General Description
The MAX1838 is a dual, current-limited USB switch with
auxiliary input supply. Each switch meets all IEC speci-
fications 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 auxil-
iary supply.
The MAX1838 has multiple protection features, includ-
ing 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 safe-
guard the input supply against overload. When pow-
ered 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 (µP) 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.
Applications
Desktops
Notebook Computers
USB Ports
Docking Stations
Features
oDual USB Switch with Auxiliary Input
oAutoreset from Fault Condition
oGuaranteed 500mA Load per Channel
oProgrammable Current Limits on Auxiliary Inputs
oReverse Current Protection
oThermal Overload Protection
oBuilt-In 20ms Fault-Blanking
o4V to 5.5V Range
oIndependent FAULT Indicator Outputs
MAX1838
Dual USB Switch with Fault Blanking
and Autoreset
________________________________________________________________
Maxim Integrated Products
1
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
GND FAULTA
ADJA
OUTA2
OUTA1
OUTB2
OUTB1
ADJB
FAULTB
TOP VIEW
MAX1838
QSOP
SEL
AUXA
AUXB
INA
INB
ON
GND
Pin Configuration
Ordering Information
19-2033; Rev 1; 2/10
PART TEMP RANGE PIN-PACKAGE
MAX1838EEE -40°C to +85°C 16 QSOP
FAULTB
FAULTA
OUTB
OUTA
ADJB
ADJA
AUXA
AUXB
INA
INB
SEL
ON
GND
AUX
IN OUTB
OUTA
10µF
MAX1838
Typical Operating Circuit
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.
MAX1838
Dual USB Switch with Fault Blanking
and Autoreset
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN__ = VAUX__ = 5V, ON = SEL = GND, ADJ_ = open, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA=
+25°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.
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= +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
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
VIN_ 4.0 5.5
Supply Voltage Range VAUX_ 4.0 5.5 V
VIN_ = 5V, each switch 95 170
Switch On-Resistance RON VIN_ = 4V, each switch 100 mΩ
AUX_ Standby Supply Current V
ON = VAUX_ 7 16 µA
IN_ Standby Supply Current V
ON = VIN_13µA
AUX_ Quiescent Supply Current V
ON = 0V, IOUT_ = 0A 66 350 µA
IN_ Quiescent Supply Current V
ON = 0V, IOUT_ = 0A, SEL = AUX_ 23 300 µA
OUT_ Off-Leakage Current V
ON = VIN_, VOUTA_ = VOUTB_ = 0V 0.02 10 µA
Reverse Leakage Current V
OUT = 5.5V, VIN_ = VAUX_ = 0V 0.02 10 µA
Undervoltage Lockout Threshold VUVLO VIN_ or VAUX_ rising edge, 3% hysteresis 3.0 3.5 3.8 V
Continuous Load Current 500 mA
VIN_ - VOUT_ = 0.5V, SEL = AUX_ 0.6 0.9 1.2
VAUX_ - VOUT_ = 0.5V, SEL = GND 0.6 0.9 1.2
Current-Limit Threshold ILIM
RADJ_ = 67kΩ0.09 0.225 0.36
A
0.3 0.48 APEAK
SEL = GND,
VOUT_ = 0V
(IOUT_ pulsing)
RADJ_ = 67kΩ
0.09 ARMS
0.6 1.2 1.8 APEAK
Short-Circuit Current Limit ISHORT
SEL = AUX_, VOUT_ = 0V 0.35 ARMS
Continuous Current-Limit
Blanking Timeout Period
From continuous current-limit condition to
FAULT_ assertion 10 20 35 ms
Short-Circuit Blanking Timeout
Period
From short-circuit current-limit condition to
FAULT_ assertion 7.5 18 35 ms
MAX1838
Dual USB Switch with Fault Blanking
and Autoreset
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VIN__ = VAUX__ = 5V, ON = SEL = GND, ADJ_ = open, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA=
+25°C.)
PARAMETER SYM BOL CONDITIONS MIN TYP MAX UNITS
Turn-On Delay
ROUT_ = 10Ω, does not include rise time
(from ON asserted to VOUT_ = 10% of
VIN_), COUT_ = 1µF
0.5 2 4.0 ms
Output Rise Time ROUT_ = 10Ω, from 10% to 90% of VOUT_,
COUT_ = 1µF 4.5 ms
Turn-Off Delay from ON
ROUT_ = 10Ω, does not include fall time
(from ON asserted to VOUT_ = 90% of
VIN_), COUT_ = 1µF
0.8 3 ms
Output Fall Time ROUT_ = 10Ω, from 90% to 10% of VOUT_,
COUT_ = 1µF 3ms
Thermal Shutdown Threshold 10°C hysteresis 160 °C
Logic Input High Voltage VIN_ = 4V to 5.5V 2 V
Logic Input Low Voltage VIN_ = 4V to 5.5V 0.8 V
Logic Input Current V
ON = 0V or 5.5V -1 1 µA
FAULT_ Output Low Voltage VIN_ = 4V, sinking 1mA 0.4 V
FAULT_ Output High Leakage
Current VIN_ = V
FAULT_ = 5.5V 1 µA
Autoreset Current In latched-off state, VOUT_ = 0V 10 25 50 mA
Autoreset Threshold In latched-off state, VOUT_ rising 0.4 0.5 0.6 V
Autoreset Blanking Time In latched-off state, VOUT_ > 1V 10 20 35 ms
AUX_ to IN_ or IN_ to AUX_
Select Delay Make-before-break (Note 1) 4.5 9 18 ms
ELECTRICAL CHARACTERISTICS
(VIN__ = VAUX__ = V ON = 5V, SEL = GND, ADJ_ = open, TA= -40°C to +85°C, unless otherwise noted. Circuit of Figure 1.) (Note 2)
PARAMETER SYM BO L CONDITIONS MIN MAX UNITS
VIN_ 4.0 5.5 V
Supply Voltage Range VAUX_ 4.0 5.5 V
Switch On-Resistance RON VIN_ = 5V, each switch 170 mΩ
AUX_ Standby Supply Current V ON = VAUX_16µA
IN_ Standby Supply Current V ON = VIN_3µA
AUX_ Quiescent Supply Current V ON = 0V, IOUT_ = 0A 350 µA
IN_ Quiescent Supply Current V ON = 0V, IOUT = 0A, SEL = AUX_ 300 µA
OUT_ Off-Leakage Current V ON = VIN_, VOUTA_ = VOUTB_ = 0V 10 µA
Reverse Leakage Current VOUT_ = 5.5V, VIN_ = VAUX_ = 0V 10 µA
Undervoltage Lockout Threshold VUVLO VIN_ or VAUX_ rising edge, 3% hysteresis 3.0 3.8 V
Continuous Load Current 500 mA
MAX1838
Dual USB Switch with Fault Blanking
and Autoreset
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(VIN__ = VAUX__ = V ON = 5V, SEL = GND, ADJ_ = open, TA= -40°C to +85°C, unless otherwise noted. Circuit of Figure 1.) (Note 2)
PARAMETER SYM BO L CONDITIONS MIN MAX UNITS
VIN_ – VOUT_ = 0.5V, SEL = AUX_ 0.6 1.2
Current-Limit Threshold ILIM VAUX_ – VOUT_ = 0.5V, SEL = GND 0.6 1.2 A
SEL = GND, VOUT_ = 0V (IOUT_ pulsing) 0.6 1.8
Short-Circuit Current Limit ISHORT
SEL = AUX_, VOUT_ = 0V 0.6 1.8
APEAK
Continuous Current-Limit
Blanking Timeout Period
From continuous current-limit condition to
FAULT_ assertion 940ms
Short-Circuit Blanking Timeout
Period
From short-circuit current-limit condition to
FAULT_ assertion 740ms
Turn-On Delay
ROUT_ = 10Ω, does not include rise time
(from ON asserted to VOUT_ = 10% of
VIN_), COUT_ = 1µF
0.5 4 ms
Turn-Off Delay from ON ROUT_ = 10Ω, does not include fall time
(from ON asserted to VOUT_ = 90% of
VIN_), COUT_ = 1µF
3ms
Logic Input High Voltage VIN_ = 4V to 5.5V 2 V
Logic Input Low Voltage VIN_ = 4V to 5.5V 0.8 V
Logic Input Current V
ON = 0V or 5.5V -1 1 µA
FAULT_ Output Low Voltage VIN_ = 4V, sinking 1mA 0.4 V
FAULT_ Output High Leakage
Current VIN_ = V
FAULT_ = 5.5V 1 µA
Autoreset Current In latched-off state, VOUT_ = 0V 10 55 mA
Autoreset Threshold In latched-off state, VOUT_ rising 0.4 0.6 V
Autoreset Blanking Time In latched-off state, VOUT_ > 1 V 9 40 ms
AUX_ to IN_ or IN_ to AUX_
Select Delay Make-before-break (Note 1) 4.5 18 ms
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 0°C to -40°C are guaranteed by design, not production tested.
MAX1838
Dual USB Switch with Fault Blanking
and Autoreset
_______________________________________________________________________________________
5
0
20
10
40
30
60
50
70
023145
AUX QUIESCENT CURRENT vs. VAUX
MAX1838 toc01
VAUX (V)
AUX QUIESCENT CURRENT (µA)
VIN = 4.5V
VIN = 5V
VIN = 5.5V
SEL = GND VIN = 0
0
10
5
20
15
25
30
023145
IN QUIESCENT CURRENT vs. VAUX
MAX1838 toc02
VAUX (V)
IN QUIESCENT CURRENT (µA)
VIN = 4.5V
VIN = 5V
VIN = 5.5V
SEL = AUX
70
100
IN + AUX QUIESCENT CURRENT
vs. TEMPERATURE
MAX1838 toc03
TEMPERATURE (°C)
QUIESCENT CURRENT (µA)
-40 20 40-20 0 60 80
80
SEL = AUX = IN
90
VAUX = 4.5V
VAUX = 5V
VAUX = 5.5V
70
80
90
100
110
IN + AUX QUIESCENT CURRENT
vs. TEMPERATURE
MAX1838 toc04
TEMPERATURE (°C)
QUIESCENT CURRENT (µA)
SEL = GND
VIN = VAUX = 5.5V
VIN = VAUX = 4.5V
VIN = VAUX = 5V
-40 20 40-20 0 60 80
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
-40 0-20 20406080
MAX1838 toc05
TEMPERATURE (°C)
SHUTDOWN CURRENT (µA)
IN + AUX QUIESCENT SHUTDOWN CURRENT
vs. TEMPERATURE
VIN = VAUX = 5.0V
VIN = VAUX = 4.5V
VIN = VAUX = 5.5V
-40 80
TEMPERATURE (°C)
SWITCH LEAKAGE CURRENT (nA)
1
10
100
1000
020-20 40 60
MAX1838 toc06
SWITCH LEAKAGE CURRENT
vs. TEMPERATURE, FULL DATA
IN = AUX = SEL = ON = GND
VOUT_ = 5.5V
VIN = VAUX = VON = VSEL = 5.5V
OUT_ = GND
0.7
0.9
0.8
1.1
1.0
1.2
1.3
-40 20 40-20 0 60 80
MAX1838 toc07
TEMPERATURE (°C)
NORMALIZED ON-RESISTANCE (Ω)
NORMALIZED SWITCH ON-RESISTANCE
vs. TEMPERATURE
IOUT_ = 500mA
0.86
0.88
0.90
0.92
0.94
MAX1838 toc08
TEMPERATURE (°C)
CONTINUOUS CURRENT LIMIT THRESHOLD (A)
-40 20-20 0 40 60 80
CONTINUOUS CURRENT LIMIT THRESHOLD
vs. TEMPERATURE
1.8
1.9
2.1
2.0
2.2
2.3
-40 0-20 20 40 60 80
TURN-ON DELAY vs. TEMPERATURE
MAX1838 toc09
TEMPERATURE (°C)
TURN-ON DELAY (ms)
SEL = GND
VIN = VAUX = 4.5V
VIN = VAUX = 5.0V
VIN = VAUX = 5.5V
Typical Operating Characteristics
(Circuit of Figure 2, VIN_ = 5V, TA= +25°C, unless otherwise noted.)
MAX1838
Dual USB Switch with Fault Blanking
and Autoreset
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN_ = 5V, TA= +25°C, unless otherwise noted.)
18.5
19.0
19.5
20.0
20.5
FAULT-BLANKING TIME vs. TEMPERATURE
MAX1838 toc10
TEMPERATURE (°C)
FAULT-BLANKING TIME (ms)
-40 20 40-20 0 60 80
VAUX = 5.0V
VAUX = 4.5V
VAUX = 5.5V
15
20
30
25
35
40
-40 0-20 20 40 60 80
AUTORESET CURRENT vs. TEMPERATURE
MAX1838 toc11
TEMPERATURE (°C)
AUTORESET CURRENT (mA)
VIN = VAUX = 5.5V
VIN = VAUX = 4.5V
0
10
5
20
15
30
25
35
0 1.0 1.5 2.00.5 2.5 3.0 3.5 4.0 4.5 5.0 5.5
AUTORESET CURRENT vs. INPUT VOLTAGE
MAX1838 toc12
INPUT VOLTAGE (V)
AUTOREST CURRENT (mA)
AUX = IN,
SEL = ON = GND
10ms/div
OVERLOAD RESPONSE INTO 2.5Ω LOAD
5V
5V
VFAULT_
IOUT_ 0.5A/div
VOUT_
0
0
0
MAX1838 toc13
0.5A/div
1ms/div
OVERLOAD RESPONSE INTO 2.5Ω LOAD
(EXPANDED TIME SCALE)
5V
5V
VFAULT_
IOUT_
VOUT_
0
0
0
MAX1838 toc14
0.5A/div
10ms/div
SHORT-CIRCUIT RESPONSE INTO 0Ω
5V
5V
VFAULT_
IOUT_
0.5A
VOUT_
0
0
1A
0
MAX1838 toc15
1ms/div
SHORT-CIRCUIT RESPONSE INTO 0Ω
(EXPANDED TIME SCALE)
5V
5V
VFAULT_
VOUT_
0
0
MAX1838 toc16
IOUT_
0.5A
0
1A
MAX1838
Dual USB Switch with Fault Blanking
and Autoreset
_______________________________________________________________________________________ 7
1ms/div
SWITCH TURN-ON TIME, COUT = 1µF
5V
5V
VOUT_
0
1V
2V
3V
4V
0
MAX1838 toc17
VON
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN_ = 5V, TA= +25°C, unless otherwise noted.)
1ms/div
SWITCH TURN-OFF TIME,
COUT = 1µF ROUT = 10Ω
MAX1838 toc18
5V
5V
VOUT_
0
1V
2V
3V
4V
0
VON
Pin Description
PIN NAME FUNCTION
1, 8 GND Ground. Connect both GND pins together.
2 SEL 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.
3, 6 AUXA, AUXB Auxiliary Input. Connect both AUX pins together and bypass with a 0.1µF capacitor to ground.
Load conditions may require additional bulk capacitance to maintain AUX voltage regulation.
4, 5 INA, INB Power Input. Connect both IN pins together and bypass with a 0.1µF capacitor to GND. Load
conditions may require additional bulk capacitance to maintain the IN voltage regulation.
7ON 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.
9FAULTB
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.
10 ADJB 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.
11, 12 OUTB1, OUTB2 Power Output B. Connect both OUTB_ pins together. Connect a 1µF or greater capacitor from
OUTB_ to ground. Load condition may require additional bulk capacitance.
13, 14 OUTA1, OUTA2 Power Output A. Connect both OUTA_ pins together. Connect a 1µF or greater capacitor from
OUTA_ to ground. Load condition may require additional bulk capacitance.
MAX1838
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 fault-
blanking and autoreset function. Each channel has an
error flag output to notify the USB controller when the
current limit, short circuit, undervoltage lockout, or ther-
mal shutdown threshold is reached. A single input
enables both channels. The MAX1838 supplies the out-
puts 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 automati-
cally 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, improv-
ing system robustness.
Input Voltage Requirements and
Undervoltage Lockout
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.
Dual USB Switch with Fault Blanking
and Autoreset
8 _______________________________________________________________________________________
CHANNEL B CONTROL
CIRCUITRY
AUX
IN
OUTB1
FAULT LOGIC
THERMAL
SHUTDOWN A
20ms
BLANKING
FAULT LOGIC
ON
SEL
IC POWER
FAULTB
N
N
ADJB
ADJA
OUTB2
CHANNEL A CONTROL
CIRCUITRY
OUTA1
OUTA2
20ms
BLANKING
THERMAL
SHUTDOWN B
FAULTA
Figure 1. Simplified Functional Diagram
Pin Description (continued)
PIN NAME FUNCTION
15 ADJA 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.
16 FAULTA
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.
Power from IN
(SEL high)
Both A and B outputs receive
current from IN. The IC is powered
from AUX.
Power from AUX
(SEL low)
Both A and B outputs receive
current from AUX. The IC is
powered from AUX. VIN_ may drop
to zero in this mode.
Full Shutdown
(ON high)
Both A and B outputs are open and
no power is available. Either VIN_ or
VAUX_ may drop to zero.
Table 1. MAX1838 Operation Modes
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
).
Autoreset Mode
The MAX1838 monitors the output to detect an over-
load 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.
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 +160°C, the
switch turns off and FAULT_ goes low immediately.
Fault blanking does not occur during thermal limit.
When the junction cools by 15°C, 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, con-
nect FAULT to AUX through a 100kΩpullup resistor.
FAULT goes low when any one of the following condi-
tions 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 +160°C.
•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/µs may cause erroneous FAULT indi-
cations.
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, con-
nect a resistor from ADJ to GND according to the fol-
lowing equation:
IOUT_ = 0.9A ×RADJ / (200kΩ+ RADJ)
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 out-
put 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 reg-
ulates 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 high-
current conditions.
The MAX1838 switches may enter current limit in nor-
mal 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 fault-
blanking 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 chan-
nel enters autoreset mode (see
Autoreset Mode
). Only
current-limit and short-circuit faults are blanked.
Thermal overload faults and input undervoltage imme-
diately 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 suc-
cessfully 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.
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
MAX1838
Dual USB Switch with Fault Blanking
and Autoreset
_______________________________________________________________________________________ 9
MAX1838
between IN and AUX. However, a fault does not trigger
since the fault blanking-time is longer than the supply
switchover time.
Applications Information
Input Power Source
AUX provides the power for the MAX1838 internal con-
trol circuitry. Limit the input-voltage slew rate to less
than 0.2V/µs 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 0.1µF ceramic capacitor is
required for local decoupling; however, higher capaci-
tor 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.
Output Capacitor
An output capacitor helps prevent parasitic inductance
from pulling OUT negative during turn-off. When start-
ing up into very large capacitive loads, the switch puls-
es the output current at 0.35ARMS 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
330µF or smaller capacitor does not trigger a fault out-
put. 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) can load the USB port. These devices
commonly connect to the port with cables, which can
add an inductive component to the load. This induc-
tance causes the output voltage at the USB port to ring
Dual USB Switch with Fault Blanking
and Autoreset
10 ______________________________________________________________________________________
CONDITION MAX1838 BEHAVIOR
If a short circuit is present
(VOUT < 1V)
• 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 0.35A-
RMS. 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.)
If an overload current is present
(VOUT > 1V)
• 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.
If thermal fault condition is present
• 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.
Table 2. MAX1838 Current Limiting and Fault Behavior
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.
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 tran-
sients 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 connect-
ed 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)2RON
For the normal operating current (IOUT = 0.5A), the max-
imum on-resistance of the switch is 170mΩ, and the
power dissipation is:
P = (0.5A)2x 0.170Ω= 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 cur-
rent limit:
P = (ILIM) (VIN - VOUT)
For a 5V input and 1V output, the maximum power dissi-
pation 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 shut-
down threshold and the switch output shuts down until
the junction temperature cools by 10°C. The duty cycle
and period are strong functions of the ambient tempera-
ture 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-over-
load-protection circuitry activates, and the output shuts
down until the junction temperature decreases by 10°C
(see
Thermal Shutdown
).
MAX1838
Dual USB Switch with Fault Blanking
and Autoreset
______________________________________________________________________________________ 11
Figure 2. Typical Application Circuit
FAULTB
FAULTA
OUTB2
OUTB1
OUTA2
OUTA1
ADJB
ADJA
AUXA
AUXB
INA
INB
SEL
ON
GND
AUX
IN OUTB
OUTA
0.1µF
0.1µF10µF
67kΩ
100kΩ
MAX1838
100kΩ
10µF
67kΩ
MAX1838
Dual USB Switch with Fault Blanking
and Autoreset
12 ______________________________________________________________________________________
Chip Information
PROCESS: BiCMOS
Package Information
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 draw-
ings may show a different suffix character, but the drawing per-
tains to the package regardless of RoHS status.
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
16 QSOP E16-5 21-0055
MAX1838
Dual USB Switch with Fault Blanking
and Autoreset
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.
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0 4/01 Initial release —
1 2/10 Removed UL Certification Pending bullet from Features section 1