19-2550; Rev 3; 2/10
Single USB Switch with Autoreset and
Fault Blanking in Tiny TDFN
The MAX1946 single current-limited switch with autoreset supplies a guaranteed 500mA load in accordance
with USB specifications. The MAX1946 operates from a
2.7V to 5.5V input supply and consumes only 40µA of
quiescent current when operating and only 3µA in shutdown. Selectable active-high/active-low control logic
and shutdown control provide additional flexibility. An
autoreset feature latches the switch off in the event of a
short circuit, saving system power. The switch reactivates upon removal of the shorted condition.
The MAX1946 provides several safety features to protect the USB port. Built-in thermal-overload protection
turns off the switch when the die temperature exceeds
+160°C. Accurate internal current-limiting circuitry protects the input supply against both overload and shortcircuit conditions. An open-drain fault signal, (FAULT),
notifies the microprocessor (µP) when a thermal overload, current-limit, undervoltage lockout (UVLO), or
short-circuit fault occurs. A 20ms fault-blanking feature
enables the circuit to ignore momentary faults, such as
those caused when hot-swapping a capacitive load,
preventing false alarms to the host system. The faultblanking feature prevents fault signals from being
issued when the device powers up the load.
The MAX1946 is available in a space-saving 8-pin
(3mm x 3mm) TDFN package and operates over the
extended (-40°C to +85°C) temperature range.
Applications
Features
Single USB Switch in 3mm x 3mm Package
Autoreset Feature Saves System Power
Guaranteed 500mA Load Current
Built-In 20ms Fault-Blanking Circuitry
Active-High or Active-Low Control Logic
Fully Compliant to USB Specifications
2.7V to 5.5V Input Voltage Range
Fault-Indicator Output
Thermal-Overload Protection
Ordering Information
PART
TEMP RANGE
PINPACKAGE
TOP MARK
MAX1946ETA+
-40°C to +85°C
8 TDFN-EP*
ACQ
+Denotes a lead(Pb)-free/RoHS-compliant package.
*EP = Exposed paddle.
USB Ports and USB Hubs
Notebook and Desktop Computers
PDAs and Palmtop Computers
Digital Cameras
Docking Stations
Typical Operating Circuit
Pin Configuration
INPUT
2.7V TO 5.5V
TOP VIEW
+
GND 2
IN
8 SEL
FAULT 1
MAX1946
FAULT
IN
7 ON
OUT 3
6 IN
OUT 4
5 IN
ACTIVE HIGH
ACTIVE LOW
SELECTABLE
ON
MAX1946
OUT
OUT
SEL
GND
TDFN
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
1
MAX1946
General Description
MAX1946
Single USB Switch with Autoreset and
Fault Blanking in Tiny TDFN
ABSOLUTE MAXIMUM RATINGS
IN, ON, OUT, SEL to GND........................................-0.3V to +6V
FAULT to GND .............................................-0.3V to (VIN + 0.3V)
IN to OUT..................................................................-0.3V to +6V
OUT Continuous Switch Current (internally limited)..............1.3A
FAULT DC Current ..............................................................10mA
Continuous Power Dissipation (TA = +70°C)
Exposed Pad Must be Soldered to PC Board
8-Pin TDFN (derate 23.8mW/°C above +70°C) .........1905mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VIN = 5V, CIN = 0.1µF, COUT = 1µF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note1)
PARAMETER
Supply Voltage Range
SYMBOL
CONDITIONS
VIN
MIN
VIN = 5, TA = +25°C
Switch On-Resistance
RON
Standby Supply Current
Quiescent Supply Current
OUT Off-Leakage Current
Undervoltage Lockout Threshold
75
ILKG
VULVO
MAX
UNITS
5.5
V
90
VIN = 5V
110
VIN = 3.3V
125
Switch disabled
IIN
TYP
2.7
Switch enabled, IOUT = 0
3
10
µA
µA
40
60
Switch disabled, VOUT = 0, TA = +25°C
0.01
1
Switch disabled, VOUT = 0, TA = +85°C
0.2
Rising edge, 3% hysteresis
Continuous Load Current
2.3
2.5
2.7
500
Continuous Current Limit
ILIM
VIN_ - VOUT = 0.5V
Short-Circuit Current Limit
ISC
VOUT = 0 (IOUT pulsing)
0.74
0.8
mΩ
µA
V
mA
1.1
1.20
1.2
1.6
A
APK
0.35
ARMS
1
V
Short-Circuit Detect Threshold
(Note 2)
Continuous Current-Limit
Blanking Timeout Period
From continuous current-limit condition to
FAULT asserted
10
20
35
ms
Short-Circuit Blanking Timeout
Period
From short-circuit current-limit condition to
FAULT asserted
7.5
18
35.0
ms
0.25
0.6
1.50
ms
Turn-On Delay
tON
ROUT = 10Ω, time from ON to 10% of VOUT,
does not include rise time
Output Rise Time
tRISE
ROUT = 10Ω, from 10% to 90% of VOUT
1.2
Turn-Off Delay
tOFF
ROUT = 10Ω, time from ON to 90% of VOUT,
does not include fall time
0.8
Output Fall Time
tFALL
Thermal-Shutdown Threshold
2
ROUT = 10Ω, from 90% to 10% of VOUT
15oC hysteresis
ms
2
ms
3
ms
160
°C
_______________________________________________________________________________________
Single USB Switch with Autoreset and
Fault Blanking in Tiny TDFN
(VIN = 5V, CIN = 0.1µF, COUT = 1µF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note1)
PARAMETER
SYMBOL
ON, SEL Input High Level
VIH
ON, SEL Input Low Level
VIL
ON, SEL Input Leakage Current
FAULT Output Low Voltage
CONDITIONS
1.6
VIN = 4V to 5.5V
2
TYP
MAX
UNITS
V
VIN = 2.7V to 4V
0.6
VIN = 4V to 5.5V
0.8
VON, VSEL = 0 or VIN, TA = +25°C
VOL
MIN
VIN = 2.7V to 4V
-1
ISINK = 1mA, VIN = 2.7V
V
+1
µA
0.5
V
FAULT Output High Leakage
Current
VIN = V FAULT = 5.5V
OUT Output Current Autoreset
Mode
In latched off state, VOUT = 0
10
25
45
mA
OUT Autoreset Threshold
In latched off state, VOUT rising
0.4
0.5
0.6
V
OUT Autoreset Blanking Time
In latched off state, VOUT > 0.5V
10
20
35
ms
MIN
TYP
MAX
UNITS
5.5
V
1
µA
ELECTRICAL CHARACTERISTICS
(VIN = 5V, CIN = 0.1µF, COUT = 1µF, TA = -40°C to +85°C, unless otherwise noted.) (Note1)
PARAMETER
Supply Voltage Range
Switch On-Resistance
SYMBOL
VIN
RON
Standby Supply Current
Quiescent Supply Current
OUT Off-Leakage Current
Undervoltage Lockout Threshold
CONDITIONS
IIN
ILKG
VULVO
2.7
VIN = 5, TA = +25°C
90
VIN = 5V
110
VIN = 3.3V
125
mΩ
Switch disabled
10
µA
Switch enabled, IOUT = 0
60
µA
1
µA
Switch disabled, VOUT = 0, TA = -40°C to +25°C
Rising edge, 3% hysteresis
Continuous Load Current
2.3
2.7
500
V
mA
Continuous Current Limit
ILIM
VIN_ - VOUT = 0.5V
0.6
1.3
A
Short-Circuit Current Limit
ISC
VOUT = 0 (IOUT pulsing)
0.8
1.6
APK
Continuous Current-Limit
Blanking Timeout Period
From continuous current-limit condition to
FAULT asserted
10
35
ms
Short-Circuit Blanking Timeout
Period
From short-circuit current-limit condition to
FAULT asserted
7.5
35.0
ms
ROUT = 10Ω. Time from ON to 10% of VOUT.
Does not include rise time.
0.25
1.50
ms
Turn-On Delay
tON
_______________________________________________________________________________________
3
MAX1946
ELECTRICAL CHARACTERISTICS (continued)
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 5V, CIN = 0.1µF, COUT = 1µF, TA = -40°C to +85°C, unless otherwise noted.) (Note1)
PARAMETER
SYMBOL
Turn-Off Delay
tOFF
ON, SEL Input High Level
VIH
ON, SEL Input Low Level
FAULT Output Low Voltage
CONDITIONS
MIN
TYP
ROUT = 10Ω, time from ON to 90% of VOUT,
does not include fall time
VIN = 2.7V to 4V
1.6
VIN = 4V to 5.5V
2
VIL
UNITS
2
ms
V
0.6
VIN = 4V to 5.5V
VON, VSEL = 0 or VIN
VOL
MAX
V
VIN = 2.7V to 4V
ON, SEL Input Leakage Current
-1
ISINK = 1mA, VIN = 2.7V
V
0.8
V
+1
µA
0.5
V
OUT Output Current Autoreset
Mode
In latched off state, VOUT = 0
10
45
mA
OUT Autoreset Threshold
In latched off state, VOUT rising
0.4
0.6
V
OUT Autoreset Blanking Time
In latched off state, VOUT > 0.5V
10
35
ms
Note 1: All parts are 100% tested at TA = +25°C. Limits over the operating temperature range are guaranteed by design.
Note 2: Short-circuit detect threshold is the output voltage at which the device transitions from short-circuit current limit to continuous
current limit.
Typical Operating Characteristics
(Circuit of Figure 2, VIN = 5V, CIN = 0.1µF, COUT = 1µF, ON = SEL, TA = +25°C, unless otherwise noted.)
20
10
0
42
VIN = 5.5V
39
VIN = 5V
36
33
VIN = 2.7V
INPUT VOLTAGE (V)
3.4
ON = GND, VSEL = 5V
3.3
3.2
3.1
3.0
2.9
2.8
2.7
2.6
30
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
4
VSEL = VON = GND
SHUTDOWN SUPPLY CURRENT (µA)
30
MAX1946 toc02
40
45
QUIESCENT SUPPLY CURRENT (µA)
VSEL = VON = GND
MAX1946 toc01
50
SHUTDOWN SUPPLY CURRENT
vs. TEMPERATURE
QUIESCENT SUPPLY CURRENT
vs. TEMPERATURE
MAX1946 toc03
QUIESCENT SUPPLY CURRENT
vs. INPUT VOLTAGE
QUIESCENT SUPPLY CURRENT (µA)
MAX1946
Single USB Switch with Autoreset and
Fault Blanking in Tiny TDFN
-40
-15
10
35
TEMPERATURE (°C)
60
85
-40
-15
10
35
TEMPERATURE (°C)
_______________________________________________________________________________________
60
85
Single USB Switch with Autoreset and
Fault Blanking in Tiny TDFN
NORMALIZED ON-RESISTANCE
vs. TEMPERATURE
10
1
0.1
1.0
0.9
0.8
0.7
0.01
10
35
60
1080
VIN = 5.5V
VIN = 5.0V
1070
1060
1050
VIN = 2.7V
1040
1030
-40
85
-15
10
35
60
85
-40
-15
10
35
60
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
TURN-ON TIME
vs. TEMPERATURE (tON + tRISE)
TURN-OFF TIME vs. TEMPERATURE
(tOFF + tFALL)
FAULT-BLANKING TIME
vs. TEMPERATURE
3.0
VIN = 2.7V
2.5
VIN = 5V
2.0
3.5
VIN = 5.5V
1.5
VIN = 5.5V
VIN = 5V
3.2
2.9
2.6
VIN = 2.7V
21.0
20.8
20.6
VIN = 2.7V
20.4
20.2
VIN = 5V
20.0
19.8
19.6
VIN = 5.5V
19.4
2.3
85
MAX1946 toc09
3.5
ROUT = 10Ω
FAULT-BLANKING TIME (ms)
3.8
MAX1946 toc08
ROUT = 10Ω
TURN-OFF TIME (ms)
4.0
-15
MAX1946 toc07
-40
TURN-ON TIME (ms)
1.1
1090
MAX1946 toc06
1.2
NORMALIZED RON
100
MAX1946 toc05
ON = OUT = GND, VSEL = 5V
SWITCH OFF-LEAKAGE (nA)
1.3
MAX1946 toc04
1000
CONTINUOUS CURRENT-LIMIT
THRESHOLD vs. TEMPERATURE
CONTINUOUS CURRENT THRESHOLD (mA)
SWITCH OFF-LEAKAGE
vs. TEMPERATURE
19.2
1.0
10
35
60
-15
10
35
60
-15
10
35
60
TEMPERATURE (°C)
FAULT OUTPUT LOW VOLTAGE
vs. TEMPERATURE
AUTORESET CURRENT
vs. TEMPERATURE
AUTORESET CURRENT
vs. INPUT VOLTAGE
245
225
VIN = 5V
VIN = 5.5V
30
25
VIN = 5V
20
15
185
VIN = 2.7V
10
VIN = 5.5V
165
-15
10
35
TEMPERATURE (°C)
60
85
31
27
23
TA = -40°C
19
TA = +25°C
15
11
TA = +85°C
7
5
145
85
MAX1946 toc12
35
35
AUTORESET CURRENT (mA)
VIN = 2.7V
205
40
AUTORESET CURRENT (mA)
285
-40
-40
85
TEMPERATURE (°C)
305
265
-40
TEMPERATURE (°C)
ISINK = 1mA
325
85
MAX1946 toc11
345
-15
MAX1946 toc10
-40
FAULT OUTPUT LOW VOLTAGE (mV)
19.0
2.0
-40
-15
10
35
TEMPERATURE (°C)
60
85
3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
INPUT VOLTAGE (V)
_______________________________________________________________________________________
5
MAX1946
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN = 5V, CIN = 0.1µF, COUT = 1µF, ON = SEL, TA = +25°C, unless otherwise noted.)
MAX1946
Single USB Switch with Autoreset and
Fault Blanking in Tiny TDFN
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN = 5V, CIN = 0.1µF, COUT = 1µF, ON = SEL, TA = +25°C, unless otherwise noted.)
OVERLOAD RESPONSE INTO 2.5Ω
(EXPANDED TIME SCALE)
OVERLOAD RESPONSE INTO 2.5Ω
MAX1946 toc13
MAX1946 toc14
A
0
A
0
B
0
B
0
C
0
C
0
0
D
D
0
10ms/div
400µs/div
A: VOUT, 5V/div
B: VFAULT, 5V/div
C: IIN, 1A/div
D: IOUT, 1A/div
A: VOUT, 5V/div
B: VFAULT, 5V/div
C: IIN, 1A/div
D: IOUT, 1A/div
SHORT-CIRCUIT RESPONSE INTO 0Ω
(EXPANDED TIME SCALE)
SHORT-CIRCUIT RESPONSE INTO 0Ω
MAX1946 toc15
MAX1946 toc16
A
0
A
0
B
0
B
0
C
0
C
CURRENT TRANSIENT FROM
SHORTING OUTPUT CAPACITANCE
0
D
0
D
0
10ms/div
A: VOUT, 5V/div
B: VFAULT, 5V/div
C: IIN, 1A/div
D: IOUT, 1A/div
6
400µs/div
A: VOUT, 5V/div
B: VFAULT, 5V/div
C: IIN, 2A/div
D: IOUT, 10A/div
_______________________________________________________________________________________
Single USB Switch with Autoreset and
Fault Blanking in Tiny TDFN
SWITCH TURN-ON TIME
(tON + tRISE)
SWITCH TURN-OFF TIME
(tOFF + tFALL)
MAX1946 toc17
MAX1946 toc18
COUT = 1µF
ROUT = 10Ω
COUT = 1µF
ROUT = 10Ω
A
0
A
0
B
0
B
0
1ms/div
1ms/div
A: VON, 2V/div
B: VOUT, 2V/div
A: VON, 2V/div
B: VOUT, 2V/div
STARTUP WAVEFORMS
COUT = 1µF
COUT = 1µF
ROUT = 10Ω
STARTUP WAVEFORMS
COUT = 330µF
MAX1946 toc19
COUT = 330µF
ROUT = 10Ω
MAX1946 toc20
0
A
B
0
B
0
C
0
C
0
A
D
0
D
0
1ms/div
400µs/div
A: VOUT, 5V/div
B: VFAULT, 5V/div
C: VOUT, 2V/div
D: IOUT, 500mA/div
0
A: VOUT, 5V/div
B: VFAULT, 5V/div
C: VOUT, 2V/div
D: IOUT, 500mA/div
_______________________________________________________________________________________
7
MAX1946
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN = 5V, CIN = 0.1µF, COUT = 1µF, ON = SEL, TA = +25°C, unless otherwise noted.)
MAX1946
Single USB Switch with Autoreset and
Fault Blanking in Tiny TDFN
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN = 5V, CIN = 0.1µF, COUT = 1µF, ON = SEL, TA = +25°C, unless otherwise noted.)
STARTUP WAVEFORMS
COUT = 100µF
UVLO RESPONSE
MAX1946 toc21
A
COUT = 100µF
ROUT = 10Ω
MAX1946 toc22
0
A
0
0
B
C
B
0
C
0
0
D
0
1ms/div
A: VOUT, 5V/div
B: VFAULT, 5V/div
C: VOUT, 2V/div
D: IOUT, 500mA/div
10ms/div
A: VIN, 2V/div
B: VOUT, 2V/div
C: VFAULT, 2V/div
Pin Description
PIN
NAME
FUNCTION
1
FAULT
Fault-Indicator Output Switch. Open-drain output asserts low when switch enters thermal shutdown,
undervoltage lockout, or a sustained (>20ms) current-limit or short-circuit condition.
2
GND
Ground
3, 4
OUT
Switch Power Output. Connect OUT pins together at the device and bypass with a 1µF ceramic
capacitor. Load conditions may require additional bulk capacitance. When disabled, OUT goes into
a high-impedance state.
5, 6
IN
Switch Power Input. Connect IN pins together at the device and bypass with a 0.1µF ceramic
capacitor to GND. Input conditions may require additional bulk capacitance to prevent pulling the
input supply down.
7
ON
ON/OFF Control Input. The active polarity of ON is set by SEL. Connect SEL high to make ON active
high. Connect SEL to GND to make ON active low.
8
SEL
Logic Input Polarity Select. SEL sets the active polarity of the ON input. Connect SEL high to make
ON active high. Connect SEL to GND to make ON active low.
—
EP
Exposed Metal Paddle. This paddle is internally connected to GND through a soft connect. For
enhanced thermal dissipation, connect EP to a large ground plane. Do not use EP as a sole ground
connection.
Detailed Description
The MAX1946 includes output current limiting, short-circuit protection, thermal shutdown, an enable input, and
fault indicator (see the Functional Diagram). A logic
input at SEL sets the active polarity of the enable input.
The fault indicator notifies the system when the currentlimit, short-circuit, undervoltage lockout, or thermalshutdown threshold is exceeded.
8
The MAX1946 operates from a 2.7V to 5.5V input
supply and supplies a minimum output current of 740mA.
A built-in current limit of 1.1A limits the output current in
the event of an overload condition. A built-in short-circuit
detection circuit pulses the output current if the output
voltage falls below 1V. This lowers RMS output current
and reduces power dissipation during continuous short
conditions.
_______________________________________________________________________________________
Single USB Switch with Autoreset and
Fault Blanking in Tiny TDFN
MAX1946
IN
ON
CHARGE
PUMP
SEL
IN
FAULT
MAX1946
ILIM
OUT
THERMAL
SHUTDOWN
25mA
FAULT LOGIC
20ms TIMERS
OSC
25kHz
UVLO
BIAS
REF
GND
Figure 1. Functional Diagram
An internal micropower charge pump generates a highside supply that drives the gate of an internal 75mΩ
NMOS switch.
On/Off Control and Undervoltage
Lockout (UVLO)
SEL sets the active polarity of the logic input of the
MAX1946. Connect ON to the same voltage as SEL to
enable OUT. Connect ON to the opposite voltage as
SEL to disable OUT (see Table 1). The output enters a
high-impedance state when disabled.
The MAX1946 includes a UVLO circuit to prevent erroneous switch operation when the input voltage goes
low during startup and brownout conditions. Input voltages less than 2.5V inhibit operation of the device.
FAULT asserts low during a UVLO condition.
Output Fault Protection and Autoreset
The MAX1946 senses the switch output voltage and
selects continuous current limiting for VOUT greater
than 1V or short-circuit current limiting for VOUT less
than 1V. When VOUT is greater than 1V, the device
operates in a continuous current-limit mode that limits
output current to 1.1A. When VOUT is less than 1V, the
device operates in short-circuit current-limit mode,
sourcing 1.2A pulses to the load. When either fault condition persists for 20ms, the output turns off and the
Table 1. On/Off Control
SEL
GND
VIN
ON
OUT BEHAVIOR
GND
ON
VIN
OFF
GND
OFF
VIN
ON
fault flag asserts. The output automatically restarts
20ms after the short or overload is removed.
The MAX1946 detects short-circuit removal by sourcing
25mA from the output and monitoring the output voltage. When the voltage at the output exceeds 0.5V for
20ms, the fault flag resets, the output turns back on,
and the 25mA current source turns off.
Thermal Shutdown
When the MAX1946 die temperature exceeds +160°C
the switch turns off and FAULT goes low. Thermal shutdown does not utilize the 20ms fault-blanking timeout
period. When the junction temperature cools by 15°C
the switch turns on again and FAULT returns high.The
switch cycles on and off if the overload condition persists, resulting in a pulsed output that reduces the average system load.
_______________________________________________________________________________________
9
MAX1946
Single USB Switch with Autoreset and
Fault Blanking in Tiny TDFN
Fault Indicator
The MAX1946 provides an open-drain fault output,
FAULT. Connect FAULT to IN through a 100kΩ pullup
resistor for most applications. FAULT asserts low when
any of the following conditions occur:
• The input voltage is below the UVLO threshold.
•
The switch junction temperature exceeds the
+160°C thermal-shutdown temperature limit.
•
The switch is in current-limit or short-circuit currentlimit mode for more than 20ms.
The FAULT output deasserts after a 20ms delay once
the fault condition is removed. Ensure that the
MAX1946 input bypass capacitance is sufficiently large
to prevent load glitches from triggering the FAULT output. Limit the input voltage slew rate to 0.2V/µs to prevent erroneous FAULT indications.
To differentiate large capacitive loads from short circuits or sustained overloads, the MAX1946 has a faultblanking circuit. When a load transient causes the
device to enter current limit, an internal counter monitors the duration of the fault. For load faults exceeding
the 20ms fault-blanking time, the switch turns off,
FAULT asserts low, and the device enters autoreset
mode (see the Output Fault Protection and Autoreset
section). Only current-limit and short-circuit faults are
blanked. Thermal-overload faults and input voltage
drops below the UVLO threshold immediately turn the
switch off and assert FAULT low.
Fault blanking allows the MAX1946 to handle USB
loads that may not be fully compliant with the USB
specification. The MAX1946 successfully powers USB
loads with additional bypass capacitance and/or large
startup currents while protecting the upstream power
source. No fault is reported if the switch brings up the
load within the 20ms blanking period. See Table 2 for a
summary of current limit and fault behavior.
Applications Information
Typical Application Circuit
Input Power Supply and Capacitance
Connect both IN inputs together. IN powers the internal
control circuitry and charge pump for the switch.
Bypass IN to GND with a 0.1µF ceramic capacitor.
When driving inductive loads or operating from inductive sources, which may occur when the MAX1946 is
powered by long leads or PC traces, larger input
bypass capacitance is required to prevent voltage
spikes from exceeding the MAX1946’s absolute maximum ratings during short-circuit events.
Table 2. Current Limiting and Fault Behavior
CONDITION
MAX1946 BEHAVIOR
Output Short Circuit
(VOUT < 1V)
• If a short is detected at the output, the switch turns off, and the blanking timer begins. FAULT
remains high during the blanking timeout period.
• If the short persists during the fault-blanking period, the output pulses at 0.35ARMS. If the short is
removed before the 18ms short-circuit blanking timeout period, the next ramped current pulse soft
starts the output. FAULT remains high.
• If the short circuit persists after the fault-blanking period, FAULT goes low, autoreset mode
begins, and the output sources 25mA.
• If the output voltage rises above 0.5V for 20ms, the switch resets, the output turns on, and FAULT
goes high (see Short-Circuit Response in the Typical Operating Characteristics.)
Output Overload Current
(VOUT > 1V)
• Output current regulates at ILIM and the blanking timer turns on. FAULT remains high during
the blanking timeout period.
• Continuous current at ILIM persists until either the 20ms blanking period expires or a thermal
fault occurs.
• If overcurrent persists after 20ms, FAULT goes low, autoreset mode is enabled, and the output
sources 25mA.
• If the output voltage rises above 0.5V for 20ms, the switch resets, the output turns on, and FAULT
goes high (see Short Overload Response in the Typical Operating Characteristics.)
Thermal Fault
(TJ > +160oC)
• A junction temperature of +160°C immediately asserts FAULT low (the blanking timeout period
does not apply for thermal faults) and turns off the switch. When the junction cools by 15°C,
the thermal fault is cleared and FAULT goes high. Note that if other fault conditions are present
when a thermal fault clears, those fault states then take effect.
10
______________________________________________________________________________________
Single USB Switch with Autoreset and
Fault Blanking in Tiny TDFN
IN
100kΩ
FAULT
IN
0.1µF
OFF
ON
MAX1946
OUT
ON
OUT
SEL
1µF
GND
*USB APPLICATIONS MAY REQUIRE
ADDITIONAL BULK CAPACITANCE
Figure 2. Typical Application Circuit
Output Capacitor
Bypass OUT to GND with a 1µF ceramic capacitor for
local decoupling. Additional bulk capacitance (up to
470µF) reduces output-voltage transients under dynamic
load conditions. Using output capacitors greater than
470µF can assert FAULT if the current limit cannot
charge the output capacitor within the 20ms faultblanking period. In addition to bulk capacitance, smallvalue (0.1µF or greater) ceramic capacitors improve
the output’s resilience to electrostatic discharge (ESD).
Driving Inductive Loads
A wide variety of devices (mice, keyboards, cameras, and
printers) typically connect to the USB port with cables,
which can add an inductive component to the load. This
inductance causes the output voltage at the USB port to
oscillate during a load step. The MAX1946 drives inductive loads, but avoid exceeding the device’s absolute
maximum ratings. The load inductance is usually relatively
small, and the MAX1946 input typically includes substantial bulk capacitance from an upstream regulator as well
as local bypass capacitors, limiting overshoot. If severe
ringing occurs due to large load inductance, clamp the
MAX1946 output below +6V and above -0.3V.
Turn-On and Turn-Off Behavior
When turned on, the MAX1946 output ramps up over
1.2ms to eliminate load transients on the upstream
power source. When turned off, the output ramps down
for 3ms. Under fault conditions, the output of the
MAX1946 turns off rapidly to provide maximum safety
for the upstream power source and downstream
devices. Internal blocks shut down to minimize supply
current when the switch is off.
P = (IOUT)2 RON
At the normal operating current (IOUT = 0.5A) and the
maximum on-resistance of the switch (90mΩ), the
power dissipation is:
P = (0.5A)2 0.09Ω = 22.5mW
The worst-case power dissipation occurs when the output current is just below the current-limit threshold
(1.2A max) with an output voltage just greater than 1V.
In this case, the power dissipated is the voltage drop
across the switch multiplied by the current limit:
P = ILIM (VIN - VOUT)
For a 5V input and 1V output, the maximum power dissipation per switch is:
P = 1.2A (5V - 1V) = 4.8W
Since the package power dissipation is 1951mW for the
8-pin TDFN, the MAX1946 die temperature exceeds the
+160°C thermal-shutdown threshold. The switch output
shuts down until the junction temperature cools by
15°C. The duty cycle and period are strong functions of
the ambient temperature and the PC board layout (see
the Thermal Shutdown section).
If the output current exceeds the current-limit threshold,
or the output voltage is pulled below the short-circuit
detect threshold, the MAX1946 enters a fault state after
20ms, at which point autoreset mode is enabled and
25mA is sourced by the output. For a 5V input, OUT
short-circuited to GND, and autoreset mode active, the
power dissipation is as follows:
P = 0.025A 5V = 0.125W
______________________________________________________________________________________
11
MAX1946
INPUT
2.7V TO 5.5V
Layout and Thermal Dissipation
Keep all traces as short as possible to reduce the effect
of undesirable parasitic inductance, and to optimize the
switch response time to output short-circuit conditions.
Place input and output capacitors no more than 5mm
from device leads. Connect IN and OUT to the power
bus with short traces. The exposed pad on the TDFN
package underside must be soldered to the PC board in
order to realize the rated package dissipation.
An active switch dissipates little power with minimal
change in package temperature. Calculate the power
dissipation for this condition as follows:
MAX1946
Single USB Switch with Autoreset and
Fault Blanking in Tiny TDFN
Package Information
Chip Information
PROCESS: BiCMOS
12
For the latest package outline information and land patterns, go
to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in
the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status.
PACKAGE TYPE
PACKAGE CODE
DOCUMENT NO.
8 TDFN-EP
T833+1
21-0137
______________________________________________________________________________________
Single USB Switch with Autoreset and
Fault Blanking in Tiny TDFN
REVISION
NUMBER
REVISION
DATE
3
2/10
DESCRIPTION
Removed UL Certification Pending bullet from Features section
PAGES
CHANGED
1
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 13
© 2010 Maxim Integrated Products
Maxim is a registered trademark of Maxim Integrated Products, Inc.
MAX1946
Revision History
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