MAX1812
Detailed Description
The MAX1812 is a dual current-limited switch designed
specifically for USB applications. It has two indepen-
dent switches, each with its own enable control input.
Each switch also has an independent error flag output
to notify the USB controller when the current-limit, short-
circuit, undervoltage-lockout, or thermal-shutdown
threshold is reached (Figure 1).
The MAX1812 operates from a +4V to +5.5V input volt-
age and guarantees a minimum output current of
500mA. A built-in current-limit of 0.9A (typ) limits the
current in the event of a heavy overload condition. The
MAX1812 has independent thermal shutdown for each
switch in the event of a prolonged overload or short-cir-
cuit condition.
Use of internal low R
ON
NMOS switches enables the
MAX1812 to fit two switches in the ultra-small 10-pin
µMAX 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 circuits permit each
switch to operate independently, improving system
robustness.
Undervoltage Lockout and
Input Voltage Requirements
The MAX1812 includes an undervoltage-lockout
(UVLO) circuit to prevent erroneous switch operation
when the input voltage goes low during startups and
brownout conditions. Operation is inhibited when V
IN_
< 3.4V.
Output Fault Protection
The MAX1812 senses the switch output voltage and
selects continuous current limiting when V
OUT
_ > 1V, or
pulsed current limiting when V
OUT_
< 1V. When V
OUT_
> 1V, the device operates in a continuous current-limit
mode, which sets the output current limit to 0.9A (typ).
When V
OUT_
< 1V, the device operates in short-circuit
current-limit mode. The MAX1812 pulses the output
current at 400Hz to limit the output current to 0.35A
(RMS).
Thermal Shutdown
The MAX1812 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
back on again. If the fault overload condition continues,
the switch will cycle on and off, resulting in a pulsed
output that saves battery power.
Fault Indicators
The MAX1812 provides an open-drain fault output
(FAULT_) for each switch. For most applications, con-
nect FAULT_ to IN_ through a 100kΩ pullup resistor.
Dual USB Switch with Fault Blanking
6 _______________________________________________________________________________________
Control Input for Switch A. Can be driven higher than IN_ without damage. A logic low turns switch A
on.
Power Input. Connect all IN_ pins together and bypass with a 0.1µF capacitor to ground. Load
conditions may require additional bulk capacitance to prevent the input from being pulled down.
Fault Indicator Output for Switch B. This open-drain output goes low when switch B is in thermal
shutdown or undervoltage lockout or in a sustained (>20ms) current-limit or short-circuit condition.
Power Output for Switch B. Connect a 1µF capacitor from OUTB to ground. Load condition may
require additional bulk capacitance. See USB requirements.
Power Output for Switch A. Connect a 1µF capacitor from OUTA to ground. Load condition may
require additional bulk capacitance. See USB requirements.
Fault Indicator Output for Switch A. This open-drain output goes low when switch A is in thermal
shutdown or undervoltage lockout or in a sustained (>20ms) current-limit or short-circuit condition.
