the switch pulses the output current at 0.35A RMS until
the output voltage rises above 1V, then the capacitor
will continue to charge at the full 0.9A current limit.
There is no limit to the output capacitor size, 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
will not trigger a fault output. In addition to bulk capaci-
tance, small value (0.1µF) ceramic capacitors improve
the output’s resilience to electrostatic discharge (ESD).
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 can cause the output voltage at the USB port to
ring during a load step. The MAX1812 is capable of dri-
ving inductive loads, but care should be taken to avoid
exceeding the device’s absolute maximum ratings.
Usually, the load inductance is relatively small, and the
MAX1812’s input includes a substantial bulk capaci-
tance from an upstream regulator as well as a local
bypass, so the amount of transient overshoot is small. If
the load inductance is very large, ringing may become
severe, and it may be necessary to clamp the
MAX1812’s output below 6V and above -0.3V.
Turn-On and Turn-Off Behavior
In normal operation, the MAX1812’s internal switches
turn on and turn off slowly under the control of the ON_
inputs. Transition times for both edges are approxi-
mately 2ms. The slow charge-pump switch-drive mini-
mizes load transients the upstream power source.
Under thermal fault and under voltage lockout, the
power device will turn off rapidly (100ns typ) to protect
the power device.
Layout and Thermal Dissipation
To optimize the switch-response time to output short-
circuit conditions, it is important to 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 the package leads. All IN_ and
OUT_ pins must be connected with short traces to the
power bus. Wide power bus planes provide superior
heat dissipation through the switch IN_ and OUT_ pins.
Under normal operating conditions, power dissipation
is small and the package can conduct heat away.
Calculate the maximum power dissipation for normal
operation as follows:
P = (I
OUT_
)
2
R
ON
P = (0.5A)
2
x 0.135Ω = 34mW per switch
where I
OUT_
is the maximum normal operating current,
and R
ON
is the on-resistance of the switch (135mΩ
max).
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
current limit:
P = (ILIM) (V
IN
- V
OUT
)
For a 5V input and 1V output, the maximum power dis-
sipation per switch is:
P = (1.2A) ( 5V - 1V) = 4.8W
Since the maximum package power dissipation is only
444mW, the MAX1812 die temperature will quickly
exceed the thermal-shutdown threshold, and the switch
output will pulse on and off. The duty cycle and period
are strong functions of the ambient temperature and
the PC board layout.
When the output is short circuited, current limiting acti-
vates and the power dissipated across the switch
increases as does junction temperature. If the fault con-
dition persists, the thermal-overload-protection circuitry
activates (see Thermal Shutdown).
Chip Information
TRANSISTOR COUNT: 2739
PROCESS: BiCMOS
MAX1812
Dual USB Switch with Fault Blanking
_______________________________________________________________________________________ 9
Figure 2. Typical Application Circuit
