MAXIM MAX1940 User Manual

General Description

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 micro-
processor 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.

Applications

Features

o Triple USB Switch in Tiny 16-Pin QSOP Package

o Autoreset Feature Saves System Power

o Guaranteed 500mA Load Current per Channel

o Built-In 20ms Fault-Blanking Circuitry

o Active-High/Active-Low Control Logic

o Fully Compliant to USB Specifications

o 4V to 5.5V Input Voltage Range

o Independent Shutdown Control

o Independent Fault Indicator Outputs

o Thermal-Overload Protection

o 3µA Standby Current

MAX1940

Triple USB Switch with Autoreset and

Fault Blanking

________________________________________________________________

Maxim Integrated Products

1

Typical Operating Circuit

Ordering Information

19-2497; Rev 1; 2/10

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.

Pin Configuration

USB Ports

USB Hubs

Notebook Computers

Desktop Computers

PDAs and Palmtop
Computers

Docking Stations

PART TEMP RANGE PIN-PACKAGE

MAX1940EEE -40°C to +85°C 16 QSOP

TOP VIEW

ONA

1

IN1

2

IN2

3

4

IN3

ONB

5

IN4

6

IN5

7

ONC

8

16

FAULTA

15

OUTA

14

GND

MAX1940

QSOP

13

12

11

10

9

OUTB

FAULTB

SEL

OUTC

FAULTC

INPUT

4V TO 5.5V

ACTIVE-HIGH/ACTIVE-LOW

SELECTABLE

MAX1940

IN1

IN2

IN3

IN4

IN5

ONA

ONB

ONC

SEL

FAULTA

FAULTB

FAULTC

OUTA

OUTB

OUTC

GND

USB
PORT A

USB
PORT B

USB
PORT C

MAX1940

Triple USB Switch with Autoreset and
Fault Blanking

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

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_, 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 (T

A

= +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

ELECTRICAL CHARACTERISTICS

(V

IN_

= 5V, C

IN_

= 0.1µF, C

OUT_

= 1µF, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)

Supply Voltage Range V

Switch On-Resistance R

Standby Supply Current All switches disabled 3 10 µA

OUT_ Off-Leakage Current I

Undervoltage Lockout Threshold V

Continuous Load Current 500 mA

Continuous Current Limit I

Short-Circuit Current Limit I

Short-Circuit Detect Threshold (Note 1) 1 V

Continuous Current-Limit
Blanking Timeout Period

Short-Circuit Blanking Timeout
Period

Turn-On Delay t

Output Rise Time t

Turn-Off Delay t

Output Fall Time t

Thermal-Shutdown Threshold 15°C hysteresis 160 °C

ON_, SEL Input High Level V

ON_, SEL Input Low Level V

ON_, SEL Input Leakage Current V

PARAMETER SYM B O L CONDITIONS MIN TYP MAX UNITS

IN_

ON

IN_

LKG

ULVO

LIM

SC

TA = 0°C to +85°C, each switch 85 135 mΩ

One switch enabled, I

Two switches enabled, I

All switches enabled, I

All switches disabled, V
T

= 0°C to +85°C

A

= 0 47 75

OUT_

= 0 55 90Quiescent Supply Current I

OUT_

= 0 63 100

OUT_

= 0,

OUT_

Rising edge, 3% hysteresis 3.0 3.4 3.8 V

V

IN_

V

OUT_

- V

= 0.5V 0.7 0.9 1.2 A

OUT_

= 0 (I

OUT_

pulsing)

From continuous current-limit condition to
FAULT_ asserted

From short-circuit current-limit condition to
FAULT_ asserted

R

= 10Ω, does not include rise time

ON

RISE

OFF

FALL

IH

IL

OUT_

(from ON_ to 10% of V

R

= 10Ω, from 10% to 90% of V

OUT_

R

= 10Ω, does not include fall time

OUT_

(from ON_ to 90% of V

R

= 10Ω, from 90% to 10% of V

OUT_

V

= 4V to 5.5V 2 V

IN_

V

= 4V to 5.5V 0.8 V

IN_

= 0 or V

ON_

IN_

OUT_

OUT_

)

OUT_

)

OUT_

4.0 5.5 V

0.02 10 µA

0.9 1.2 1.6 A

0.35 A

10 20 35 ms

7.5 18 35.0 ms

0.5 1.2 4.0 ms

2.5 ms

0.8 3 ms

2.5 ms

-1 +1 µA

µA

PK

RMS

MAX1940

Triple USB Switch with Autoreset and

Fault Blanking

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(V

IN_

= 5V, C

IN_

= 0.1µF, C

OUT_

= 1µF, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)

ELECTRICAL CHARACTERISTICS

(V

IN_

= 5V, C

IN_

= 0.1µF, C

OUT_

= 1µF, TA= -40°C to +85°C, unless otherwise noted.) (Note 2)

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.

FAULT_ Output Low Voltage V
FAULT_ Output High Leakage

Current

OUT_ Autoreset Current In latched off state, V

PARAMETER SYM B O L CONDITIONS MIN TYP MAX UNITS

I

OL

SINK

V

IN_

= 1mA, V

= V

FAULT_

= 4V 0.4 V

IN_

= 5.5V 1 µA

= 0 102545mA

OUT_

OUT_ Autoreset Threshold In latched off state, OUT_ rising 0.4 0.5 0.6 V

OUT_ Autoreset Blanking Time In latched off state, V

> 0.5V 10 20 35 ms

OUT_

Supply Voltage Range V

Switch On-Resistance R

Standby Supply Current All switches disabled 10 µA

OUT_ Off-Leakage Current I

Undervoltage Lockout Threshold V

Continuous Load Current 500 mA

Continuous Current Limit I

Short-Circuit Current Limit I

Continuous Current-Limit
Blanking Timeout Period

Short-Circuit Blanking Timeout
Period

Turn-On Delay t

Turn-Off Delay t

ON_, SEL Input High Level V

ON_, SEL Input Low Level V

ON_, SEL Input Leakage Current V

FAULT_ Output Low Voltage V

FAULT_ Output High Leakage

Current

OUT_ Autoreset Current In latched off state, V

OUT_ Autoreset Threshold In latched off state, OUT_ rising 0.4 0.6 V

OUT_ Autoreset Blanking Time In latched off state, V

PARAMETER SYM B O L CONDITIONS MIN MAX UNITS

IN_

ON

IN_

LKG

ULVO

LIM

SC

Each switch 135 mΩ

One switch enabled, I

Two switches enabled, I

All switches enabled, I

All switches disabled, V

= 0 75

OUT_

= 0 90Quiescent Supply Current I

OUT_

= 0 100

OUT_

= 0 10 µA

OUT_

Rising edge, 3% hysteresis 3.0 3.8 V

V

IN_

V

OUT_

- V

= 0.5V 0.7 1.2 A

OUT_

= 0 (I

pulsing) 0.9 1.6 A

OUT_

From continuous current-limit condition to
FAULT_ asserted

From short-circuit current-limit condition to
FAULT_ asserted

R

= 10Ω, does not include rise time

ON

OFF

IH

IL

OL

OUT_

(from ON_ to 10% of V

R

= 10Ω, does not include fall time

OUT_

(from ON_ to 90% of V

V

= 4V to 5.5V 2 V

IN_

V

= 4V to 5.5V 0.8 V

IN_

= 0 or V

ON_

I

SINK

V

IN_

= 1mA, V

= V

IN_

= 4V 0.4 V

IN_

= 5.5V 1 µA

FAULT_

)

OUT_

)

OUT_

= 0 10 50 mA

OUT_

> 0.5V 10 35 ms

OUT_

4.0 5.5 V

10 35 ms

7.5 35.0 ms

0.5 4.0 ms

3ms

-1 +1 µA

µA

PK

MAX1940

Triple USB Switch with Autoreset and
Fault Blanking

4 _______________________________________________________________________________________

Typical Operating Characteristics

(Circuit of Figure 2, V

IN_

= 5V, C

IN_

= 0.1µF, C

OUT_

= 1µF, ON_ = SEL, TA= +25°C, unless otherwise noted.)

QUIESCENT SUPPLY CURRENT

vs. INPUT VOLTAGE

MAX1940 toc01

INPUT VOLTAGE (V)

QUIESCENT SUPPLY CURRENT (µA)

5.04.54.03.53.02.52.01.51.00.5

10

20

30

40

50

60

70

0

05.5

ALL SWITCHES ACTIVE

QUIESCENT SUPPLY CURRENT

vs. TEMPERATURE

MAX1940 toc02

TEMPERATURE (°C)

QUIESCENT SUPPLY CURRENT (µA)

603510-15

61

62

63

64

65

66

67

68

69

70

60

-40 85

ALL SWITCHES ACTIVE

V

IN_

= 5.5V

V

IN_

= 5V

V

IN_

= 4.5V

SHUTDOWN SUPPLY CURRENT

vs. TEMPERATURE

MAX1940 toc03

TEMPERATURE (°C)

SHUTDOWN SUPPLY CURRENT (µA)

603510-15

2.6

2.7

2.8

2.9

3.0

2.5

-40 85

ON_ = GND, V

SEL

= 5V

SWITCH OFF-LEAKAGE (ONE SWITCH)

vs. TEMPERATURE

MAX1940 toc04

TEMPERATURE (°C)

SWITCH OFF-LEAKAGE (nA)

603510-15

0.1

1

10

100

1000

0.01

-40 85

ON_ = OUT_, V

SEL

= 5V

CONTINUOUS CURRENT-LIMIT THRESHOLD

vs. TEMPERATURE

MAX1940 toc06

TEMPERATURE (°C)

CONTINUOUS CURRENT-LIMIT THRESHOLD (mA)

603510-15

904

908

912

916

920

900

-40 85

V

IN_

= 5.0V

V

IN_

= 5.5V

V

IN_

= 4.5V

FAULT-BLANKING TIME

vs. TEMPERATURE

MAX1940 toc09

TEMPERATURE (°C)

FAULT-BLANKING TIME (ms)

603510-15

19.5

20.0

20.5

21.0

21.5

22.0

19.0

-40 85

V

IN_

= 5.5V

V

IN_

= 5V

V

IN_

= 4.5V

NORMALIZED ON-RESISTANCE

1.3

vs. TEMPERATURE

TURN-ON TIME (tON + t

RISE

)

vs. TEMPERATURE

3.5
R

= 10Ω

OUT_

3.4

V

= 5V

IN_

3.3

3.2

TURN-ON TIME (ms)

3.1

3.0

-40 85

V

= 4.5V

IN_

V

IN_

TEMPERATURE (°C)

MAX1940 toc07

= 5.5V

603510-15

1.2

1.1

1.0

0.9

NORMALIZED ON-RESISTANCE

0.8

0.7

-40 85
TEMPERATURE (°C)

TURN-OFF TIME (t

OFF

+ t

vs. TEMPERATURE

3.3
R

= 10Ω

OUT_

3.2

3.1

3.0

2.9

TURN-OFF TIME (ms)

2.8

2.7

2.6

-40 85

V

V

IN_

= 5V

IN_

TEMPERATURE (°C)

= 5.5V

V

603510-15

FALL

IN_

603510-15

MAX1940 toc05

)

MAX1940 toc08

= 4.5V

MAX1940

Triple USB Switch with Autoreset and

Fault Blanking

_______________________________________________________________________________________

5

Typical Operating Characteristics (continued)

(Circuit of Figure 2, V

IN_

= 5V, C

IN_

= 0.1µF, C

OUT_

= 1µF, ON_ = SEL, TA= +25°C, unless otherwise noted.)

FAULT OUTPUT LOW VOLTAGE

vs. TEMPERATURE

250

RPU = 4.7kΩ

240
230
220

V

210
200
190
180
170
160
150
140
130

FAULT OUTPUT LOW VOLTAGE (mV)

120
110
100

= 5.5V

IN_

-40 85

V

= 5V

IN_

V

IN_

TEMPERATURE (°C)

OVERLOAD RESPONSE INTO 2.5Ω

A

B

C

D

10ms/div

5V/div

C: V
D: I

FAULT_

OUT_

A: V
B: V

IN_
OUT_

5V/div

SHORT-CIRCUIT RESPONSE INTO 0Ω

(EXPANDED TIME SCALE)

A

B

C

= 4.5V

5V/div

1A/div

603510-15

MAX1940 toc13

MAX1940 toc16

AUTORESET CURRENT

vs. INPUT VOLTAGE

TA = -40°C

TA = +25°C

TA = +85°C

INPUT VOLTAGE (V)

SHORT-CIRCUIT RESPONSE INTO 0Ω

5V/div

10ms/div

C: V

FAULT_

D: I

OUT_

1A/div

A: V
B: V

IN_
OUT_

5V/div

SWITCH TURN-ON TIME (tON + t

V

= 5V

SEL

= 10Ω

R

OUT_

= 1µF

C

OUT_

38

36

MAX1940 toc10

34

32

30

28

26

24

AUTORESET CURRENT (mA)

22

20

18

5V

0

0

0

5V

0

5V

A

B

C

D

A

B

AUTORESET CURRENT

vs. TEMPERATURE

V

= 5.5V

IN_

V

= 5V

IN_

V

= 4.5V

IN_

-40 85
TEMPERATURE (°C)

603510-15

OVERLOAD RESPONSE INTO 2.5Ω

(EXPANDED TIME SCALE)

400µs/div

5V/div

C: V
D: I

FAULT_

OUT_

1A/div

A: V
B: V

IN_
OUT_

5V/div

SWITCH TURN-ON TIME (tON + t

V

= 5V

SEL

= 10Ω

R

OUT_

= 1µF

C

OUT_

5V/div

MAX1940 toc14

RISE

MAX1940 toc17

)

40

35

MAX1940 toc11

30

25

20

15

AUTORESET CURRENT (mA)

10

5

0

3.5 5.5

5V

0

5V

0

5V

5V

A

B

C

D

A

B

5V/div

MAX1940 toc15

FALL

MAX1940 toc18

MAX1940 toc12

5.35.13.7 3.9 4.1 4.5 4.74.3 4.9

5V

5V

5V

0

)

5V

D

400µs/div

5V/div

C: V
D: I

FAULT_

IN_

2A/div

5V/div

A: V
B: V

IN_
OUT_

5V/div

0

A: V

5V/div

ON_

2V/div

B: V

OUT_

1ms/div

A: V
B: V

ON_
OUT_

5V/div

2V/div

1ms/div

0

MAX1940

Triple USB Switch with Autoreset and
Fault Blanking

6 _______________________________________________________________________________________

Typical Operating Characteristics (continued)

(Circuit of Figure 2, V

IN_

= 5V, C

IN_

= 0.1µF, C

OUT_

= 1µF, ON_ = SEL, TA= +25°C, unless otherwise noted.)

SEL

ON_
FAULT_

= 5V, R

5V/div

STARTUP TIME

= 330µF

C

OUT

= 10Ω, C

OUT_

1ms/div

C: V

5V/div

D: I

OUT_

OUT_

= 330µF

OUT_

5V/div

500mA/div

MAX1940 toc20

5V

5V

0

500mA

STARTUP TIME

= 1µF

C

OUT

V

= 5V, R

SEL

A

B

C

D

A: V

ON_

B: V

FAULT_

5V/div

5V/div

OUT_

= 10Ω, C

400µs/div

C: V

OUT_

D: I

OUT_

= 1µF

OUT_

5V/div

500mA/div

MAX1940 toc19

5V

5V
5V

500mA

V

A

B

C

D

A: V
B: V

STARTUP TIME

= 100µF

C

OUT

V

= 5V, R

SEL

A

B

C

D

A: V

ON_

B: V

FAULT_

5V/div

5V/div

OUT_

= 10Ω, C

1ms/div

C: V

OUT_

D: I

OUT_

= 100µF

OUT_

2V/div

500mA/div

MAX1940 toc21

5V

5V
5V

500mA

UNDERVOLTAGE LOCKOUT RESPONSE

R

= 10Ω, C

OUT_

A

B

C

5V/div

A: V

IN_

5V/div

B: V

OUT_

OUT_

10ms/div

C: V

= 1µF

FAULT_

5V/div

MAX1940 toc22

0

0

0

MAX1940

Triple USB Switch with Autoreset and

Fault Blanking

_______________________________________________________________________________________ 7

Pin Description

PIN NAME FUNCTION

1 ONA Control Input for Switch A. The active polarity of ONA is set by SEL (see Table 1).

2 IN1

3 IN2

4 IN3

5 ONB Control Input for Switch B. The active polarity of ONB is set by SEL (see Table 1).

6 IN4

7 IN5

8 ONC Control Input for Switch C. The active polarity of ONC is set by SEL (see Table 1).

9 FAULTC

10 OUTC

11 SEL

12 FAULTB

13 OUTB

14 GND Ground

15 OUTA

16 FAULTA

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

MAX1940

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

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-R

ON

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

8 _______________________________________________________________________________________

Figure 1. Functional Diagram

ONA

ONB

IN1

SEL

MAX1940

THERMAL

SHUTDOWN

FAULT LOGIC
20ms TIMERS

UVLO

CHARGE

PUMP

OSC

25kHz

SWITCH B

(SEE SWITCH A)

BIAS

25mA

IN2

ILIM

SWITCH A

(EXPANDED)

SWITCH C

(SEE SWITCH A)

IN_

FAULTA

OUTA

OUTB

FAULTB

OUTC

FAULTC

ONC

REF

IN2 IN3 IN4 IN5GND

On/Off Control and

Undervoltage Lockout

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 V

OUT_

greater

than 1V, or short-circuit current limiting for V

OUT_

less

than 1V. When V

OUT_

is greater than 1V, the device
operates in a continuous current-limit mode that limits
output current to 0.9A. When V

OUT_

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.

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.

MAX1940

Triple USB Switch with Autoreset and

Fault Blanking

_______________________________________________________________________________________ 9

Table 1. On/Off Control

SEL ON_ OUT_ BEHAVIOR

GND

V

IN

GND ON

V

IN

GND OFF

V

IN

OFF

ON

MAX1940

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

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

10 ______________________________________________________________________________________

Table 2. Current-Limiting and Fault Behavior

CONDITION 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 0.35A

Output Short Circuit

< 1V)

(V

OUT_

Output Overload Current

> 1V)

(V

OUT_

Thermal Fault
(T

> +160°C)

J

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 I
blanking timeout period.
Continuous current at I
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 output turns on and FAULT_ goes high (see Overload
Response into 2.5Ω in the Typical Operating Characteristics.)

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.

LIM

and the blanking timer turns on. FAULT_ remains high during the

LIM

persists until either the 20ms blanking period expires or a thermal fault occurs.

. If the short is

RMS

power bus with short traces. Wide power bus planes at
IN_ and OUT provide superior heat dissipation as well.

An active switch dissipates little power with minimal
change in package temperature. Calculate the power
dissipation for this condition as follows:

P = (I

OUT_

)2x R

ON

At the normal operating current (I

OUT_

= 0.5A) and the
maximum on-resistance of the switch (135mΩ), the
power dissipation is:

P = (0.5A)2x 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 = I

LIM

x (VIN- V

OUT

)

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

MAX1940

Triple USB Switch with Autoreset and

Fault Blanking

______________________________________________________________________________________ 11

Figure 2. Typical Application Circuit

100kΩ

MAX1940

IN1

INPUT

4V TO 5.5V

O.1µF

OFF

ON

IN2

IN3

IN4

IN5

ONA

ONB

ONC

SEL

*USB APPLICATIONS MAY REQUIRE
ADDITIONAL BULK CAPACITANCE

FAULTA

FAULTB

FAULTC

OUTA

OUTB

OUTC

GND

100kΩ

100kΩ

USB
PORT A

1µF*

USB
PORT B

1µF*

USB
PORT C

1µF*

MAX1940

Triple USB Switch with Autoreset and
Fault Blanking

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 draw­ings may show a different suffix character, but the drawing per­tains to the package regardless of RoHS status.

PACKAGE TYPE PACKAGE CODE DOCUMENT NO.

16 QSOP E16-5

21-0055

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.

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

0 7/02 Initial release —

1 2/10 Removed UL Certification Pending bullet from Features section 1

REVISION

DATE

DESCRIPTION

PAGES

CHANGED