LM3544M-L National Semiconductor, LM3544M-L Datasheet - Page 8

• Manufacturer Part Number: LM3544M-L
• Description: IC,Power Control/Management,SOP,16PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM3544M-L

• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

www.national.com
Functional Description
Power Switches
The power switches that comprise the four ports of the
LM3544 are N-Channel MOSFETs. They have a typical
on-state drain-to-source resistance of 90 m when the input
voltage is 5 V. When enabled, each switch will supply a 500
mA minimum current to its load. In the unlikely event that a
switch is enabled and the output voltage of that switch is
pulled above the input voltage, the bi-directional nature of
the switch results in current to flow from the output to the
input. When a switch is disabled, current flow through the
switch is prevented in both directions.
Charge Pump and Driver
The gate voltages of the high-side NFET power switches are
supplied by an internal charge-pump and driver circuit
combination.
switched-capacitor circuit that efficiently generates voltages
above the LM3544 input supply. The charge pump output is
used to supply a transconductance amplifier driver circuit
that controls the gate voltages of the power switches. Rise
and fall times on the gates are typically kept between 2 ms
and 4 ms to limit large current surges and associated
electromagnetic interference (EMI).
ENABLE (EN
The LM3544 comes in two versions: an active-high enable
version, LM3544-H, and an active-low enable version,
LM3544-L. In the LM3544-H, the EN
logic inputs that, when asserted, turn on the associated
power supply switch(es). Power supply switches are
controlled by the EN
LM3544-L. With all four ports disabled on either version of
the LM3544, less than 5 µA of supply current is consumed.
Both types of enable inputs, active-high and active-low, are
TTL and CMOS logic compatible.
Input and Output
The power supply to the control circuitry and the drains of the
power-switch MOSFETs are connected to the two input pins,
IN1 and IN2. These two pins are connected externally in
most standard applications. The two ground nodes GND1
and GND2 must be connected externally in all applications.
Pins OUT1, OUT2, OUT3, and OUT4 are connections to the
source nodes of the power-switch MOSFETs. In a typical
application circuit, current flows through the switches from
IN1 and IN2 to OUT
Undervoltage Lockout (UVLO)
Undervoltage Lockout (UVLO) prevents the MOSFET
switches from turning on until the input voltage exceeds a
typical value of 1.8V.
If the input voltage drops below the UVLO threshold, the
MOSFET switches are opened and fault flags are activated.
UVLO flags function only when one or more of the ports is
enabled. If a port is enabled in a UVLO condition, flags
corresponding to the enabled port and its dual (port 1 is
paired with port 2, port 3 is paired with port 4) are asserted.
Current Limit and Foldback
The current limit circuit is designed to protect the system
supply, the LM3544 switches, and the load from potential
damage resulting from excessive currents. If a direct short
occurs on an output of the LM3544, the input capacitor(s)
x
The
or EN
x
x
toward the load.
charge
)
x
active-low logic inputs in the
pump
x
pins are active-high
is
a
low-current
8
rapidly discharge through the part, activating current limit
circuitry. The threshold for activating current limiting is 2.0A
(typ.). Protection is achieved by momentarily opening the
MOSFET switch and then gradually turning it on. Turn-on is
halted when the current through the switch reaches the
current-limit level of 1.0A (typ.) The current is held at this
level until either the excessive load/short is removed or the
part overheats and thermal shutdown occurs (see Thermal
Shutdown section, below). The fault flag of a switch is
asserted whenever the switch is current limiting.
If a port on the LM3544 is enabled into a short condition, the
output current of that port will rise to the current-limit level
and hold there.
When a port is in a current-limit condition, the LM3544
senses the output voltage on that port and, if it is less than
1.0V (typ.), will reduce the output current through that port.
This operation is shown in Figure 2 , below. The current
reduction, or foldback, reduces power dissipation through
the overloaded MOSFET switch. An additional advantage of
the foldback feature is the reduction of power required from
the source supply when one or more output ports are
shorted.
Thermal Shutdown
The LM3544 is internally protected against excessive power
dissipation by a two-stage thermal protection circuit. If the
device temperature rises to approximately 145˚C, the
thermal shutdown circuitry turns off any switch that is current
limited. Non-overloaded switches continue to function
normally. If the die temperature rises above 160˚C, all
switches are turned off and all four fault flag outputs are
activated. Hysteresis ensures that a switch turned off by
thermal shutdown will not be turned on again until the die
temperature 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.
The thermal shutdown function is shown graphically in
Figure 3 and Figure 4 .
FIGURE 2. Short-Circuit Output Current (with
Foldback) vs. Output Voltage
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