MAX1838EEE-T [MAXIM]
Power Supply Support Circuit, Fixed, 1 Channel, BICMOS, PDSO16, 0.150 INCH, 0.025 INCH PTICH, QSOP-16;
| 型号： | MAX1838EEE-T |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | Power Supply Support Circuit, Fixed, 1 Channel, BICMOS, PDSO16, 0.150 INCH, 0.025 INCH PTICH, QSOP-16 开关 |
| 文件： | 总13页 (文件大小：316K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-2033; Rev 1; 2/10
Dual USB Switch with Fault Blanking
and Autoreset
MAX138
General Description
Features
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.
o Dual USB Switch with Auxiliary Input
o Autoreset from Fault Condition
o Guaranteed 500mA Load per Channel
o Programmable Current Limits on Auxiliary Inputs
o Reverse Current Protection
o Thermal Overload Protection
o Built-In 20ms Fault-Blanking
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.
o 4V to 5.5V Range
o Independent FAULT Indicator Outputs
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.
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
MAX1838EEE
-40°C to +85°C
16 QSOP
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
Typical Operating Circuit
Notebook Computers
USB Ports
Docking Stations
Pin Configuration
AUX
IN
AUXA
AUXB
FAULTB
FAULTA
TOP VIEW
GND
SEL
1
2
3
4
5
6
7
8
16 FAULTA
15 ADJA
MAX1838
OUTB
OUTA
INA
INB
OUTB
10µF
AUXA
INA
14 OUTA2
13 OUTA1
12 OUTB2
11 OUTB1
10 ADJB
MAX1838
SEL
ON
OUTA
INB
ADJB
ADJA
AUXB
ON
GND
GND
9
FAULTB
QSOP
________________________________________________________________ Maxim Integrated Products
1
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.
Dual USB Switch with Fault Blanking
and Autoreset
ABSOLUTE MAXIMUM RATINGS
Operating Temperature Range ...........................-40°C to +85°C
IN_, AUX_, ON, OUT_, SEL, FAULT_,
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
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 (T = +70°C)
A
MAX138
16-Pin QSOP (derate 8.3mW/°C above +70°C)..........667mW
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
+25°C.)
(V
= V
= 5V, ON = SEL = GND, ADJ_ = open, T = 0°C to +85°C, unless otherwise noted. Typical values are at T
=
IN__
AUX__
A
A
PARAMETER
SYMBOL
CONDITIONS
MIN
4.0
TYP
MAX
5.5
UNITS
V
IN_
Supply Voltage Range
Switch On-Resistance
V
V
4.0
5.5
AUX_
V
V
V
V
V
V
V
V
V
_ = 5V, each switch
95
100
7
170
IN
R
mΩ
ON
_ = 4V, each switch
IN
AUX_ Standby Supply Current
IN_ Standby Supply Current
AUX_ Quiescent Supply Current
IN_ Quiescent Supply Current
OUT_ Off-Leakage Current
Reverse Leakage Current
= V
= V
_
16
3
µA
µA
µA
µA
µA
µA
V
ON
ON
ON
ON
ON
OUT
AUX
_
1
IN
= 0V, I
= 0V, I
_ = 0A
OUT
66
350
300
10
_ = 0A, SEL = AUX_
OUT
23
= V _, V
_ = V _ = 0V
OUTA OUTB
0.02
0.02
3.5
IN
= 5.5V, V _ = V
_ = 0V
AUX
10
IN
Undervoltage Lockout Threshold
Continuous Load Current
V
_ or V _ rising edge, 3% hysteresis
AUX
3.0
500
0.6
3.8
UVLO
IN
mA
V
V
_ - V
_ = 0.5V, SEL = AUX_
_ = 0.5V, SEL = GND
OUT
0.9
0.9
1.2
1.2
IN
OUT
Current-Limit Threshold
I
A
_ - V
0.6
LIM
AUX
R _ = 67kΩ
ADJ
0.09
0.225
0.36
SEL = GND,
0.3
0.48
1.8
A
PEAK
V
(I
_ = 0V
_ pulsing)
R
_ = 67kΩ
ADJ
OUT
OUT
0.09
A
RMS
Short-Circuit Current Limit
I
SHORT
0.6
1.2
A
PEAK
SEL = AUX_, V
_ = 0V
OUT
0.35
A
RMS
Continuous Current-Limit
Blanking Timeout Period
From continuous current-limit condition to
FAULT_ assertion
10
20
18
35
35
ms
Short-Circuit Blanking Timeout
Period
From short-circuit current-limit condition to
FAULT_ assertion
7.5
ms
2
_______________________________________________________________________________________
Dual USB Switch with Fault Blanking
and Autoreset
MAX138
ELECTRICAL CHARACTERISTICS (continued)
+25°C.)
(V
= V
= 5V, ON = SEL = GND, ADJ_ = open, T = 0°C to +85°C, unless otherwise noted. Typical values are at T
=
IN__
AUX__
A
A
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
R
OUT_
= 10Ω, does not include rise time
Turn-On Delay
(from ON asserted to V
_ = 10% of
0.5
2
4.0
ms
ms
ms
ms
OUT
V
_), C _ = 1µF
OUT
IN
R _ = 10Ω, from 10% to 90% of V
OUT
,
OUT_
Output Rise Time
Turn-Off Delay from ON
Output Fall Time
4.5
0.8
C
_ = 1µF
OUT
R
_ = 10Ω, does not include fall time
= 90% of
OUT
(from ON asserted to V
3
OUT_
V
_), C _ = 1µF
OUT
IN
R _ = 10Ω, from 90% to 10% of V
OUT
_,
OUT
3
C
_ = 1µF
OUT
Thermal Shutdown Threshold
Logic Input High Voltage
Logic Input Low Voltage
Logic Input Current
10°C hysteresis
160
°C
V
V
V
V
V
_ = 4V to 5.5V
_ = 4V to 5.5V
2
IN
0.8
1
V
IN
= 0V or 5.5V
-1
µA
V
ON
FAULT_ Output Low Voltage
_ = 4V, sinking 1mA
0.4
IN
FAULT_ Output High Leakage
Current
V
_ = V
= 5.5V
1
µA
IN
FAULT_
Autoreset Current
In latched-off state, V
In latched-off state, V
In latched-off state, V
_ = 0V
_ rising
_ > 1V
10
0.4
10
25
0.5
20
50
0.6
35
mA
V
OUT
OUT
OUT
Autoreset Threshold
Autoreset Blanking Time
ms
AUX_ to IN_ or IN_ to AUX_
Select Delay
Make-before-break (Note 1)
4.5
9
18
ms
ELECTRICAL CHARACTERISTICS
(V
= V
= V
= 5V, SEL = GND, ADJ_ = open, T = -40°C to +85°C, unless otherwise noted. Circuit of Figure 1.) (Note 2)
A
IN__
AUX__
ON
PARAMETER
SYMBOL
CONDITIONS
MIN
4.0
MAX
5.5
5.5
170
16
UNITS
V
V
_
IN
Supply Voltage Range
V
_
AUX
4.0
V
Switch On-Resistance
R
V
V
V
V
V
V
V
V
_ = 5V, each switch
mΩ
µA
µA
µA
µA
µA
µA
V
ON
IN
AUX_ Standby Supply Current
IN_ Standby Supply Current
AUX_ Quiescent Supply Current
IN_ Quiescent Supply Current
OUT_ Off-Leakage Current
Reverse Leakage Current
Undervoltage Lockout Threshold
Continuous Load Current
= V
= V
_
ON
ON
ON
ON
ON
OUT
AUX
_
3
IN
= 0V, I
= 0V, I
_ = 0A
OUT
350
300
10
= 0A, SEL = AUX_
OUT
= V _ V
_ = V
_ = 0V
IN , OUTA
OUTB
_ = 5.5V, V _ = V
_ = 0V
AUX
10
IN
V
_ or V _ rising edge, 3% hysteresis
AUX
3.0
3.8
UVLO
IN
500
mA
_______________________________________________________________________________________
3
Dual USB Switch with Fault Blanking
and Autoreset
ELECTRICAL CHARACTERISTICS (continued)
(V
= V
= V
= 5V, SEL = GND, ADJ_ = open, T = -40°C to +85°C, unless otherwise noted. Circuit of Figure 1.) (Note 2)
A
IN__
AUX__
ON
PARAMETER
SYMBOL
CONDITIONS
MIN
0.6
MAX
1.2
UNITS
V
V
_ – V
_ = 0.5V, SEL = AUX_
OUT
IN
Current-Limit Threshold
I
A
LIM
_ – V
AUX
_ = 0.5V, SEL = GND
OUT
0.6
1.2
SEL = GND, V
_ = 0V (I
_ pulsing)
OUT
0.6
0.6
1.8
1.8
OUT
Short-Circuit Current Limit
I
A
PEAK
SHORT
MAX138
SEL = AUX_, V
_ = 0V
OUT
Continuous Current-Limit
Blanking Timeout Period
From continuous current-limit condition to
FAULT_ assertion
9
7
40
40
ms
ms
Short-Circuit Blanking Timeout
Period
From short-circuit current-limit condition to
FAULT_ assertion
R _ = 10Ω, does not include rise time
OUT
Turn-On Delay
(from ON asserted to V
_ = 10% of
0.5
4
3
ms
ms
OUT
V
_), C _ = 1µF
OUT
IN
R _ = 10Ω, does not include fall time
OUT
Turn-Off Delay from ON
(from ON asserted to V
_ = 90% of
OUT
V
_), C _ = 1µF
OUT
IN
Logic Input High Voltage
Logic Input Low Voltage
Logic Input Current
V
V
V
V
_ = 4V to 5.5V
_ = 4V to 5.5V
2
V
V
IN
0.8
1
IN
= 0V or 5.5V
-1
µA
V
ON
FAULT_ Output Low Voltage
_ = 4V, sinking 1mA
0.4
IN
FAULT_ Output High Leakage
Current
V
_ = V
= 5.5V
1
µA
IN
FAULT_
Autoreset Current
In latched-off state, V
In latched-off state, V
In latched-off state, V
_ = 0V
10
0.4
9
55
0.6
40
mA
V
OUT
OUT
OUT
Autoreset Threshold
Autoreset Blanking Time
_ rising
_ > 1 V
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.
4
_______________________________________________________________________________________
Dual USB Switch with Fault Blanking
and Autoreset
MAX138
Typical Operating Characteristics
(Circuit of Figure 2, V
= 5V, T = +25°C, unless otherwise noted.)
A
IN_
IN + AUX QUIESCENT CURRENT
vs. TEMPERATURE
AUX QUIESCENT CURRENT vs. V
IN QUIESCENT CURRENT vs. V
AUX
AUX
70
60
50
40
30
20
10
0
30
25
20
15
10
5
100
90
V
= 5V
IN
V
= 5.5V
IN
V
= 4.5V
IN
SEL = AUX
SEL = AUX = IN
V
= 0
IN
V
= 5V
IN
SEL = GND
V
= 5.5V
IN
V
= 5.5V
V
= 4.5V
AUX
IN
80
V
= 5V
AUX
V
= 4.5V
AUX
0
70
0
1
2
3
4
5
0
1
2
3
4
5
-40
-20
0
20
40
60
80
V
(V)
V
(V)
TEMPERATURE (°C)
AUX
AUX
IN + AUX QUIESCENT CURRENT
vs. TEMPERATURE
IN + AUX QUIESCENT SHUTDOWN CURRENT
vs. TEMPERATURE
SWITCH LEAKAGE CURRENT
vs. TEMPERATURE, FULL DATA
110
100
90
10.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
1000
100
10
SEL = GND
V
= V
= 5.0V
AUX
IN
V
= V
= 4.5V
AUX
IN
IN = AUX = SEL = ON = GND
V
= V
= 5.5V
AUX
IN
V
V
= 5.5V
OUT_
= V
= 5V
AUX
IN
80
V
= V
= V = V = 5.5V
AUX ON SEL
IN
OUT_ = GND
V
= V
= 4.5V
AUX
IN
V
= V
= 5.5V
IN
AUX
0
70
1
-40
-20
0
20
40
60
80
-40
-20
20
40
60
80
-40
-20
0
20
40
60
80
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
NORMALIZED SWITCH ON-RESISTANCE
vs. TEMPERATURE
CONTINUOUS CURRENT LIMIT THRESHOLD
vs. TEMPERATURE
TURN-ON DELAY vs. TEMPERATURE
0.94
0.92
0.90
0.88
0.86
1.3
1.2
1.1
1.0
0.9
0.8
0.7
2.3
2.2
2.1
2.0
1.9
1.8
SEL = GND
V
= V
= 5.0V
AUX
IN
V
= V
= 4.5V
AUX
I
= 500mA
IN
OUT_
V
= V
= 5.5V
0
IN
AUX
-40
-20
0
20
40
60
80
-40
-20
0
20
40
60
80
-40
-20
20
40
60
80
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
_______________________________________________________________________________________
5
Dual USB Switch with Fault Blanking
and Autoreset
Typical Operating Characteristics (continued)
(Circuit of Figure 2, V
= 5V, T = +25°C, unless otherwise noted.)
A
IN_
AUTORESET CURRENT vs. INPUT VOLTAGE
FAULT-BLANKING TIME vs. TEMPERATURE
AUTORESET CURRENT vs. TEMPERATURE
35
30
25
20
15
10
5
40
35
30
25
20
15
20.5
20.0
19.5
19.0
18.5
MAX138
V
= V
= 5.5V
AUX
IN
AUX = IN,
SEL = ON = GND
V
= 4.5V
AUX
V
= 5.5V
AUX
V
= V
= 4.5V
40
IN
AUX
V
= 5.0V
AUX
0
-40
-20
0
20
60
80
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE (V)
-40
-20
0
20
40
60
80
TEMPERATURE (°C)
TEMPERATURE (°C)
OVERLOAD RESPONSE INTO 2.5Ω LOAD
OVERLOAD RESPONSE INTO 2.5Ω LOAD
(EXPANDED TIME SCALE)
MAX1838 toc13
MAX1838 toc14
5V
V
0
5V
V
0
OUT_
OUT_
5V
V
5V
V
FAULT_
FAULT_
0
0
0.5A/div
0.5A/div
0.5A/div
I
I
OUT_
OUT_
0
0
10ms/div
1ms/div
SHORT-CIRCUIT RESPONSE INTO 0Ω
(EXPANDED TIME SCALE)
SHORT-CIRCUIT RESPONSE INTO 0Ω
MAX1838 toc16
MAX1838 toc15
5V
OUT_
0
5V
OUT_
V
V
0
5V
FAULT_
5V
V
V
FAULT_
0
0
1A
0.5A
1A
0.5A
I
I
OUT_
OUT_
0
0
1ms/div
10ms/div
6
_______________________________________________________________________________________
Dual USB Switch with Fault Blanking
and Autoreset
MAX138
Typical Operating Characteristics (continued)
(Circuit of Figure 2, V
= 5V, T = +25°C, unless otherwise noted.)
A
IN_
SWITCH TURN-OFF TIME,
= 1µF R = 10Ω
START-UP TIME INTO 10Ω LOAD
SWITCH TURN-ON TIME, C
= 1µF
C
OUT
OUT
OUT
MAX1838 toc19
MAX1838 toc17
MAX1838 toc18
5V
V
0
5V
V
0
5V
V
0
ON
ON
ON
4V
OUT_
2V
5V
4V
5V
4V
V
0
3V
V
3V
V
OUT_
OUT_
2V
1V
0
2V
1V
0
0.5A
I
OUT_
0
1ms/div
1ms/div
1ms/div
Pin Description
PIN
NAME
FUNCTION
1, 8
GND
Ground. Connect both GND pins together.
Select Input. Control input to select from either V
power from IN source, low for power from AUX source.
or V
for both outputs. Pull SEL high for
AUX_
IN_
2
SEL
AUXA, AUXB
INA, INB
ON
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.
3, 6
4, 5
7
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.
Control Input. A logic low at ON turns on the device. A logic high shuts off both outputs and all
power to internal circuits. Driving ON higher than IN does not cause damage.
Fault Indicator Output for Channel B. This open-drain output goes low when channel B is in
thermal shutdown or in undervoltage lockout or in a sustained (>20ms) current-limit or short-circuit
condition.
9
FAULTB
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.
10
ADJB
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.
11, 12
13, 14
OUTB1, OUTB2
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.
_______________________________________________________________________________________
7
Dual USB Switch with Fault Blanking
and Autoreset
Pin Description (continued)
PIN
NAME
FUNCTION
Current-Limit Adjust for Channel A. When operating from auxiliary input, adjust the current limit by
connecting a resistor from ADJA to GND as shown in Programmable Current Limit.
15
ADJA
Fault Indicator Output for Channel A. This open-drain output goes low when channel A is in
thermal shutdown or undervoltage lockout or in a sustained (>20ms) current-limit or short-circuit
condition.
16
FAULTA
MAX138
The MAX1838 operates from 4V to 5.5V input voltage
range and guarantees a 500mA (min) output current. A
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).
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-R
NMOS switches enables the
ON
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.
FAULT LOGIC
THERMAL
SHUTDOWN A
FAULTA
20ms
BLANKING
N
Input Voltage Requirements and
Undervoltage Lockout
SEL
ADJA
CHANNEL A CONTROL
CIRCUITRY
The MAX1838 operates in one of three modes (Table 1).
The device receives its supply current from the AUX
input. When V
falls below the 3.8V undervoltage
AUX_
lockout voltage, the MAX1838 shuts down to prevent
erroneous switch operation.
OUTA1
OUTA2
IC POWER
AUX
IN
Table 1. MAX1838 Operation Modes
OUTB2
OUTB1
Both A and B outputs receive
current from IN. The IC is powered
Power from IN
(SEL high)
from AUX.
ADJB
CHANNEL B CONTROL
CIRCUITRY
Both A and B outputs receive
current from AUX. The IC is
ON
Power from AUX
(SEL low)
FAULTB
powered from AUX. V
to zero in this mode.
may drop
IN_
20ms
BLANKING
N
Both A and B outputs are open and
no power is available. Either V or
Full Shutdown
(ON high)
THERMAL
SHUTDOWN B
IN_
FAULT LOGIC
V
may drop to zero.
AUX_
Figure 1. Simplified Functional Diagram
_______________________________________________________________________________________
8
Dual USB Switch with Fault Blanking
and Autoreset
MAX138
Output Fault Protection
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:
The MAX1838 senses the switch output voltage and
selects 0.9A continuous current limiting when V
is
OUT
greater than 1V or short-circuit current limiting when
is less than 1V (see Behavior During Current Limit
V
OUT
and Fault Blanking).
I
_ = 0.9A × R
/ (200kΩ + R
)
OUT
ADJ
ADJ
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.
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 V
is
OUT
greater than 1V (current-limit mode), the MAX1838 reg-
ulates the output current to 0.9A. If V is less than 1V
OUT
(short-circuit mode), the MAX1838 pulses the output,
decreasing the current to 0.35A (Table 2). Note that
a thermal overload may result from either of these high-
current conditions.
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.
RMS
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 Indicators
The MAX1838 provides an open-drain fault output
(FAULT) for each channel. For most applications, con-
nect FAULT to AUX through a 100kΩ pullup resistor.
FAULT goes low when any one of the following condi-
tions occurs:
• V
_ is below the undervoltage-lockout (UVLO)
AUX
threshold.
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.
• V _ is below the undervoltage-lockout threshold
IN
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).
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
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.
_______________________________________________________________________________________
9
Dual USB Switch with Fault Blanking
and Autoreset
between IN and AUX. However, a fault does not trigger
since the fault blanking-time is longer than the supply
switchover time.
Output Capacitor
An output capacitor helps prevent parasitic inductance
from pulling OUT negative during turn-off. When start-
ing up into very large capacitive loads, the switch puls-
Applications Information
es the output current at 0.35A
until the output
RMS
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).
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.
MAX138
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.
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
Table 2. MAX1838 Current Limiting and Fault Behavior
CONDITION
MAX1838 BEHAVIOR
• If a short circuit is detected at the output, the channel turns off, the blanking timer
begins, and FAULT stays high.
• If the short circuit persists during the fault-blanking period, the output pulses at 0.35A
-
. When the short is removed before the 15ms short-circuit blanking timeout period,
RMS
If a short circuit is present
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.)
(V
OUT
< 1V)
• Output current regulates at I , the blanking timer turns on, and FAULT stays high.
LIM
• Continuous current at I
persists until the overload is removed, a thermal fault occurs,
LIM
If an overload current is present
(V > 1V)
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.
OUT
• FAULT immediately goes low (the blanking timer does not apply to thermal faults), and
the channel turns off. When the junction cools by 15oC, the thermal fault condition is
removed, autoreset mode is enabled, the output sources 25mA, and FAULT remains
low. If the voltage on that output is above 0.5V for 20ms, the channel resets, the output
turns on, and FAULT goes high.
If thermal fault condition is present
10 ______________________________________________________________________________________
Dual USB Switch with Fault Blanking
and Autoreset
MAX138
during a load step. The MAX1838 is capable of driving
inductive loads, but avoid exceeding the device’s
absolute maximum ratings. Usually the load inductance
is relatively small, and the MAX1838’s input includes a
substantial bulk capacitance from an upstream regulator
as well as local bypass capacitors, limiting overshoot. If
severe ringing occurs due to large load inductance,
clamp the MAX1838’s output below 6V and above -0.3V.
For the normal operating current (I
= 0.5A), the max-
OUT
imum on-resistance of the switch is 170mΩ, and the
power dissipation is:
2
P = (0.5A) x 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:
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.
P = (I ) (V - V )
OUT
LIM
IN
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.
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:
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).
2
P = (I
) R
OUT
ON
100kΩ
100kΩ
AUX
IN
AUXA
AUXB
FAULTB
FAULTA
0.1µF
MAX1838
OUTB
OUTA
INA
INB
OUTB2
OUTB1
0.1µF
10µF
OUTA2
OUTA1
SEL
ON
10µF
ADJB
ADJA
GND
67kΩ
67kΩ
Figure 2. Typical Application Circuit
______________________________________________________________________________________ 11
Dual USB Switch with Fault Blanking
and Autoreset
Package Information
Chip 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.
PROCESS: BiCMOS
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
16 QSOP
E16-5
21-0055
MAX138
12 ______________________________________________________________________________________
Dual USB Switch with Fault Blanking
and Autoreset
MAX138
Revision History
REVISION
NUMBER
REVISION
DATE
PAGES
DESCRIPTION
CHANGED
0
1
4/01
2/10
Initial release
—
1
Removed UL Certification Pending bullet from Features section
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.
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