LM3525
Functional Description
The LM3525-H and LM3525-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 cir-
cuitry and the source of the output MOSFET.
OUT (Output) is 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
OUT
is greater than V
IN
when the switch is enabled, cur-
rent will flow from OUT to IN since the MOSFET is bidirec-
tional.
THERMAL SHUTDOWN
LM3525 is internally protected against excessive power dis-
sipation. In the event of a shorted output or heavy loads that
could elevate the die temperature to above approximately
135˚C, the thermal shutdown circuit of the LM3525 will be
activated and the power switch turned off.
The switch is turned on after die temperature drops by 10˚C.
This built-in temperature hysteresis prevents undesirable os-
cillation of the thermal protection circuit and allows the de-
vice to reset itself after it is cooled down.
UNDERVOLTAGE LOCKOUT
UVLO prevents the MOSFET switch from turning on until in-
put voltage exceeds 1.9V.
UVLO shuts off the MOSFET switch and signals the fault flag
if input voltage drops below 1.8V. UVLO functions 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 limit the maximum current to 1.0A typical.
FAULT FLAG
The fault flag is an open-drain output capable of sinking
10 mA load current to typically 60 mV above ground.
The fault flag is active (pulled low) when any of the following
conditions are present: undervoltage, current limit, or ther-
mal shutdown.
A 1ms delay in reporting fault condition prevents erroneous
fault flags and eliminates the need for external RC delay net-
work.
In selecting a ferrite bead, the DC resistance of the wire used
must be kept to minimum to reduce the voltage drop.
A 0.01 µF ceramic capacitor is recommended on each port
directly between the V
bus
and ground pins to prevent EMI
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 sup-
ply the inrush current to the peripheral, charging its bulk ca-
pacitance to V
bus
. This current is initially supplied by the in-
put capacitor. A 33 µF 16V tantalum capacitor is
recommended.
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 inrush
current is internally limited to 1.0A.
TRANSIENT OVER-CURRENT DELAY
In USB applications, it is required that output bulk capaci-
tance is utilized to support hot-plug events. During hot-plug
events, inrush currents may also cause the flag to go active.
Since these conditions are not valid over-current faults, the
USB controller must ignore the flag during these events.
High transient current is also generated when switch is en-
abled and large values of capacitance at the output have to
be rapidly charged. The inrush 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 LM3525. The duration of the inrush current depends
on the size of the output capacitance and load current. Since
this is not a valid fault condition, the LM3525 delays the gen-
eration of the fault flag for 1 ms. If condition persists due to
other causes such as a short, a fault flag is generated after
a 1 ms delay has elapsed.
The LM3525’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.
DS101055-24
FIGURE 1. Transient Fitter
PCB LAYOUT CONSIDERATIONS
In order to meet the USB requirements for voltage drop,
droop and EMI, each component used in this circuit must be
evaluated for its contribution to the circuit performance as
shown in
Figure 2.
The PCB layout rules and guidelines
must be followed.
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. Fer-
rite beads in series with all power and ground lines are rec-
ommended to eliminate or significantly reduce EMI.
•
Place the switch as close to the USB connector as pos-
sible. Keep all V
bus
traces as short as possible and use at
least 50-mil, 1 ounce copper for all V
bus
traces. Solder
plating the traces will reduce the trace resistance.
7
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