Functional Description
Application Information
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.
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. Fer-
rite beads in series with all power and ground lines are rec-
ommended to eliminate or significantly reduce EMI.
INPUT AND OUTPUT
IN (Input) is the power supply connection to the control cir-
cuitry 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.
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 Vbus and ground pins to prevent EMI
damage to other components during the hot-detachment.
If VOUT is greater than VIN when the switch is enabled, cur-
rent will flow from OUT to IN since the MOSFET is bidirec-
tional.
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 sup-
ply the in-rush current to the peripheral, charging its bulk ca-
pacitance to Vbus. This current is initially supplied by the in-
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 lim-
ited. Non-overloaded switches continue to function normally.
If the die temperature rises above 150˚C, both switches are
turned off and both fault flag outputs are activated. Hyster-
esis ensures that a switch turned off by thermal shutdown
will not be turned on again until the die temperature is re-
duced 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.
put capacitor.
recommended.
A 33 µF 16V tantalum capacitor is
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.
UNDERVOLTAGE LOCKOUT
UVLO prevents the MOSFET switch from turning on until in-
put voltage exceeds 1.8V (typical).
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 ex-
ceed 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 gen-
eration of the fault flag for 1 ms. If the condition persists due
to other causes such as a short, a fault flag is generated af-
ter a 1 ms delay has elapsed.
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.
The LM3526’s 1 ms delay in issuing the fault flag is adequate
for most applications. If longer delays are required, an RC fil-
ter as shown in Figure 1 may be used.
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 VIN pin.
Pulling the flag pins to voltages higher than VIN will forward
bias this diode and will cause an increase in supply current.
This diode will also clamp the voltage on the flag pins to a di-
ode drop above VIN
.
The fault flag is active (pulled low) when any of the following
conditions are present: under-voltage, current limit, or ther-
mal shutdown.
DS101097-24
A 1ms (typ.) delay in reporting the fault condition prevents er-
roneous fault flags and eliminates the need for an external
RC delay network.
FIGURE 1.
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