE (tOFF + tFALL)
ROUT = 10 3.5 TURN-OFF TIME (ms) 3.2 2.9 2.6 2.3 2.0 VIN = 5V VIN = 5.5V
MAX1946 toc08
FAULT-BLANKING TIME vs. TEMPERATURE
20.8 FAULT-BLANKING TIME (ms) 20.6 20.4 20.2 20.0 19.8 19.6 19.4 19.2 VIN = 5.5V VIN = 5V VIN = 2.7V
MAX1946 toc09
4.0 3.5 TURN-ON TIME (ms) 3.0
3.8
21.0
VIN = 2.7V 2.5 2.0 1.5 1.0 -40 -15 10 35 60 85 TEMPERATURE (C) VIN = 5V VIN = 5.5V
VIN = 2.7V
19.0 -40 -15 10 35 60 85 -40 -15 10 35 60 85 TEMPERATURE (C) TEMPERATURE (C)
FAULT OUTPUT LOW VOLTAGE vs. TEMPERATURE
FAULT OUTPUT LOW VOLTAGE (mV) 325 305 285 265 245 225 205 185 165 145 -40 -15 10 35 60 85 TEMPERATURE (C) VIN = 5.5V 10 5 -40 VIN = 5V VIN = 2.7V ISINK = 1mA
MAX1946 toc10
AUTORESET CURRENT vs. TEMPERATURE
MAX1946 toc11
AUTORESET CURRENT vs. INPUT VOLTAGE
MAX1946 toc12
345
40 35 AUTORESET CURRENT (mA) 30 25 20 15 VIN = 2.7V VIN = 5V VIN = 5.5V
35 31 AUTORESET CURRENT (mA) 27 23 19 15 11 7 TA = +85C TA = -40C
TA = +25C
-15
10
35
60
85
TEMPERATURE (C)
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)
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Single USB Switch with Autoreset and Fault Blanking in Tiny QFN MAX1946
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN = 5V, CIN = 0.1F, COUT = 1F, ON = SEL, TA = +25C, unless otherwise noted.)
OVERLOAD RESPONSE INTO 2.5
MAX1946 toc13
OVERLOAD RESPONSE INTO 2.5 (EXPANDED TIME SCALE)
MAX1946 toc14
A
0
A
0
B
0
B
0
C
0
C
0
D 10ms/div A: VOUT, 5V/div B: VFAULT, 5V/div C: IIN, 1A/div D: IOUT, 1A/div
0
D 400s/div A: VOUT, 5V/div B: VFAULT, 5V/div C: IIN, 1A/div D: IOUT, 1A/div
0
SHORT-CIRCUIT RESPONSE INTO 0
MAX1946 toc15
SHORT-CIRCUIT RESPONSE INTO 0 (EXPANDED TIME SCALE)
MAX1946 toc16
A
0
A
0
B
0
B
0
C C 0 CURRENT TRANSIENT FROM SHORTING OUTPUT CAPACITANCE D 400s/div A: VOUT, 5V/div B: VFAULT, 5V/div C: IIN, 2A/div D: IOUT, 10A/div
0
D 10ms/div A: VOUT, 5V/div B: VFAULT, 5V/div C: IIN, 1A/div D: IOUT, 1A/div
0
0
6
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Single USB Switch with Autoreset and Fault Blanking in Tiny QFN MAX1946
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN = 5V, CIN = 0.1F, COUT = 1F, ON = SEL, TA = +25C, unless otherwise noted.) SWITCH TURN-ON TIME (tON + tRISE)
MAX1946 toc17
SWITCH TURN-OFF TIME (tOFF + tFALL)
MAX1946 toc18
COUT = 1F ROUT = 10
COUT = 1F ROUT = 10
A
0
A
0
B 1ms/div A: VON, 2V/div B: VOUT, 2V/div
0
B
0 1ms/div A: VON, 2V/div B: VOUT, 2V/div
STARTUP WAVEFORMS COUT = 1F
MAX1946 toc19
STARTUP WAVEFORMS COUT = 330F
MAX1946 toc20
A
COUT = 1F ROUT = 10
0
A
COUT = 330F ROUT = 10
0
B
0
B
0
C
0
C
0
D 400s/div A: VOUT, 5V/div B: VFAULT, 5V/div C: VOUT, 2V/div D: IOUT, 500mA/div
0
D 1ms/div A: VOUT, 5V/div B: VFAULT, 5V/div C: VOUT, 2V/div D: IOUT, 500mA/div
0
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Single USB Switch with Autoreset and Fault Blanking in Tiny QFN MAX1946
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN = 5V, CIN = 0.1F, COUT = 1F, ON = SEL, TA = +25C, unless otherwise noted.)
STARTUP WAVEFORMS COUT = 100F
MAX1946 toc21
UVLO RESPONSE
MAX1946 toc22
A
COUT = 100F ROUT = 10
0 A 0
B
0 B 0
C
0 C 0
D 1ms/div A: VOUT, 5V/div B: VFAULT, 5V/div C: VOUT, 2V/div D: IOUT, 500mA/div
0 10ms/div A: VIN, 2V/div B: VOUT, 2V/div C: VFAULT, 2V/div
Pin Description
PIN 1 2 3, 4 NAME FAULT GND OUT FUNCTION 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. Ground Switch Power Output. Connect OUT pins together at the device and bypass with a 1F ceramic capacitor. Load conditions may require additional bulk capacitance. When disabled, OUT goes into a high-impedance state. Switch Power Input. Connect IN pins together at the device and bypass with a 0.1F ceramic capacitor to GND. Input conditions may require additional bulk capacitance to prevent pulling the input supply down. 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. 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.
5, 6
IN
7 8
ON SEL
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. The MAX1946 operates from a 2.7V to 5.5V input supply and supplies a minimum output current of 740mA.
8
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. An internal micropower charge pump generates a highside supply that drives the gate of an internal 75m NMOS switch.
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Single USB Switch with Autoreset and Fault Blanking in Tiny QFN MAX1946
IN
ON SEL IN
CHARGE PUMP FAULT
MAX1946
ILIM THERMAL SHUTDOWN 25mA FAULT LOGIC 20ms TIMERS OSC 25kHz BIAS OUT
UVLO
REF
GND
Figure 1. Functional Diagram
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.
Table 1. On/Off Control
SEL GND VIN ON GND VIN GND VIN OUT BEHAVIOR ON OFF OFF ON
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 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 +160C 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 15C 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.
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Single USB Switch with Autoreset and Fault Blanking in Tiny QFN MAX1946
Fault Indicator
The MAX1946 provides an open-drain fault output, FAULT. Connect FAULT to IN through a 100k 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 +160C thermal-shutdown temperature limit. The switch is in current-limit or short-circuit currentlimit mode for more than 20ms. 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.
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/s 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
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 0.1F 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 * 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 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.) * A junction temperature of +160C immediately asserts FAULT low (the blanking timeout period does not apply for thermal faults) and turns off the switch. When the junction cools by 15C, 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.
Output Short Circuit (VOUT < 1V)
Output Overload Current (VOUT > 1V)
Thermal Fault (TJ > +160oC)
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Single USB Switch with Autoreset and Fault Blanking in Tiny QFN
INPUT 2.7V TO 5.5V IN IN OFF ON ON SEL GND OUT 1F FAULT 100k
0.1F
MAX1946
OUT
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 QFN 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: P = (IOUT)2 RON At the normal operating current (IOUT = 0.5A) and the maximum on-resistance of the switch (90m), the power dissipation is: P = (0.5A)2 0.09 = 22.5mW
MAX1946
*USB APPLICATIONS MAY REQUIRE ADDITIONAL BULK CAPACITANCE
Figure 2. Typical Application Circuit
Output Capacitor Bypass OUT to GND with a 1F ceramic capacitor for local decoupling. Additional bulk capacitance (up to 470F) reduces output-voltage transients under dynamic load conditions. Using output capacitors greater than 470F can assert FAULT if the current limit cannot charge the output capacitor within the 20ms faultblanking period. In addition to bulk capacitance, smallvalue (0.1F 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. 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.
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 thin QFN, the MAX1946 die temperature exceeds the +160C thermal-shutdown threshold. The switch output shuts down until the junction temperature cools by 15C. 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
Chip Information
TRANSISTOR COUNT: 2004 PROCESS: BiCMOS
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11
Single USB Switch with Autoreset and Fault Blanking in Tiny QFN MAX1946
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.
PACKAGE OUTLINE, 6, 8 & 10L, QFN THIN (DUAL), EXPOSED PAD, 3x3x0.80 mm
21-0137
C
12
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6, 8, &10L, QFN THIN.EPS
Single USB Switch with Autoreset and Fault Blanking in Tiny QFN
Package Information (continued)
(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.
MAX1946
COMMON DIMENSIONS SYMBOL A D E A1 L k A2 MIN. 0.70 2.90 2.90 0.00 0.20 MAX. 0.80 3.10 3.10 0.05 0.40
0.25 MIN 0.20 REF.
PACKAGE VARIATIONS PKG. CODE T633-1 T833-1 T1033-1 N 6 8 10 D2 1.50-0.10 1.50-0.10 1.50-0.10 E2 2.30-0.10 2.30-0.10 2.30-0.10 e 0.95 BSC 0.65 BSC 0.50 BSC JEDEC SPEC MO229 / WEEA MO229 / WEEC MO229 / WEED-3 b 0.40-0.05 0.30-0.05 0.25-0.05 [(N/2)-1] x e 1.90 REF 1.95 REF 2.00 REF
PACKAGE OUTLINE, 6, 8 & 10L, QFN THIN (DUAL), EXPOSED PAD, 3x3x0.80 mm
21-0137
C
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 (c) 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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Price & Availability of MAX1946

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