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19-2033; Rev 0; 4/01
Dual USB Switch with Fault Blanking and Autoreset
General Description
The MAX1838 is a dual, current-limited USB switch with auxiliary input supply. Each switch meets all IEC specifications for USB ports and is guaranteed to supply 500mA from either of two input supplies. A control pin selects the power source from either the main or auxiliary supply. The MAX1838 has multiple protection features, including independent thermal-shutdown on each channel to limit junction temperature in case of a prolonged short or overload condition. The device has accurate internal current-limit and reverse-current protection to safeguard the input supply against overload. When powered from the auxiliary supply, the current limit on each channel is set independently with an external resistor. An autoreset feature latches the switch off if the output is shorted, thereby saving system power. When the short is removed, the switch automatically turns back on. Independent fault signals (FAULTA and FAULTB) notify the microprocessor (P) that the internal current limit has been reached. A 20ms fault-blanking feature prevents the MAX1838 from issuing false alarms to the host during power-up or when hot-swapping into a capacitive load. The MAX1838 is available in a space-saving 16-pin QSOP. For other USB current-limited switches, refer to the MAX1693, MAX1694, MAX1812, and MAX1823. o Autoreset from Fault Condition o Guaranteed 500mA Load per Channel o Programmable Current Limits on Auxiliary Inputs o Reverse Current Protection o Thermal Overload Protection o Built-In 20ms Fault-Blanking o 4V to 5.5V Range o Independent FAULT Indicator Outputs o UL Listing Pending
Features
o Dual USB Switch with Auxiliary Input
MAX1838
Ordering Information
PART MAX1838EEE TEMP. RANGE -40C to +85C PIN-PACKAGE 16 QSOP
Applications
Desktops Notebook Computers USB Ports Docking Stations
Typical Operating Circuit
Pin Configuration
TOP VIEW
GND 1 SEL 2 AUXA 3 INA 4 INB 5 AUXB 6 ON 7 GND 8 16 FAULTA
AUX
AUXA AUXB
FAULTB FAULTA
IN
15 ADJA 14 OUTA2
INA INB
MAX1838
OUTB 10F
OUTB
MAX1838
13 OUTA1 12 OUTB2 11 OUTB1 10 ADJB 9 FAULTB
OUTA SEL ON GND OUTA ADJB ADJA
QSOP ________________________________________________________________ Maxim Integrated Products 1
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.
Dual USB Switch with Fault Blanking and Autoreset MAX1838
ABSOLUTE MAXIMUM RATINGS
IN_, AUX_, ON, OUT_, SEL, FAULT_, ADJ_ to GND .........................................................-0.3V to +6V IN_, AUX_ to OUTA_; IN_, AUX_ to OUTB_.................-6V to +6V AUX_ to IN_ .................................................................-6V to +6V Maximum Continuous Switch Current (per channel, internally limited).............................................1.2A FAULT_ Current ..................................................................20mA Continuous Power Dissipation (TA = +70C) 16-Pin QSOP (derate 8.3mW/C above +70C)..........667mW 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__ = VAUX__ = 5V, ON = SEL = GND, ADJ_ = open, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Supply Voltage Range Switch On-Resistance AUX_ Standby Supply Current IN_ Standby Supply Current AUX_ Quiescent Supply Current IN_ Quiescent Supply Current OUT_ Off-Leakage Current Reverse Leakage Current Undervoltage Lockout Threshold Continuous Load Current VIN_ - VOUT_ = 0.5V, SEL = AUX_ Current-Limit Threshold ILIM VAUX_ - VOUT_ = 0.5V, SEL = GND RADJ_ = 67k SEL = GND, VOUT_ = 0 (IOUT_ pulsing) RADJ_ = 67k 0.09 0.6 1.2 0.35 10 7.5 20 18 35 35 1.8 ARMS APEAK ARMS ms ms VUVLO SYMBOL VIN_ VAUX_ RON VIN_ = 5V, each switch VIN_ = 4V, each switch V ON = VAUX_ V ON = VIN_ V ON = 0, IOUT_ = 0 V ON = 0, IOUT_ = 0, SEL = AUX_ V ON = VIN_, VOUTA_ = VOUTB_ = 0 V OUT = 5.5V, VIN_ = VAUX_ = 0 VIN_ or VAUX_ rising edge, 3% hysteresis 3.0 500 0.6 0.6 0.09 0.9 0.9 0.225 0.3 1.2 1.2 0.36 0.48 APEAK A CONDITIONS MIN 4.0 4.0 95 100 7 1 66 23 0.02 0.02 3.5 16 3 350 300 10 10 3.8 TYP MAX 5.5 5.5 170 UNITS V m A A A A A A V mA
Short-Circuit Current Limit
ISHORT
SEL = AUX_, VOUT_ = 0 Continuous Current-Limit Blanking Timeout Period Short-Circuit Blanking Timeout Period From continuous current-limit condition to FAULT_ assertion From short-circuit current-limit condition to FAULT_ assertion
2
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Dual USB Switch with Fault Blanking and Autoreset
ELECTRICAL CHARACTERISTICS (continued)
(VIN__ = VAUX__ = 5V, ON = SEL = GND, ADJ_ = open, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Turn-On Delay SYMBOL CONDITIONS ROUT_ = 10, does not include rise time (from ON asserted to VOUT_ = 10% of VIN_), COUT_ = 1F ROUT_ = 10, from 10% to 90% of VOUT_, COUT_ = 1F ROUT_ = 10, does not include fall time (from ON asserted to VOUT_ = 90% of VIN_), COUT_ = 1F ROUT_ = 10, from 90% to 10% of VOUT_, COUT_ = 1F 10C hysteresis VIN_ = 4V to 5.5V VIN_ = 4V to 5.5V V ON = 0 or 5.5V VIN_ = 4V, sinking 1mA VIN_ = V FAULT_ = 5.5V In latched-off state, VOUT_ = 0 In latched-off state, VOUT_ rising In latched-off state, VOUT_ > 1V Make-before-break (Note 1) 10 0.4 10 4.5 25 0.5 20 9 -1 2 0.8 1 0.4 1 50 0.6 35 18 MIN 0.5 TYP 2 MAX 4.0 UNITS ms
MAX1838
Output Rise Time
4.5
ms
Turn-Off Delay from ON
0.8
3
ms
Output Fall Time Thermal Shutdown Threshold Logic Input High Voltage Logic Input Low Voltage Logic Input Current FAULT_ Output Low Voltage FAULT_ Output High Leakage Current Autoreset Current Autoreset Threshold Autoreset Blanking Time AUX_ to IN_ or IN_ to AUX_ Select Delay
3 160
ms C V V A V A mA V ms ms
ELECTRICAL CHARACTERISTICS
(VIN__ = VAUX__ = V ON = 5V, SEL = GND, ADJ_ = open, TA = -40C to +85C, unless otherwise noted. Circuit of Figure 1.) (Note 2)
PARAMETER Supply Voltage Range Switch On-Resistance AUX_ Standby Supply Current IN_ Standby Supply Current AUX_ Quiescent Supply Current IN_ Quiescent Supply Current OUT_ Off-Leakage Current Reverse Leakage Current Undervoltage Lockout Threshold Continuous Load Current VUVLO SYMBOL VIN_ VAUX_ RON VIN_ = 5V, each switch V ON = VAUX_ V ON = VIN_ V ON = 0, IOUT_ = 0 V ON = 0, IOUT = 0, SEL = AUX_ V ON = VIN_, VOUTA_ = VOUTB_ = 0 VOUT_ = 5.5V, VIN_ = VAUX_ = 0 VIN_ or VAUX_ rising edge, 3% hysteresis 3.0 500 CONDITIONS MIN 4.0 4.0 MAX 5.5 5.5 170 16 3 350 300 10 10 3.8 UNITS V V m A A A A A A V mA
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3
Dual USB Switch with Fault Blanking and Autoreset MAX1838
ELECTRICAL CHARACTERISTICS (continued)
(VIN__ = VAUX__ = V ON = 5V, SEL = GND, ADJ_ = open, TA = -40C to +85C, unless otherwise noted. Circuit of Figure 1.) (Note 2)
PARAMETER Current-Limit Threshold SYMBOL ILIM CONDITIONS VIN_ - VOUT_ = 0.5V, SEL = AUX_ VAUX_ - VOUT_ = 0.5V, SEL = GND SEL = GND, VOUT_ = 0 (IOUT_ pulsing) SEL = AUX_, VOUT_ = 0 Continuous Current-Limit Blanking Timeout Period Short-Circuit Blanking Timeout Period Turn-On Delay From continuous current-limit condition to FAULT_ assertion From short-circuit current-limit condition to FAULT_ assertion ROUT_ = 10, does not include rise time (from ON asserted to VOUT_ = 10% of VIN_), COUT_ = 1F ROUT_ = 10, does not include fall time (from ON asserted to VOUT_ = 90% of VIN_), COUT_ = 1F VIN_ = 4V to 5.5V VIN_ = 4V to 5.5V V ON = 0 or 5.5V VIN_ = 4V, sinking 1mA VIN_ = V FAULT_ = 5.5V In latched-off state, VOUT_ = 0 In latched-off state, VOUT_ rising In latched-off state, VOUT_ > 1 V Make-before-break (Note 1) 10 0.4 9 4.5 -1 2 0.8 1 0.4 1 55 0.6 40 18 MIN 0.6 0.6 0.6 0.6 9 7 MAX 1.2 1.2 1.8 1.8 40 40 ms ms UNITS A
Short-Circuit Current Limit
ISHORT
APEAK
0.5
4
ms
Turn-Off Delay from ON
3
ms V V A V A mA V ms ms
Logic Input High Voltage Logic Input Low Voltage Logic Input Current FAULT_ Output Low Voltage FAULT_ Output High Leakage Current Autoreset Current Autoreset Threshold Autoreset Blanking Time AUX_ to IN_ or IN_ to AUX_ Select Delay
Note 1: SEL delay includes both make and break delay. The host system should wait for at least this interval before turning off the appropriate input supply. Note 2: Specifications from 0C to -40C are guaranteed by design, not production tested.
4
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Dual USB Switch with Fault Blanking and Autoreset
Typical Operating Characteristics
(Circuit of Figure 2, VIN_ = 5V, TA = +25C, unless otherwise noted.)
IN + AUX QUIESCENT CURRENT vs. TEMPERATURE
MAX1838 toc02 MAX1838 toc03
MAX1838
AUX QUIESCENT CURRENT vs. VAUX
VIN = 5V VIN = 4.5V SEL = GND VIN = 0 VIN = 5.5V
MAX1838 toc01
IN QUIESCENT CURRENT vs. VAUX
30 25 20 15 10 5 0 70 0 1 2 3 VAUX (V) 4 5 -40 VIN = 4.5V SEL = AUX VIN = 5V VIN = 5.5V 100
70 AUX QUIESCENT CURRENT (A) 60 50 40 30 20 10 0 0 1 2 3 VAUX (V) 4 5
IN QUIESCENT CURRENT (A)
SEL = AUX = IN QUIESCENT CURRENT (A) 90 VAUX = 5.5V 80 VAUX = 5V VAUX = 4.5V
-20
0
20
40
60
80
TEMPERATURE (C)
IN + AUX QUIESCENT CURRENT vs. TEMPERATURE
MAX1838 toc04
IN + AUX QUIESCENT SHUTDOWN CURRENT vs. TEMPERATURE
9.5 SHUTDOWN CURRENT (A) 9.0 8.5 8.0 7.5 7.0 6.5 6.0 80 -40 VIN = VAUX = 5.5V 1 -20 0 20 40 60 80 -40 VIN = VAUX = 4.5V VIN = VAUX = 5.0V
MAX1838 toc05
SWITCH LEAKAGE CURRENT vs. TEMPERATURE, FULL DATA
MAX1838 toc06
110 SEL = GND QUIESCENT CURRENT (A) 100 VIN = VAUX = 5.5V 90 VIN = VAUX = 5V 80 VIN = VAUX = 4.5V 70 -40 -20 0 20 40 60 TEMPERATURE (C)
10.0
1000 SWITCH LEAKAGE CURRENT (nA)
100 IN = AUX = SEL = ON = GND VOUT_ = 5.5V
10 VIN = VAUX = VON = VSEL = 5.5V OUT_ = GND
-20
0
20
40
60
80
TEMPERATURE (C)
TEMPERATURE (C)
NORMALIZED SWITCH ON-RESISTANCE vs. TEMPERATURE
MAX1838 toc07
CONTINUOUS CURRENT LIMIT THRESHOLD vs. TEMPERATURE
MAX1838 toc08
TURN-ON DELAY vs. TEMPERATURE
SEL = GND 2.2 TURN-ON DELAY (ms)
MAX1838 toc09
1.2 1.1 IOUT_ = 500mA 1.0 0.9 0.8 0.7 -40 -20 0 20 40 60 80 TEMPERATURE (C)
CONTINUOUS CURRENT LIMIT THRESHOLD (A)
1.3 NORMALIZED ON-RESISTANCE ()
0.94
2.3
0.92
2.1
VIN = VAUX = 5.0V
VIN = VAUX = 4.5V
0.90
2.0
0.88 1.9 VIN = VAUX = 5.5V
0.86 -40 -20 0 20 40 60 80 TEMPERATURE (C)
1.8 -40
-20
0
20
40
60
80
TEMPERATURE (C)
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5
Dual USB Switch with Fault Blanking and Autoreset MAX1838
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN_ = 5V, TA = +25C, unless otherwise noted.)
FAULT-BLANKING TIME vs. TEMPERATURE
MAX1838 toc10
AUTORESET CURRENT vs. TEMPERATURE
MAX1838 toc11
AUTORESET CURRENT vs. INPUT VOLTAGE
MAX1838 toc12
20.5
40
35 30 AUTOREST CURRENT (mA) 25 20 15 10 5 0 AUX = IN, SEL = ON = GND
FAULT-BLANKING TIME (ms)
20.0
AUTORESET CURRENT (mA)
35 VIN = VAUX = 5.5V 30
19.5
VAUX = 4.5V VAUX = 5.5V
25
19.0 VAUX = 5.0V 18.5 -40 -20 0 20 40 60 80 TEMPERATURE (C)
20
VIN = VAUX = 4.5V
15 -40 -20 0 20 40 60 80 TEMPERATURE (C)
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)
OVERLOAD RESPONSE INTO 2.5 LOAD
MAX1838 toc13
OVERLOAD RESPONSE INTO 2.5 LOAD (EXPANDED TIME SCALE)
MAX1838 toc14
5V VOUT_ 0 5V VFAULT_ 0
5V VOUT_ 0 5V VFAULT_ 0
0.5A/div IOUT_ 0.5A/div 0 10ms/div 1ms/div
0.5A/div IOUT_ 0
SHORT-CIRCUIT RESPONSE INTO 0
MAX1838 toc15
SHORT-CIRCUIT RESPONSE INTO 0 (EXPANDED TIME SCALE)
MAX1838 toc16
5V VOUT_ 0 5V VFAULT_ 0 1A 0.5A IOUT_ 0 10ms/div 1ms/div
5V VOUT_ 0 5V VFAULT_ 0 1A 0.5A IOUT_ 0
6
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Dual USB Switch with Fault Blanking and Autoreset
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN_ = 5V, TA = +25C, unless otherwise noted.)
SWITCH TURN-ON TIME, COUT = 1F SWITCH TURN-OFF TIME, COUT = 1F ROUT = 10
5V VON 0 5V 4V 3V VOUT_ 2V 1V 0 1ms/div 1ms/div
MAX1838
START-UP TIME INTO 10 LOAD
5V VON 0 5V 4V 3V VOUT_ 2V 1V 0 1ms/div IOUT_ 0
MAX1838 toc19
MAX1838 toc17
MAX1838 toc18
5V VON 0 4V VOUT_ 2V 0 0.5A
Pin Description
PIN 1, 8 2 3, 6 4, 5 7 NAME GND SEL AUXA, AUXB INA, INB ON Ground. Connect both GND pins together. Select Input. Control input to select from either VIN_ or VAUX_ for both outputs. Pull SEL high for power from IN source, low for power from AUX source. Auxiliary Input. Connect both AUX pins together and bypass with a 0.1F capacitor to ground. Load conditions may require additional bulk capacitance to maintain AUX voltage regulation. Power Input. Connect both IN pins together and bypass with a 0.1F capacitor to GND. Load conditions may require additional bulk capacitance to maintain the IN voltage regulation. Control Input. A logic low at ON turns on the device. A logic high shuts off both outputs and all power to internal circuits. Driving ON higher than IN does not cause damage. Fault Indicator Output for Channel B. This open-drain output goes low when channel B is in thermal shutdown or in undervoltage lockout or in a sustained (>20ms) current-limit or short-circuit condition. Current-Limit Adjust for Channel B. When operating from auxiliary input, adjust the current limit by connecting a resistor from ADJB to GND as shown in Programmable Current Limit. Power Output B. Connect both OUTB_ pins together. Connect a 1F or greater capacitor from OUTB_ to ground. Load condition may require additional bulk capacitance. Power Output A. Connect both OUTA_ pins together. Connect a 1F or greater capacitor from OUTA_ to ground. Load condition may require additional bulk capacitance. FUNCTION
9
FAULTB
10 11, 12 13, 14
ADJB OUTB1, OUTB2 OUTA1, OUTA2
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7
Dual USB Switch with Fault Blanking and Autoreset MAX1838
Pin Description (continued)
PIN 15 NAME ADJA FUNCTION Current-Limit Adjust for Channel A. When operating from auxiliary input, adjust the current limit by connecting a resistor from ADJA to GND as shown in Programmable Current Limit. Fault Indicator Output for Channel A. This open-drain output goes low when channel A is in thermal shutdown or undervoltage lockout or in a sustained (>20ms) current-limit or short-circuit condition.
16
FAULTA
Detailed Description
The MAX1838 is a dual, current-limited switch with auxiliary inputs designed for USB applications. It has two independent channels, each with its own faultblanking and autoreset function. Each channel has an error flag output to notify the USB controller when the current limit, short circuit, undervoltage lockout, or thermal shutdown threshold is reached. A single input enables both channels. The MAX1838 supplies the outputs from one of two inputs, IN or AUX, selected by SEL (Table 1).
The MAX1838 operates from 4V to 5.5V input voltage range and guarantees a 500mA (min) output current. A 0.9A built-in current limit limits the current in the event of a heavy overload condition. When powering the switches from the auxiliary input, each channel has an adjustable current limit. The MAX1838 has independent thermal shutdown for each channel in the event of a prolonged overload or short-circuit condition. The autoreset function monitors the overload and automatically turns the switch on when the overload is removed. Use of internal, low-RON NMOS switches enables the MAX1838 to provide four switches in the small 16-pin QSOP package. Internal micropower charge pumps generate the gate drive of these high-side switches. Separate current limiting and thermal shutdown circuits permit each switch to operate independently, improving system robustness.
THERMAL SHUTDOWN A
FAULT LOGIC FAULTA 20ms BLANKING N
Input Voltage Requirements and Undervoltage Lockout
SEL CHANNEL A CONTROL CIRCUITRY ADJA
IC POWER AUX IN
OUTA1 OUTA2
The MAX1838 operates in one of three modes (Table 1). The device receives its supply current from the AUX input. When VAUX_ falls below the 3.8V undervoltage lockout voltage, the MAX1838 shuts down to prevent erroneous switch operation.
OUTB2 OUTB1
Table 1. MAX1838 Operation Modes
Power from IN (SEL high) Both A and B outputs receive current from IN. The IC is powered from AUX. Both A and B outputs receive current from AUX. The IC is powered from AUX. VIN_ may drop to zero in this mode. Both A and B outputs are open and no power is available. Either VIN_ or VAUX_ may drop to zero.
ON
CHANNEL B CONTROL CIRCUITRY
ADJB
FAULTB 20ms BLANKING THERMAL SHUTDOWN B N
Power from AUX (SEL low)
FAULT LOGIC
Full Shutdown (ON high)
Figure 1. Simplified Functional Diagram 8 _______________________________________________________________________________________
Dual USB Switch with Fault Blanking and Autoreset
Output Fault Protection
The MAX1838 senses the switch output voltage and selects 0.9A continuous current limiting when VOUT is greater than 1V or short-circuit current limiting when VOUT is less than 1V (see Behavior During Current Limit and Fault Blanking).
Programmable Current Limit
When using the auxiliary input (AUX), each channel has an adjustable current limit. For maximum current limit (0.9A), open ADJ. For current limits below 0.9A, connect a resistor from ADJ to GND according to the following equation: IOUT_ = 0.9A x RADJ / (200k + RADJ)
MAX1838
Autoreset Mode
The MAX1838 monitors the output to detect an overload condition. If an output fault is detected for more than the 20ms blanking time, OUT_ latches, FAULT goes low, and OUT sources 25mA to the load. If the voltage at OUT exceeds 0.5V for 20ms, the fault resets, the 25mA current source shuts down, and the channel turns back on. For proper autoreset function, the output load current must remain below 25mA when the output voltage is lower than 0.5V. Toggle ON to manually reset the output.
Behavior During Current Limit and Fault Blanking
The MAX1838 limits switch current in the following ways. When ON is high, both channels are off. When ON is low, both channels can supply a continuous output current of at least 500mA. When the output current exceeds the 0.9A threshold, the MAX1838 will limit the current depending on the output voltage. If VOUT is greater than 1V (current-limit mode), the MAX1838 regulates the output current to 0.9A. If VOUT is less than 1V (short-circuit mode), the MAX1838 pulses the output, decreasing the current to 0.35ARMS (Table 2). Note that a thermal overload may result from either of these highcurrent conditions. The MAX1838 switches may enter current limit in normal operation when powering up or driving heavy capacitive loads. To differentiate these conditions from short circuits or sustained overloads that may damage the device, the MAX1838 has an independent faultblanking circuit in each channel. When a load transient causes the device to enter current limit, an internal counter starts to monitor the duration of the fault. If the load fault persists beyond the 20ms fault-blanking time, the channel turns off, FAULT asserts low, and the channel enters autoreset mode (see Autoreset Mode). Only current-limit and short-circuit faults are blanked. Thermal overload faults and input undervoltage immediately cause the channel to turn off and FAULT to assert low. Fault blanking allows the MAX1838 to accommodate USB loads that may not be fully compliant with the USB specifications. USB loads with additional bypass capacitance and/or large startup currents can be successfully powered while protecting the upstream power source. If the switch is able to bring up the load within the 20ms blanking period, no fault is indicated.
Thermal Shutdown
The MAX1838 features independent thermal shutdown for each channel, allowing one channel to deliver power even if the other channel has a fault condition. When the junction temperature exceeds +160C, the switch turns off and FAULT_ goes low immediately. Fault blanking does not occur during thermal limit. When the junction cools by 15C, the channel turns on again. If the fault overload condition continues, the channel cycles on and off, resulting in a pulsed output.
Fault Indicators
The MAX1838 provides an open-drain fault output (FAULT) for each channel. For most applications, connect FAULT to AUX through a 100k pullup resistor. FAULT goes low when any one of the following conditions occurs: * VAUX_ is below the undervoltage-lockout (UVLO) threshold. * VIN_ is below the undervoltage-lockout threshold and SEL = AUX. * The switch junction temperature exceeds the thermal shutdown temperature limit of +160C. * The switch is in current-limit or short-circuit limit mode after the fault-blanking period is exceeded (autoreset). After the fault condition is removed, the FAULT output deasserts after a 20ms delay. Ensure that the MAX1838 has adequate input bypass capacitance to prevent glitches from triggering FAULT outputs. Input glitches greater than 0.2V/s may cause erroneous FAULT indications.
Switching Supplies
To select IN as the source for both channels, drive SEL to a logic high. To select AUX, drive SEL low. To minimize switching transients, the MAX1838 switches between input supplies (IN and AUX) with a make-before-break sequence. Both switches are on simultaneously for up to 9ms before the initial supply is disconnected. When switching from IN to AUX, a significant current may flow
9
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Dual USB Switch with Fault Blanking and Autoreset MAX1838
between IN and AUX. However, a fault does not trigger since the fault blanking-time is longer than the supply switchover time.
Output Capacitor
An output capacitor helps prevent parasitic inductance from pulling OUT negative during turn-off. When starting up into very large capacitive loads, the switch pulses the output current at 0.35A RMS until the output voltage rises above 1V, then the capacitor continues to charge at the full 0.9A current limit. There is no limit to the size of output capacitor, but to prevent a startup fault assertion, the capacitor must charge up within the fault-blanking delay period. Typically starting up into a 330F or smaller capacitor does not trigger a fault output. In addition to bulk capacitance, small value (0.1F or greater) ceramic capacitors improve the output's resilience to electrostatic discharge (ESD).
Applications Information
Input Power Source
AUX provides the power for the MAX1838 internal control circuitry. Limit the input-voltage slew rate to less than 0.2V/s to prevent erroneous FAULT indications.
Input Capacitor
To limit the input voltage drop during momentary output short-circuit conditions, connect a capacitor from both AUX and IN to ground. A 0.1F ceramic capacitor is required for local decoupling; however, higher capacitor values will further reduce the voltage drop at the input (Figure 2). When driving inductive loads, a larger capacitance prevents voltage spikes from exceeding the MAX1838's absolute maximum ratings.
Driving Inductive Loads
A wide variety of devices (mice, keyboards, cameras, and printers) can load the USB port. These devices commonly connect to the port with cables, which can add an inductive component to the load. This inductance causes the output voltage at the USB port to ring
Table 2. MAX1838 Current Limiting and Fault Behavior
CONDITION MAX1838 BEHAVIOR * If a short circuit is detected at the output, the channel turns off, the blanking timer begins, and FAULT stays high. * If the short circuit persists during the fault-blanking period, the output pulses at 0.35ARMS. When the short is removed before the 15ms short-circuit blanking timeout period, the next ramped current pulse soft-starts the output. The FAULT flag stays high. * If the short circuit persists after the fault-blanking period, FAULT goes low, autoreset mode is enabled, and the output sources 25mA. If the voltage on that output is above 0.5V for 20ms, the channel resets, the output turns on, and FAULT goes high (see Short-Circuit Response in the Typical Operating Characteristics.) * Output current regulates at ILIM, the blanking timer turns on, and FAULT stays high. * Continuous current at ILIM persists until the overload is removed, a thermal fault occurs, or the 20ms continuous current-limit timeout period is reached. * If overcurrent is still present at 20ms, FAULT goes low, autoreset mode is enabled, and the output sources 25mA. If the voltage on that output is above 0.5V for 20ms, the channel resets, the output turns on, and FAULT goes high. * FAULT immediately goes low (the blanking timer does not apply to thermal faults), and the channel turns off. When the junction cools by 15oC, the thermal fault condition is removed, autoreset mode is enabled, the output sources 25mA, and FAULT remains low. If the voltage on that output is above 0.5V for 20ms, the channel resets, the output turns on, and FAULT goes high.
If a short circuit is present (VOUT < 1V)
If an overload current is present (VOUT > 1V)
If thermal fault condition is present
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Dual USB Switch with Fault Blanking and Autoreset
during a load step. The MAX1838 is capable of driving inductive loads, but avoid exceeding the device's absolute maximum ratings. Usually the load inductance is relatively small, and the MAX1838's input includes a 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 MAX1838's output below 6V and above -0.3V. For the normal operating current (IOUT = 0.5A), the maximum on-resistance of the switch is 170m, and the power dissipation is: P = (0.5A)2 x 0.170 = 43mW per switch The worst-case power dissipation occurs when the switch is in current limit and the output is 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) (VIN - VOUT) For a 5V input and 1V output, the maximum power dissipation per switch is: P = (1.2A) (5.5V - 1V) = 5.4W Since the package power dissipation is only 667mW, the MAX1838 die temperature will exceed the thermal shutdown threshold and the switch output shuts down until the junction temperature cools by 10C. The duty cycle and period are strong functions of the ambient temperature and the PC board layout. A short circuit at the output causes the power dissipated across the switch and the junction temperature to increase. If the fault condition persists, the thermal-overload-protection circuitry activates, and the output shuts down until the junction temperature decreases by 10C (see Thermal Shutdown).
MAX1838
Turn-On and Turn-Off Behavior
In absence of faults, the MAX1838's internal switches turn on and off slowly under the control of the ON input. The slow charge-pump switch-drive minimizes load transients on the upstream power source. Under thermal fault and undervoltage lockout, the power device turns off rapidly to protect the switch and the power source.
Layout and Thermal Dissipation
To optimize the switch response to output short-circuit conditions, keep all traces as short as possible to reduce the effect of undesirable parasitic inductance. Place input and output capacitors no more than 5mm from device leads. IN, AUX, and OUT pins must be connected with short traces to the power bus. Wide power bus planes provide superior switch heat dissipation. While the switches are on, power dissipation is small and the package temperature rise is minimal. Calculate the power dissipation for this condition as follows: P = (IOUT)2 RON
Chip Information
TRANSISTOR COUNT: 3227 PROCESS: BiCMOS
100k AUX 0.1F MAX1838 AUXA AUXB FAULTB FAULTA 100k
IN 0.1F
INA INB
OUTB2 OUTB1 10F OUTA2 OUTA1 10F ADJB ADJA 67k 67k
OUTB
OUTA
SEL ON GND
Figure 2. Typical Application Circuit ______________________________________________________________________________________ 11
Dual USB Switch with Fault Blanking and Autoreset MAX1838
Package Information
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 (c) 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
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