lm3526mx-l National Semiconductor Corporation, lm3526mx-l Datasheet - Page 8

• Manufacturer Part Number: lm3526mx-l
• Description: Dual Port Usb Power Switch And Over-current Protection
• Manufacturer: National Semiconductor Corporation

www.national.com
Functional Description
The LM3526-H and LM3526-L are high side P-Channel
switches with active-high and active-low enable inputs, re-
spectively. Fault conditions turn-off and inhibit turn-on of the
output transistor and activate the open-drain error flag tran-
sistor sinking current to the ground.
INPUT AND OUTPUT
IN (Input) is the power supply connection to the control
circuitry and the source of the output MOSFET.
OUT (Output) is the connection to the drain of the output
MOSFET. In a typical application circuit, current flows
through the switch from IN to OUT towards the load.
If V
current will flow from OUT to IN since the MOSFET is
bidirectional.
THERMAL SHUTDOWN
The LM3526 is internally protected against excessive power
dissipation by a two-stage thermal protection circuit. If the
device temperature rises to approximately 145˚C, the ther-
mal shutdown circuitry turns off any switch that is current
limited. Non-overloaded switches continue to function nor-
mally. If the die temperature rises above 150˚C, both
switches are turned off and both fault flag outputs are acti-
vated. Hysteresis ensures that a switch turned off by thermal
shutdown will not be turned on again until the die tempera-
ture is reduced to 135˚C. Shorted switches will continue to
cycle off and on, due to the rising and falling die temperature,
until the short is removed.
UNDERVOLTAGE LOCKOUT
UVLO prevents the MOSFET switch from turning on until
input voltage exceeds 1.8V (typical).
If input voltage drops below 1.8V (typical), UVLO shuts off
the MOSFET switch and signals the fault flag. UVLO func-
tions only when device is enabled.
CURRENT LIMIT
The current limit circuit is designed to protect the system
supply, the MOSFET switches and the load from damage
caused by excessive currents. The current limit threshold is
set internally to allow a minimum of 500 mA through the
MOSFET but limits the output current to approximately 1.0A
typical.
FAULT FLAG
The fault flag is an open-drain output capable of sinking
10 mA load current to typically 100 mV above ground.
A parasitic diode exists between the flag pins and V
Pulling the flag pins to voltages higher than V
bias this diode and will cause an increase in supply current.
This diode will also clamp the voltage on the flag pins to a
diode drop above V
The fault flag is active (pulled low) when any of the following
conditions are present: under-voltage, current limit, or ther-
mal shutdown.
A 1ms (typ.) delay in reporting the fault condition prevents
erroneous fault flags and eliminates the need for an external
RC delay network.
OUT
is greater than V
IN
.
IN
when the switch is enabled,
IN
will forward
IN
pin.
8
Application Information
FILTERING
The USB specification indicates that “no less than 120 µF
tantalum capacitors” must be used on the output of each
downstream port. This bulk capacitance provides the short-
term transient current needed during a hot plug-in. Current
surges caused by the input capacitance of the down stream
device could generate undesirable EMI signals. Ferrite
beads in series with all power and ground lines are recom-
mended to eliminate or significantly reduce EMI.
In selecting a ferrite bead, the DC resistance of the wire used
must be kept to a minimum to reduce the voltage drop.
A 0.01 µF ceramic capacitor is recommended on each port
directly between the V
damage to other components during the hot-detachment.
Adequate capacitance must be connected to the input of the
device to limit the input voltage drop during a hot-plug event
to less than 330 mV. For a few tens of µs, the host must
supply the in-rush current to the peripheral, charging its bulk
capacitance to V
input capacitor. A 33 µF 16V tantalum capacitor is recom-
mended.
In choosing the capacitors, special attention must be paid to
the Effective Series Resistance, ESR, of the capacitors to
minimize the IR drop across the capacitor’s ESR.
SOFT START
To eliminate the upstream voltage droop caused by the high
in-rush current drawn by the output capacitors, the maximum
in-rush current is internally limited to 1.5A.
TRANSIENT OVER-CURRENT DELAY
High transient current is also generated when the switch is
enabled and large values of capacitance at the output have
to be rapidly charged. The in-rush currents created could
exceed the short circuit current limit threshold of the device
forcing it into the current limit mode. The capacitor is
charged with the maximum available short circuit current set
by the LM3526. The duration of the in-rush current depends
on the size of the output capacitance and load current. Since
this is not a valid fault condition, the LM3526 delays the
generation of the fault flag for 1 ms. If the condition persists
due to other causes such as a short, a fault flag is generated
after a 1 ms delay has elapsed.
The LM3526’s 1 ms delay in issuing the fault flag is adequate
for most applications. If longer delays are required, an RC
filter as shown in Figure 1 may be used.
bus
. This current is initially supplied by the
bus
FIGURE 1.
and ground pins to prevent EMI
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