MAX1558/MAX1558H
Dual, 3mm x 3mm, 1.2A/Programmable-Current
USB Switches with Autoreset
10 ______________________________________________________________________________________
Applications Information
Setting the Current Limit
A resistor from ISET to ground programs the current-
limit value for both outputs. Use a resistor between
26kΩ and 86kΩ to set the current limit according to
the formula:
I
LIM(TYPICAL)
= 36400 / R
ISET
(Amps)
Do not use R
ISET
values below 26kΩ because the maxi-
mum current rating of the device may be exceeded.
R
ISET
values larger than 60kΩ are not recommended
and do not provide a lower limit current than 500mA.
If the output drops below 1V (typ), the MAX1558 shifts
to a short-circuit current-limit threshold that is 30%
above the programmed level given by the I
LIM(TYPICAL)
formula. If the short-circuit threshold is exceeded, the
switch shuts off immediately (no 20ms delay) and
ramps the current back up in approximately 3ms. If the
short persists and the current ramps all the way up to
the short-circuit limit again, the switch again turns off. If
the short still persists, the output pulses this way for
20ms, at which time the switch turns off and autoreset
mode begins.
Input Capacitor
INA and INB provide the power for all control and
charge-pump circuitry and must be connected together
externally. Connect a capacitor from IN_ to ground to
limit the input-voltage drop during momentary output
short-circuit conditions. A 0.1µF ceramic capacitor is
required for local decoupling; higher capacitor values
further reduce the voltage drop at the input. When dri-
ving inductive loads, a larger capacitance prevents
voltage spikes from exceeding the MAX1558/
MAX1558Hs’ absolute maximum ratings.
Output Capacitor
Place a 1µF or greater capacitor at each output for
noise immunity. When starting up into very large capac-
itive loads, the switch may pulse the output current at
the short-circuit current-limit program level until the out-
put voltage rises above 1V. Then, the capacitor contin-
ues to charge at the full, continuous current-limit
program level. There is no limit to the output capacitor
size, but to prevent a startup fault assertion, the capaci-
tor must charge up within the fault-blanking delay peri-
od. Typically, starting up into a 500µF or smaller
capacitor does not trigger a fault output. In addition to
bulk capacitance, small-value (0.1µF or greater) ceram-
ic capacitors improve the output’s resilience to electro-
static discharge (ESD).
Driving Inductive Loads
A wide variety of devices (mice, keyboards, cameras,
and printers) can load the USB port. These devices com-
monly connect to the port with cables, which can add an
inductive component to the load. This inductance caus-
es the output voltage at the USB port to ring during a
load step. The MAX1558/MAX1558H are capable of dri-
ving inductive loads, but avoid exceeding the devices’
absolute maximum ratings. Usually the load inductance
is relatively small, and the MAX1558/MAX1558Hs’
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 MAX1558/MAX1558Hs’ out-
put below +6V and above -0.3V.
Turn-On and Turn-Off Behavior
In the absence of faults, the MAX1558/MAX1558Hs’
internal switches turn on and off slowly under the control
of the ON_ inputs. Transition times for both edges are
approximately 4ms. The slow charge-pump switch drive
minimizes load transients on the upstream power source.
Under thermal fault and UVLO, the power device turns
off rapidly (100ns) to protect the power device.
Layout and Thermal Dissipation
To optimize the switch response time to output short-
circuit conditions, keep all traces as short as possible
to reduce the effect of undesirable parasitic induc-
tance. Place input and output capacitors no more than
5mm from device 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. While the
switches are on, power dissipation is small and the
package temperature change is minimal. Calculate the
power dissipation for this condition as follows:
P = (I
OUT
_)
2
R
ON
For the maximum operating current (I
OUT
_ = 1.2A) and
the maximum on-resistance of the switch (125mΩ), the
power dissipation is:
P = (1.2A)
2
x 0.125Ω = 180mW per switch
The worst-case power dissipation occurs when the
switch is in current limit and the output is greater than
1V. The instantaneous power dissipated in each switch
is the voltage drop across the switch multiplied by the
current limit. The fault-blanking circuit turns the output
off if the fault persists for 20ms, while the autoreset cir-
cuit can turn it back on after 20ms in the off state. Thus,
the average worst-case power is approximately 50% of
the instantaneous value.
P = 0.5 x (I
LIM
) x (V
IN_
- V
OUT_
)
