MAXIM MAX1940

19-2497; Rev 0; 7/02
Triple USB Switch with Autoreset and
Fault Blanking
The MAX1940 triple current-limited switch with autoreset supplies a guaranteed 500mA load per channel in
accordance with USB specifications. The MAX1940
operates from a 4V to 5.5V input supply and consumes
only 60µA of quiescent current when operating and only
3µA in standby. Selectable active-high/active-low control logic and independent shutdown controls for each
channel provide additional flexibility. An autoreset feature latches the switch off in the event of a short circuit,
saving system power. The switch reactivates upon
removal of the shorted condition.
The MAX1940 provides several safety features to protect the USB port. Built-in thermal-overload protection
turns off the switch when the die temperature exceeds
+160°C. Accurate internal current-limiting circuitry protects the input supply against both overload and shortcircuit conditions. Independent open-drain fault signals
(FAULTA, FAULTB, and FAULTC) notify the microprocessor when a thermal-overload, current-limit,
undervoltage lockout (UVLO), or short-circuit fault
occurs. A 20ms fault-blanking feature enables the circuit to ignore momentary faults, such as those caused
when hot-swapping a capacitive load, preventing false
alarms to the host system. The fault-blanking feature
also prevents fault signals from being issued when the
device powers up the load.
The MAX1940 is available in a space-saving 16-pin
QSOP package and operates over the extended (-40°C
to +85°C) temperature range.
Features
♦ Triple USB Switch in Tiny 16-Pin QSOP Package
♦ Autoreset Feature Saves System Power
♦ Guaranteed 500mA Load Current per Channel
♦ Built-In 20ms Fault-Blanking Circuitry
♦ Active-High/Active-Low Control Logic
♦ Fully Compliant to USB Specifications
♦ 4V to 5.5V Input Voltage Range
♦ Independent Shutdown Control
♦ Independent Fault Indicator Outputs
♦ Thermal-Overload Protection
♦ 3µA Standby Current
♦ UL Certification Pending
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
MAX1940EEE
-40°C to +85°C
16 QSOP
Applications
USB Hubs
PDAs and Palmtop
Computers
Notebook Computers
Docking Stations
USB Ports
Typical Operating Circuit
MAX1940
Desktop Computers
Pin Configuration
INPUT
4V TO 5.5V
IN1
FAULTA
IN2
FAULTB
IN3
FAULTC
TOP VIEW
ONA 1
IN1 2
IN5
15 OUTA
OUTA
USB
PORT A
OUTB
USB
PORT B
OUTC
USB
PORT C
14 GND
IN2 3
IN3 4
IN4
16 FAULTA
MAX1940
ONA
13 OUTB
12 FAULTB
ONB 5
IN4 6
11 SEL
IN5 7
10 OUTC
9
ONC 8
FAULTC
ACTIVE-HIGH/ACTIVE-LOW
SELECTABLE
ONB
ONC
SEL
GND
QSOP
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1
MAX1940
General Description
MAX1940
Triple USB Switch with Autoreset and
Fault Blanking
ABSOLUTE MAXIMUM RATINGS
IN_, ON_, OUT_, SEL, FAULT_, to GND...................-0.3V to +6V
IN1, IN2 to OUTA......................................................-0.3V to +6V
IN2, IN3 to OUTB......................................................-0.3V to +6V
IN4, IN5 to OUTC .....................................................-0.3V to +6V
OUT_ Continuous Switch Current
(per channel, internally limited).........................................1.4A
FAULT_ DC Current ............................................................20mA
Continuous Power Dissipation (TA = +70°C)
16-Pin QSOP (derate 8.3mW/°C above +70°C)...........667mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VIN_ = 5V, CIN_ = 0.1µF, COUT_ = 1µF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
Supply Voltage Range
VIN_
Switch On-Resistance
RON
Standby Supply Current
Quiescent Supply Current
OUT_ Off-Leakage Current
Undervoltage Lockout Threshold
IIN_
ILKG
VULVO
CONDITIONS
Short-Circuit Current Limit
TYP
4.0
TA = 0°C to +85°C, each switch
85
MAX
V
135
mΩ
µA
All switches disabled
3
10
47
75
Two switches enabled, IOUT_ = 0
55
90
All switches enabled, IOUT_ = 0
63
100
0.02
10
3.4
3.8
All switches disabled, VOUT_ = 0,
TA = 0°C to +85°C
Rising edge, 3% hysteresis
3.0
500
ILIM
ISC
VIN_ - VOUT_ = 0.5V
VOUT_ = 0 (IOUT_ pulsing)
UNITS
5.5
One switch enabled, IOUT_ = 0
Continuous Load Current
Continuous Current Limit
MIN
µA
µA
V
mA
0.7
0.9
1.2
A
0.9
1.2
1.6
APK
0.35
ARMS
1
V
Short-Circuit Detect Threshold
(Note 1)
Continuous Current-Limit
Blanking Timeout Period
From continuous current-limit condition to
FAULT_ asserted
10
20
35
ms
Short-Circuit Blanking Timeout
Period
From short-circuit current-limit condition to
FAULT_ asserted
7.5
18
35.0
ms
0.5
1.2
4.0
ms
Turn-On Delay
tON
ROUT_ = 10Ω, does not include rise time
(from ON_ to 10% of VOUT_)
Output Rise Time
tRISE
ROUT_ = 10Ω, from 10% to 90% of VOUT_
2.5
Turn-Off Delay
tOFF
ROUT_ = 10Ω, does not include fall time
(from ON_ to 90% of VOUT_)
0.8
Output Fall Time
tFALL
ROUT_ = 10Ω, from 90% to 10% of VOUT_
2.5
ms
160
°C
Thermal-Shutdown Threshold
15°C hysteresis
ON_, SEL Input High Level
VIH
VIN_ = 4V to 5.5V
ON_, SEL Input Low Level
VIL
VIN_ = 4V to 5.5V
ON_, SEL Input Leakage Current
2
VON_ = 0 or VIN_
ms
3
2
-1
_______________________________________________________________________________________
ms
V
0.8
V
+1
µA
Triple USB Switch with Autoreset and
Fault Blanking
(VIN_ = 5V, CIN_ = 0.1µF, COUT_ = 1µF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
FAULT_ Output Low Voltage
SYMBOL
VOL
FAULT_ Output High Leakage
Current
OUT_ Autoreset Current
CONDITIONS
MIN
TYP
MAX
UNITS
ISINK = 1mA, VIN_ = 4V
0.4
V
VIN_ = V FAULT_ = 5.5V
1
µA
mA
In latched off state, VOUT_ = 0
10
25
45
OUT_ Autoreset Threshold
In latched off state, OUT_ rising
0.4
0.5
0.6
V
OUT_ Autoreset Blanking Time
In latched off state, VOUT_ > 0.5V
10
20
35
ms
MIN
MAX
UNITS
4.0
5.5
V
Each switch
135
mΩ
All switches disabled
10
µA
One switch enabled, IOUT_ = 0
75
ELECTRICAL CHARACTERISTICS
(VIN_ = 5V, CIN_ = 0.1µF, COUT_ = 1µF, TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
SYMBOL
Supply Voltage Range
VIN_
Switch On-Resistance
RON
Standby Supply Current
Quiescent Supply Current
OUT_ Off-Leakage Current
Undervoltage Lockout Threshold
IIN_
ILKG
VULVO
CONDITIONS
Two switches enabled, IOUT_ = 0
90
All switches enabled, IOUT_ = 0
100
All switches disabled, VOUT_ = 0
10
Rising edge, 3% hysteresis
Continuous Load Current
3.0
3.8
500
Continuous Current Limit
ILIM
Short-Circuit Current Limit
ISC
µA
µA
V
mA
VIN_ - VOUT_ = 0.5V
0.7
1.2
A
VOUT_ = 0 (IOUT_ pulsing)
0.9
1.6
APK
Continuous Current-Limit
Blanking Timeout Period
From continuous current-limit condition to
FAULT_ asserted
10
35
ms
Short-Circuit Blanking Timeout
Period
From short-circuit current-limit condition to
FAULT_ asserted
7.5
35.0
ms
0.5
4.0
ms
3
ms
Turn-On Delay
tON
ROUT_ = 10Ω, does not include rise time
(from ON_ to 10% of VOUT_)
Turn-Off Delay
tOFF
ROUT_ = 10Ω, does not include fall time
(from ON_ to 90% of VOUT_)
ON_, SEL Input High Level
VIH
VIN_ = 4V to 5.5V
ON_, SEL Input Low Level
VIL
VIN_ = 4V to 5.5V
2
V
0.8
V
+1
µA
ISINK = 1mA, VIN_ = 4V
0.4
V
FAULT_ Output High Leakage
Current
VIN_ = V FAULT_ = 5.5V
1
µA
OUT_ Autoreset Current
In latched off state, VOUT_ = 0
10
50
mA
OUT_ Autoreset Threshold
In latched off state, OUT_ rising
0.4
0.6
V
OUT_ Autoreset Blanking Time
In latched off state, VOUT_ > 0.5V
10
35
ms
ON_, SEL Input Leakage Current
FAULT_ Output Low Voltage
VON_ = 0 or VIN_
VOL
-1
Note 1: Short-circuit detect threshold is the output voltage at which the device transitions from short-circuit current limit to
continuous current limit.
Note 2: Specifications to -40°C are guaranteed by design, not production tested.
_______________________________________________________________________________________
3
MAX1940
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(Circuit of Figure 2, VIN_ = 5V, CIN_ = 0.1µF, COUT_ = 1µF, ON_ = SEL, TA = +25°C, unless otherwise noted.)
40
30
20
10
68
VIN_ = 5.5V
67
66
65
VIN_ = 5V
64
63
62
VIN_ = 4.5V
3.0
ON_ = GND, VSEL = 5V
2.9
2.8
2.7
2.6
61
0
2.5
60
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
-40
-15
10
35
60
-40
85
-15
10
35
60
85
TEMPERATURE (°C)
TEMPERATURE (°C)
SWITCH OFF-LEAKAGE (ONE SWITCH)
vs. TEMPERATURE
NORMALIZED ON-RESISTANCE
vs. TEMPERATURE
CONTINUOUS CURRENT-LIMIT THRESHOLD
vs. TEMPERATURE
10
1
0.1
1.2
1.1
1.0
0.9
0.8
0.01
0.7
-15
10
35
60
85
-15
10
35
VIN_ = 5.0V
912
908
904
VIN_ = 4.5V
900
-40
85
-15
10
35
60
TEMPERATURE (°C)
TEMPERATURE (°C)
TURN-ON TIME (tON + tRISE)
vs. TEMPERATURE
TURN-OFF TIME (tOFF + tFALL)
vs. TEMPERATURE
FAULT-BLANKING TIME
vs. TEMPERATURE
VIN_ = 5V
3.2
3.1
VIN_ = 5.5V
VIN_ = 5.5V
3.2
TURN-OFF TIME (ms)
3.4
ROUT_ = 10Ω
3.1
VIN_ = 5V
3.0
2.9
2.8
VIN_ = 4.5V
2.6
-15
10
35
TEMPERATURE (°C)
60
85
21.5
VIN_ = 4.5V
21.0
20.5
20.0
VIN_ = 5.5V
VIN_ = 5V
19.5
2.7
3.0
22.0
85
MAX1940 toc09
3.3
FAULT-BLANKING TIME (ms)
VIN_ = 4.5V
ROUT_ = 10Ω
-40
60
VIN_ = 5.5V
916
TEMPERATURE (°C)
3.5
3.3
-40
MAX1940 toc07
-40
920
MAX1940 toc06
MAX1940 toc05
100
1.3
NORMALIZED ON-RESISTANCE
MAX1940 toc04
ON_ = OUT_, VSEL = 5V
CONTINUOUS CURRENT-LIMIT THRESHOLD (mA)
INPUT VOLTAGE (V)
1000
SWITCH OFF-LEAKAGE (nA)
ALL SWITCHES ACTIVE
SHUTDOWN SUPPLY CURRENT (µA)
50
69
MAX1940 toc02
60
70
MAX1940 toc08
QUIESCENT SUPPLY CURRENT (µA)
ALL SWITCHES ACTIVE
QUIESCENT SUPPLY CURRENT (µA)
MAX1940 toc01
70
4
SHUTDOWN SUPPLY CURRENT
vs. TEMPERATURE
QUIESCENT SUPPLY CURRENT
vs. TEMPERATURE
MAX1940 toc03
QUIESCENT SUPPLY CURRENT
vs. INPUT VOLTAGE
TURN-ON TIME (ms)
MAX1940
Triple USB Switch with Autoreset and
Fault Blanking
-40
-15
10
35
TEMPERATURE (°C)
60
19.0
85
-40
-15
10
35
TEMPERATURE (°C)
_______________________________________________________________________________________
60
85
Triple USB Switch with Autoreset and
Fault Blanking
AUTORESET CURRENT (mA)
VIN_ = 5V
36
VIN_ = 5.5V
VIN_ = 4.5V
VIN_ = 5.5V
34
32
VIN_ = 5V
30
28
26
24
22
18
-40
-15
10
35
60
85
35
30
TA = -40°C
25
20
15
TA = +85°C
10
0
-40
-15
TEMPERATURE (°C)
10
35
60
85
3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
TEMPERATURE (°C)
MAX1940 toc13
SHORT-CIRCUIT RESPONSE INTO 0Ω
MAX1940 toc14
5V
A
INPUT VOLTAGE (V)
OVERLOAD RESPONSE INTO 2.5Ω
(EXPANDED TIME SCALE)
OVERLOAD RESPONSE INTO 2.5Ω
TA = +25°C
5
VIN_ = 4.5V
20
40
MAX1940 toc12
38
AUTORESET CURRENT (mA)
RPU = 4.7kΩ
MAX1940 toc11
250
240
230
220
210
200
190
180
170
160
150
140
130
120
110
100
AUTORESET CURRENT
vs. INPUT VOLTAGE
AUTORESET CURRENT
vs. TEMPERATURE
MAX1940 toc10
FAULT OUTPUT LOW VOLTAGE (mV)
FAULT OUTPUT LOW VOLTAGE
vs. TEMPERATURE
MAX1940 toc15
5V
A
5V
A
5V
B
0
B
0
C
5V
B
C
5V
C
0
D
D
D
0
A: VIN_ 5V/div
B: VOUT_ 5V/div
0
10ms/div
C: VFAULT_ 5V/div
D: IOUT_ 1A/div
A: VIN_ 5V/div
B: VOUT_ 5V/div
SHORT-CIRCUIT RESPONSE INTO 0Ω
(EXPANDED TIME SCALE)
SWITCH TURN-ON TIME (tON + tFALL)
VSEL = 5V
ROUT_ = 10Ω
COUT_ = 1µF
VSEL = 5V
ROUT_ = 10Ω
COUT_ = 1µF
MAX1940 toc17
5V
MAX1940 toc18
5V
A
B
A: VIN_ 5V/div
B: VOUT_ 5V/div
10ms/div
C: VFAULT_ 5V/div
D: IOUT_ 1A/div
SWITCH TURN-ON TIME (tON + tRISE)
MAX1940 toc16
A
0
400µs/div
C: VFAULT_ 5V/div
D: IOUT_ 1A/div
5V
A
0
5V
C
5V
B
B
0
D
0
A: VIN_ 5V/div
B: VOUT_ 5V/div
400µs/div
C: VFAULT_ 5V/div
D: IIN_ 2A/div
1ms/div
A: VON_ 5V/div
B: VOUT_ 2V/div
1ms/div
A: VON_ 5V/div
B: VOUT_ 2V/div
_______________________________________________________________________________________
5
MAX1940
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN_ = 5V, CIN_ = 0.1µF, COUT_ = 1µF, ON_ = SEL, TA = +25°C, unless otherwise noted.)
MAX1940
Triple USB Switch with Autoreset and
Fault Blanking
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN_ = 5V, CIN_ = 0.1µF, COUT_ = 1µF, ON_ = SEL, TA = +25°C, unless otherwise noted.)
STARTUP TIME
COUT = 1µF
STARTUP TIME
COUT = 330µF
MAX1940 toc19
VSEL = 5V, ROUT_ = 10Ω, COUT_ = 1µF
MAX1940 toc20
VSEL = 5V, ROUT_ = 10Ω, COUT_ = 330µF
5V
A
5V
A
B
5V
5V
C
B
5V
C
0
500mA
D
500mA
D
400µs/div
C: VOUT_ 5V/div
A: VON_ 5V/div
B: VFAULT_ 5V/div D: IOUT_ 500mA/div
STARTUP TIME
COUT = 100µF
1ms/div
C: VOUT_ 5V/div
A: VON_ 5V/div
B: VFAULT_ 5V/div D: IOUT_ 500mA/div
UNDERVOLTAGE LOCKOUT RESPONSE
MAX1940 toc21
MAX1940 toc22
VSEL = 5V, ROUT_ = 10Ω, COUT_ = 100µF
ROUT_ = 10Ω, COUT_ = 1µF
5V
A
A
0
B
5V
5V
B
0
C
C
0
500mA
D
1ms/div
C: VOUT_ 2V/div
A: VON_ 5V/div
B: VFAULT_ 5V/div D: IOUT_ 500mA/div
6
A: VIN_ 5V/div
B: VOUT_ 5V/div
10ms/div
C: VFAULT_ 5V/div
_______________________________________________________________________________________
Triple USB Switch with Autoreset and
Fault Blanking
PIN
NAME
1
ONA
FUNCTION
Control Input for Switch A. The active polarity of ONA is set by SEL (see Table 1).
2
IN1
Power Input. Connect all IN_ inputs together and bypass with a 0.1µF ceramic capacitor to GND. Load
conditions might require additional bulk capacitance to prevent pulling IN_ down.
3
IN2
Power Input. Connect all IN_ inputs together and bypass with a 0.1µF ceramic capacitor to GND. Load
conditions might require additional bulk capacitance to prevent pulling IN_ down.
4
IN3
Power Input. Connect all IN_ inputs together and bypass with a 0.1µF ceramic capacitor to GND. Load
conditions might require additional bulk capacitance to prevent pulling IN_ down.
5
ONB
6
IN4
Power Input. Connect all IN_ inputs together and bypass with a 0.1µF ceramic capacitor to GND. Load
conditions might require additional bulk capacitance to prevent pulling IN_ down.
7
IN5
Power Input. Connect all IN_ inputs together and bypass with a 0.1µF ceramic capacitor to GND. Load
conditions might require additional bulk capacitance to prevent pulling IN_ down.
8
ONC
Control Input for Switch B. The active polarity of ONB is set by SEL (see Table 1).
Control Input for Switch C. The active polarity of ONC is set by SEL (see Table 1).
Fault Indicator Output for Switch C. Open-drain output asserts low when switch C enters thermal shutdown,
undervoltage lockout, or a sustained (>20ms) current-limit or short-circuit condition.
9
FAULTC
10
OUTC
11
SEL
12
FAULTB
13
OUTB
Power Output for Switch B. Bypass OUTB to GND with a 1µF ceramic capacitor. Load conditions might
require additional bulk capacitance. When disabled, OUTB goes into a high-impedance state.
14
GND
Ground
15
OUTA
Power Output for Switch A. Bypass OUTA to GND with a 1µF ceramic capacitor. Load conditions might
require additional bulk capacitance. When disabled, OUTA goes into a high-impedance state.
16
FAULTA
Power Output for Switch C. Bypass OUTC to GND with a 1µF ceramic capacitor. Load conditions might
require additional bulk capacitance. When disabled, OUTC goes into a high-impedance state.
Logic Input Polarity Select. SEL sets the active polarity of the ON_ inputs. Connect SEL high to set activehigh inputs. Connect SEL to GND to set active-low inputs.
Fault Indicator Output for Switch B. Open-drain output asserts low when switch B enters thermal shutdown,
undervoltage lockout, or enters a sustained (>20ms) current-limit or short-circuit condition.
Fault Indicator Output for Switch A. Open-drain output asserts low when switch A enters thermal shutdown,
undervoltage lockout, or a sustained (>20ms) current-limit or short-circuit condition.
_______________________________________________________________________________________
7
MAX1940
Pin Description
MAX1940
Triple USB Switch with Autoreset and
Fault Blanking
IN_
ONA
SWITCH A
(EXPANDED)
CHARGE
PUMP
IN2
IN1
FAULTA
SEL
ILIM
OUTA
THERMAL
SHUTDOWN
25mA
MAX1940
FAULT LOGIC
20ms TIMERS
OUTB
SWITCH B
(SEE SWITCH A)
UVLO
FAULTB
OSC
25kHz
OUTC
SWITCH C
(SEE SWITCH A)
BIAS
FAULTC
ONB
REF
ONC
GND
IN2 IN3
IN4
IN5
Figure 1. Functional Diagram
Detailed Description
The MAX1940 triple current-limited USB power switch
provides three independent switches, each with its own
enable-control input and fault indicator (see Figure 1).
A logic input sets the active polarity of the enable control inputs. The fault indicators notify the system when
the current-limit, short-circuit, undervoltage lockout, or
thermal-shutdown threshold is exceeded.
The MAX1940 operates from a 4V to 5.5V input supply
and supplies a minimum output current of 700mA per
channel. A built-in current limit of 0.9A (typ) limits the
output current in the event of an overload condition.
Built-in short-circuit detection pulses the output current if
8
the output voltage falls below 1V, resulting in lower RMS
output current and reduced power dissipation in the
device. Independent thermal shutdown allows normal
operation to continue if one channel experiences a prolonged overload or short-circuit condition.
Low-RON NMOS switches enable the MAX1940 to provide three switches in the space-saving 16-pin QSOP
package. An internal micropower charge pump generates the high-side supply needed for driving the gates
of these high-side switches. Separate current-limiting
and thermal-shutdown circuitry permits each switch to
operate independently, improving system robustness.
_______________________________________________________________________________________
Triple USB Switch with Autoreset and
Fault Blanking
SEL sets the active polarity of the logic inputs of the
MAX1940. Connect ON_ to the same voltage as SEL to
enable the respective OUT_ switch. Connect ON_ to
the opposite voltage as SEL to disable the respective
output (see Table 1). The output of a disabled switch
enters a high-impedance state.
The MAX1940 includes a UVLO circuit to prevent erroneous switch operation when the input voltage goes low
during startup and brownout conditions. Input voltages of
less than 3.4V inhibit operation of the device. FAULT_
asserts low during an undervoltage lockout condition.
Output Fault Protection and
Autoreset
The MAX1940 senses the switch output voltage and
selects continuous current limiting for VOUT_ greater
than 1V, or short-circuit current limiting for VOUT_ less
than 1V. When VOUT_ is greater than 1V, the device
operates in a continuous current-limit mode that limits
output current to 0.9A. When VOUT_ is less than 1V, the
device operates in short-circuit current-limit mode,
sourcing 1.2A pulses to the load. When either fault condition persists for 20ms, the output turns off and its fault
flag is asserted. The output automatically restarts 20ms
after the short or overload is removed.
The MAX1940 detects short-circuit removal by sourcing
25mA from the output and monitoring the output voltage. When the voltage at the output exceeds 0.5V for
20ms, the fault flag resets, the output turns back on,
and the 25mA current source turns off. Active loads are
not expected to have measurable current when supplied with less than 0.5V.
Thermal Shutdown
Independent thermal shutdown for each channel permits normal operation of two switches to continue while
a third experiences a thermal fault. The switch turns off
and the FAULT_ output asserts low immediately when
the junction temperature exceeds +160°C. Thermal
shutdown does not utilize the 20ms fault-blanking timeout period. The switch turns on again and FAULT_
returns high when the junction temperature cools by
+15°C. The switch cycles on and off if the overload
condition persists, resulting in a pulsed output that
reduces system power.
Table 1. On/Off Control
SEL
GND
VIN
ON_
OUT_ BEHAVIOR
GND
ON
VIN
OFF
GND
OFF
VIN
ON
Fault Indicators
The MAX1940 provides an independent open-drain fault
output (FAULT_) for each switch. Connect FAULT_ to IN_
through a 100kΩ pullup resistor for most applications.
FAULT_ asserts low when any of the following conditions
occur:
• The input voltage is below the UVLO threshold.
• The switch junction temperature exceeds the +160°C
thermal-shutdown temperature limit.
• The switch is in current-limit or short-circuit currentlimit mode after the fault-blanking period (20ms)
expires.
The FAULT_ output deasserts after a 20ms delay once
the fault condition is removed. Ensure that the MAX1940
input bypass capacitance prevents glitches from triggering the FAULT_ outputs. Limit the input voltage slew rate
to 0.2V/µs to prevent erroneous FAULT_ indications.
To differentiate large capacitive loads from short circuits or sustained overloads, the MAX1940 has an
independent fault-blanking circuit for each switch.
When a load transient causes the device to enter current limit, an internal counter monitors the duration of
the fault. For load faults exceeding the 20ms faultblanking time, the switch turns off, FAULT_ asserts low,
and the device enters autoreset mode (see the Output
Fault Protection and Autoreset Mode section). Only current-limit and short-circuit faults are blanked. Thermal
overload faults and input voltage drops below the
UVLO threshold immediately turn the switch off and
assert FAULT_ low.
Fault blanking allows the MAX1940 to handle USB
loads that might not be fully compliant with USB specifications. The MAX1940 successfully powers USB loads
with additional bypass capacitance and/or large startup currents while protecting the upstream power
source. No fault is reported if the switch brings up the
load within the 20ms blanking period. See Table 2 for a
summary of current-limit and fault behavior.
_______________________________________________________________________________________
9
MAX1940
On/Off Control and
Undervoltage Lockout
MAX1940
Triple USB Switch with Autoreset and
Fault Blanking
Table 2. Current-Limiting and Fault Behavior
CONDITION
Output Short Circuit
(VOUT_ < 1V)
MAX1940 BEHAVIOR
If a short is detected at the output, the channel turns off, and the blanking timer begins. FAULT_ remains
high during the blanking timeout period.
If the short persists during the fault-blanking period, the output pulses at 0.35ARMS. If the short is
removed before the 18ms short-circuit blanking timeout period, the next ramped current pulse soft-starts
the output. FAULT_ remains high.
If the short circuit persists after the fault-blanking period, FAULT_ goes low, autoreset mode begins, and
the output sources 25mA.
If the output voltage rises above 0.5V for 20ms, the output turns on and FAULT_ goes high (see ShortCircuit Response in the Typical Operating Characteristics.)
Output current regulates at ILIM and the blanking timer turns on. FAULT_ remains high during the
blanking timeout period.
Continuous current at ILIM persists until either the 20ms blanking period expires or a thermal fault occurs.
Output Overload Current
If overcurrent persists after 20ms, FAULT_ goes low, autoreset mode is enabled, and the output sources
(VOUT_ > 1V)
25mA.
If the output voltage rises above 0.5V for 20ms, the output turns on and FAULT_ goes high (see Overload
Response into 2.5Ω in the Typical Operating Characteristics.)
Thermal Fault
(TJ > +160°C)
A junction temperature of +160°C immediately asserts FAULT_ low (the blanking timeout period does not
apply for thermal faults) and turns off the switch. When the junction cools by 15°C, the thermal fault is
cleared and FAULT_ goes high. Note that if other fault conditions are present when a thermal fault clears,
those fault states take effect.
Applications Information
Input Power Supply and Capacitance
Connect all IN_ inputs together externally. IN_ powers
the internal control circuitry and charge pump for each
switch. Bypass IN_ to GND with a 0.1µF ceramic
capacitor. When driving inductive loads or operating
from inductive sources, which may occur when the
MAX1940 is powered by long leads or PC traces, larger
input bypass capacitance is required to prevent voltage spikes from exceeding the MAX1940’s absolute
maximum ratings during short-circuit events.
Output Capacitor
Bypass OUT_ to GND with a 1µF ceramic capacitor for
local decoupling. Additional bulk capacitance (up to
470µF) reduces output-voltage transients under
dynamic load conditions. Using output capacitors
greater than 470µF might assert FAULT_ if the current
limit cannot charge the output capacitor within the
20ms fault-blanking period. In addition to bulk capacitance, small-value (0.1µF or greater) ceramic capacitors improve the output’s resilience to electrostatic
discharge (ESD).
Driving Inductive Loads
A wide variety of devices (mice, keyboards, cameras,
and printers) typically connect to the USB port with
10
cables, which might add an inductive component to the
load. This inductance causes the output voltage at the
USB port to oscillate during a load step. The MAX1940
drives inductive loads, but avoid exceeding the
device’s absolute maximum ratings. Usually, the load
inductance is relatively small, and the MAX1940’s input
includes a substantial bulk capacitance from an
upstream regulator as well as local bypass capacitors,
limiting overshoot. If severe ringing occurs because of
large load inductance, clamp the MAX1940 outputs
below +6V and above -0.3V.
Turn-On and Turn-Off Behavior
The MAX1940’s slow turn-on and turn-off minimizes
load transients on the upstream power source. Under
fault conditions, the outputs of the MAX1940 turn off
rapidly to provide maximum safety for the upstream
power source and downstream devices. Internal blocks
shut down to minimize supply current when all three
channels are off.
Layout and Thermal Dissipation
Keep all traces as short as possible to reduce the
effect of undesirable parasitic inductance and optimize
the switch response time to output short-circuit conditions. Place input and output capacitors no more than
5mm from device leads. Connect IN_ and OUT_ to the
______________________________________________________________________________________
Triple USB Switch with Autoreset and
Fault Blanking
100kΩ
MAX1940
INPUT
4V TO 5.5V
100kΩ
IN1
FAULTA
IN2
FAULTB
IN3
FAULTC
100kΩ
O.1µF
IN4
OUTA
IN5
USB
PORT A
1µF*
ONA
OFF
OUTB
ONB
ON
USB
PORT B
1µF*
ONC
SEL
OUTC
GND
USB
PORT C
1µF*
*USB APPLICATIONS MAY REQUIRE
ADDITIONAL BULK CAPACITANCE
Figure 2. Typical Application Circuit
power bus with short traces. Wide power bus planes at
IN_ and OUT provide superior heat dissipation as well.
P = (IOUT_)2 x RON
At the normal operating current (IOUT_ = 0.5A) and the
maximum on-resistance of the switch (135mΩ), the
power dissipation is:
P = (0.5A)2 x 0.135Ω = 34mW per switch.
The worst-case power dissipation occurs when the output
current is just below the current-limit threshold (1.2A max)
with an output voltage greater than 1V. In this case, the
power dissipated in each switch is the voltage drop
across the switch multiplied by the current limit:
P = ILIM x (VIN - VOUT)
For a 5V input and 1V output, the maximum power dissipation per switch is:
P = 1.2A x (5V - 1V) = 4.8W
Because the package power dissipation is 667mW, the
MAX1940 die temperature exceeds the +160°C thermal
shutdown threshold, and the switch output shuts down
until the junction temperature cools by +15°C. The duty
cycle and period are strong functions of the ambient
temperature and the PC board layout (see the Thermal
Shutdown section).
If the output current exceeds the current-limit threshold,
or the output voltage is pulled below the short-circuit
detect threshold, the MAX1940 enters a fault state after
20ms, at which point autoreset mode is enabled and
25mA is sourced by the output. For a 5V input, OUT_
short-circuited to GND, and autoreset mode active, the
power dissipation is as follows:
P = 0.025A x 5V = 0.125W
Chip Information
TRANSISTOR COUNT: 4259
PROCESS: BiCMOS
______________________________________________________________________________________
11
MAX1940
An active switch dissipates little power with minimal
change in package temperature. Calculate the power
dissipation for this condition as follows:
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
QSOP.EPS
MAX1940
Triple USB Switch with Autoreset and
Fault Blanking
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2002 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.