MAX1823/MAX1823A/MAX1823B/MAX1823H
Dual USB Switch with Fault
Blanking and Autoreset
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Detailed Description
The MAX1823 is a dual, current-limited switch designed
for USB applications. It has two independent switches,
each with its own enable control input and autoreset
function. Each switch has an error-flag output to notify
the USB controller when the current-limit, short-circuit,
undervoltage-lockout, or thermal-shutdown threshold is
reached (Figure 2).
The MAX1823 operates from a 4V to 5.5V input voltage
range and guarantees a minimum 720mA output current.
A built-in 0.9A current limit limits the current in the event
of a heavy-overload condition. The MAX1823 has inde-
pendent thermal shutdown for each switch 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 low-R
ON
NMOS switches enables the MAX1823
to provide two switches in the ultra-small 10-pin µMAX
package. An internal micropower charge pump gener-
ates the high-side supply needed for driving the gates
of these high-side switches. Separate current-limiting
and thermal-shutdown circuits permit each switch to
operate independently, improving system robustness.
Undervoltage-Lockout and
Input-Voltage Requirements
The MAX1823 includes a UVLO circuit to prevent erro-
neous switch operation when the input voltage goes
low during startup and brownout conditions. Operation
is inhibited when V
IN_
is less than 3.4V.
Output Fault Protection
The MAX1823 senses the switch output voltage and
selects continuous current limiting when V
OUT
is greater
than 1V or short-circuit current limiting when V
OUT
is less
than 1V. When V
OUT
is greater than 1V, the device oper-
ates in a continuous current-limit mode that sets the out-
put current limit to 0.9A. When V
OUT
is less than 1V, the
device operates in short-circuit current-limit mode,
sourcing 0.35A
RMS
current pulses to the load.
Autoreset Mode
If an output fault is detected for more than the 20ms
blanking time, the output latches off, the FAULT_ output
goes low, and a 25mA current is forced at the output. If
the voltage on the output exceeds 0.5V for 20ms, the fault
resets, the 25mA current source shuts down, and the out-
put turns on. The device monitors the output voltage so
that a short-circuit condition can be detected. Active
loads are not expected to have measurable currents
when the supply is below 0.5V. The MAX1823/MAX1823A
can also be reset from fault manually by toggling ON_
(ON_ for the MAX1823B/MAX1823H) for that channel.
Thermal Shutdown
The MAX1823 features independent thermal shutdown
for each switch channel, allowing one switch to deliver
power even if the other switch has a fault condition.
When the junction temperature exceeds +160°C, the
switch turns off and the FAULT_ output goes low imme-
diately; fault blanking does not occur during thermal
limit. When the junction cools by 15°C, the switch turns
on again. If the fault overload condition continues, the
switch cycles on and off, resulting in a pulsed output,
saving battery power.
Reverse Current Blocking
The USB specification does not allow an output device
to source current back into the USB port. However, the
MAX1823A/MAX1823B are designed to safely power
noncompliant devices. When disabled, each output is
switched to a high-impedance state, blocking reverse
current flow from the output back to the input. However,
during normal operation with the device enabled, the
MAX1823A/MAX1823B are bidirectional switches.
Fault Indicators
The MAX1823 provides an open-drain fault output
(FAULT_) for each switch. For most applications, connect
FAULT_ to IN_ through a 100kΩ pullup resistor. FAULT_
goes 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 limit
mode after the fault-blanking period is exceeded.
• The switch is in autoreset mode.
ADDITIONAL BULK CAPACITANCE.
