MAX1838(2001) 查看數據表（PDF） - Maxim Integrated

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MAX1838
(Rev.:2001)

Dual USB Switch with Fault Blanking and Autoreset

Maxim Integrated 

Dual USB Switch with Fault Blanking

and Autoreset

Output Fault Protection

The MAX1838 senses the switch output voltage and

selects 0.9A continuous current limiting when VOUT is

greater than 1V or short-circuit current limiting when

VOUT is less than 1V (see Behavior During Current Limit

and Fault Blanking).

Autoreset Mode

The MAX1838 monitors the output to detect an over-

load condition. If an output fault is detected for more

than the 20ms blanking time, OUT_ latches, FAULT

goes low, and OUT sources 25mA to the load. If the

voltage at OUT exceeds 0.5V for 20ms, the fault resets,

the 25mA current source shuts down, and the channel

turns back on. For proper autoreset function, the output

load current must remain below 25mA when the output

voltage is lower than 0.5V. Toggle ON to manually reset

the output.

Thermal Shutdown

The MAX1838 features independent thermal shutdown

for each channel, allowing one channel to deliver

power even if the other channel has a fault condition.

When the junction temperature exceeds +160°C, the

switch turns off and FAULT_ goes low immediately.

Fault blanking does not occur during thermal limit.

When the junction cools by 15°C, the channel turns on

again. If the fault overload condition continues, the

channel cycles on and off, resulting in a pulsed output.

Fault Indicators

The MAX1838 provides an open-drain fault output

(FAULT) for each channel. For most applications, con-

nect FAULT to AUX through a 100kΩ pullup resistor.

FAULT goes low when any one of the following condi-

tions occurs:

• VAUX_ is below the undervoltage-lockout (UVLO)

threshold.

• VIN_ is below the undervoltage-lockout threshold

and SEL = AUX.

• The switch junction temperature exceeds the thermal

shutdown temperature limit of +160°C.

• The switch is in current-limit or short-circuit limit

mode after the fault-blanking period is exceeded

(autoreset).

After the fault condition is removed, the FAULT output

deasserts after a 20ms delay. Ensure that the MAX1838

has adequate input bypass capacitance to prevent

glitches from triggering FAULT outputs. Input glitches

greater than 0.2V/µs may cause erroneous FAULT indi-

cations.

Programmable Current Limit

When using the auxiliary input (AUX), each channel has

an adjustable current limit. For maximum current limit

(0.9A), open ADJ. For current limits below 0.9A, con-

nect a resistor from ADJ to GND according to the fol-

lowing equation:

IOUT_ = 0.9A × RADJ / (200kΩ + RADJ)

Behavior During Current Limit

and Fault Blanking

The MAX1838 limits switch current in the following

ways. When ON is high, both channels are off. When

ON is low, both channels can supply a continuous out-

put current of at least 500mA. When the output current

exceeds the 0.9A threshold, the MAX1838 will limit the

current depending on the output voltage. If VOUT is

greater than 1V (current-limit mode), the MAX1838 reg-

ulates the output current to 0.9A. If VOUT is less than 1V

(short-circuit mode), the MAX1838 pulses the output,

decreasing the current to 0.35ARMS (Table 2). Note that

a thermal overload may result from either of these high-

current conditions.

The MAX1838 switches may enter current limit in nor-

mal operation when powering up or driving heavy

capacitive loads. To differentiate these conditions from

short circuits or sustained overloads that may damage

the device, the MAX1838 has an independent fault-

blanking circuit in each channel. When a load transient

causes the device to enter current limit, an internal

counter starts to monitor the duration of the fault. If the

load fault persists beyond the 20ms fault-blanking time,

the channel turns off, FAULT asserts low, and the chan-

nel enters autoreset mode (see Autoreset Mode). Only

current-limit and short-circuit faults are blanked.

Thermal overload faults and input undervoltage imme-

diately cause the channel to turn off and FAULT to

assert low.

Fault blanking allows the MAX1838 to accommodate

USB loads that may not be fully compliant with the USB

specifications. USB loads with additional bypass

capacitance and/or large startup currents can be suc-

cessfully powered while protecting the upstream power

source. If the switch is able to bring up the load within

the 20ms blanking period, no fault is indicated.

Switching Supplies

To select IN as the source for both channels, drive SEL to

a logic high. To select AUX, drive SEL low. To minimize

switching transients, the MAX1838 switches between

input supplies (IN and AUX) with a make-before-break

sequence. Both switches are on simultaneously for up to

9ms before the initial supply is disconnected. When

switching from IN to AUX, a significant current may flow

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