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PDF LM3481 Data sheet ( Hoja de datos )
| Número de pieza | LM3481 | |
| Descripción | High Efficiency Low-Side N-Channel Controller | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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November 16, 2007
LM3481
High Efficiency Low-Side N-Channel Controller for
Switching Regulators
General Description
The LM3481 is a versatile Low-Side N-FET high performance
controller for switching regulators. It is suitable for use in
topologies requiring a low-side FET, such as boost, flyback,
SEPIC, etc. The LM3481 can be operated at extremely high
switching frequencies in order to reduce the overall solution
www.DataSheest4izUe..cTohme switching frequency of the LM3481 can be adjusted
to any value between 100 kHz and 1 MHz by using a single
external resistor or by synchronizing it to an external clock.
Current mode control provides superior bandwidth and tran-
sient response in addition to cycle-by-cycle current limiting.
Current limit can be programmed with a single external resis-
tor.
The LM3481 has built in protection features such as thermal
shutdown, short-circuit protection and over voltage protec-
tion. Power saving shutdown mode reduces the total supply
current to 5µA and allows power supply sequencing. Internal
soft-start limits the inrush current at start-up.
Key Specifications
■ Wide supply voltage range of 2.97V to 48V
■ 100 kHz to 1 MHz Adjustable and Synchronizable clock
frequency
■ ±1.5% (over temperature) internal reference
■ 10 µA shutdown current (over temperature)
Features
■ 10-lead MSOP package
■ Internal push-pull driver with 1A peak current capability
■ Current limit and thermal shutdown
■ Frequency compensation optimized with a capacitor and
a resistor
■ Internal softstart
■ Current Mode Operation
■ Adjustable Undervoltage Lockout with Hysteresis
■ Pulse Skipping at Light Loads
Applications
■ Distributed Power Systems
■ Notebook, PDA, Digital Camera, and other Portable
Applications
■ Offline Power Supplies
■ Set-Top Boxes
Typical Application Circuit
Typical SEPIC Converter
© 2007 National Semiconductor Corporation 201365
201365a5
www.national.com
1 page  
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings indicates conditions for which the device is
intended to be functional, but does not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.
The guaranteed specifications apply only for the test conditions.
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5kΩ resistor into each pin.
Note 3: All limits are guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%
tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate
Average Outgoing Quality Level (AOQL).
Note 4: Typical numbers are at 25°C and represent the most likely norm.
Note 5: The FA/SYNC/SD pin should be pulled to VIN through a resistor to turn the regulator off. The voltage on the FA/SYNC/SD pin must be above the max
limit for the Output = High longer than 30 µs to keep the regulator off and must be below the minimum limit for Output = Low to keep the regulator on.
Note 6: The drive pin voltage, VDR, is equal to the input voltage when input voltage is less than 6V. VDR is equal to 6V when the input voltage is greater than or
equal to 6V.
Note 7: The over-voltage protection is specified with respect to the feedback voltage. This is because the over-voltage protection tracks the feedback voltage.
The over-voltage threshold can be calculated by adding the feedback voltage (VFB) to the over-voltage protection specification.
Note 8: For this test, the FA/SYNC/SD Pin is pulled to ground using a 40 kΩ resistor .
Note 9: For this test, the FA/SYNC/SD Pin is pulled to 3V using a 40 kΩ resistor.
Typical Performance Characteristics
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Comp Pin Voltage vs. Load Current
Unless otherwise specified, VIN = 12V, TJ = 25°C.
Switching Frequency vs. RFA
20136546
Efficiency vs. Load Current (3.3VIN and 12VOUT)
20136547
Efficiency vs. Load Current (5VIN and 12VOUT)
20136548
5
20136549
www.national.com
5 Page 
It is good design practice to only add as much slope com-
pensation as needed to avoid subharmonic oscillation. Addi-
tional slope compensation minimizes the influence of the
sensed current in the control loop. With very large slope com-
pensation the control loop characteristics are similar to a
voltage mode regulator which compares the error voltage to
a saw tooth waveform rather than the inductor current.
FREQUENCY ADJUST/SYNCHRONIZATION/SHUTDOWN
The switching frequency of the LM3481 can be adjusted be-
tween 100 kHz and 1 MHz using a single external resistor.
This resistor must be connected between the FA/SYNC/SD
pin and ground, as shown in Figure 7. Please refer to the typ-
ical performance characteristics to determine the value of the
resistor required for a desired switching frequency.
The following equation can also be used to estimate the fre-
quency adjust resistor.
Where fS is in kHz and RFA in kΩ.
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20136513
FIGURE 5. Increasing the Slope of the Compensation
Ramp
The LM3481 can be synchronized to an external clock. The
external clock must be connected between the FA/SYNC/SD
pin and ground, as shown in Figure 8. The frequency adjust
resistor may remain connected while synchronizing a signal,
therefore if there is a loss of signal, the switching frequency
will be set by the frequency adjust resistor.
It is also necessary to have the width of the synchronization
pulse narrower than the duty cycle of the converter and to
have the synchronization pulse width ≥ 300 ns.
The FA/SYNC/SD pin also functions as a shutdown pin. If a
high signal (refer to the electrical characteristics section for
definition of high signal) appears on the FA/SYNC/SD pin, the
LM3481 stops switching and goes into a low current mode.
The total supply current of the IC reduces to 5 µA, typically,
under these conditions.
Figure 9 and Figure 10 shows an implementation of a shut-
down function when operating in frequency adjust mode and
synchronization mode respectively. In frequency adjust
mode, connecting the FA/SYNC/SD pin to ground forces the
clock to run at a certain frequency. Pulling this pin high shuts
down the IC. In frequency adjust or synchronization mode, a
high signal for more than 30 µs shuts down the IC.
20136551
FIGURE 6. ΔVSL vs RSL
11 www.national.com
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| PDF Descargar | [ Datasheet LM3481.PDF ] | |
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