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PDF LM3478MM Data sheet ( Hoja de datos )
| Número de pieza | LM3478MM | |
| Descripción | High Efficiency Low-Side N-Channel Controller for Switching Regulator | |
| Fabricantes | National Semiconductor | |
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May 2003
LM3478
High Efficiency Low-Side N-Channel Controller for
Switching Regulator
General Description
The LM3478 is a versatile Low-Side N-FET switching regu-
lator controller. It is suitable for use in topologies requiring
low side FET, such as boost, flyback, SEPIC, etc. Moreover,
the LM3478 can be operated at extremely high switching
frequency in order to reduce the overall solution size. The
switching frequency of LM3478 can be adjusted to any value
between 100kHz and 1MHz by using a single external resis-
tor. Current mode control provides superior bandwidth and
transient response, besides cycle-by-cycle current limiting.
Output current can be programmed with a single external
resistor.
The LM3478 has built in features such as thermal shutdown,
short-circuit protection, over voltage protection, etc. 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
n Wide supply voltage range of 2.97V to 40V
n 100kHz to 1MHz Adjustable clock frequency
n ±2.5% (over temperature) internal reference
n 10µA shutdown current (over temperature)
Features
n 8-lead Mini-SO8 (MSOP-8) package
n Internal push-pull driver with 1A peak current capability
n Current limit and thermal shutdown
n Frequency compensation optimized with a capacitor and
a resistor
n Internal softstart
n Current Mode Operation
n Undervoltage Lockout with hysteresis
Applications
n Distributed Power Systems
n Battery Chargers
n Offline Power Supplies
n Telecom Power Supplies
n Automotive Power Systems
Typical Application Circuit
Typical High Efficiency Step-Up (Boost) Converter
10135501
© 2003 National Semiconductor Corporation DS101355
www.national.com
1 page  
Electrical Characteristics (Continued)
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the device
is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics.
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/SD pin should be pulled to VIN through a resistor to turn the regulator off. The voltage on the FA/SD pin must be above the maximum limit for Output
= High to keep the regulator off and must be below the limit for Output = Low to keep the regulator on.
Note 6: The voltage on the drive pin, VDR is equal to the input voltage when input voltage is less than 7.2V. VDR is equal to 7.2V when the input voltage is greater
than or equal to 7.2V.
Note 7: The limits for the maximum duty cycle can not be specified since the part does not permit less than 100% maximum duty cycle operation.
Note 8: The over-voltage protection is specified with respect to the feedback voltage. This is because the over-voltage protection tracks the feedback voltage. The
overvoltage protection threshold is given by adding the feedback voltage, VFB to the over-voltage protection specification.
Note 9: For this test, the FA/SD pin is pulled to ground using a 40K resistor.
Note 10: For this test, the FA/SD pin is pulled to 5V using a 40K resistor.
Typical Performance Characteristics Unless otherwise specified, VIN = 12V, TJ = 25˚C.
IQ vs Input Voltage (Shutdown)
ISupply vs Input Voltage (Non-Switching)
10135503
ISupply vs VIN (Switching)
10135534
Switching Frequency vs RFA
10135535
5
10135504
www.national.com
5 Page 
Functional Description (Continued)
FIGURE 3. Compensation Ramp Avoids Sub-Harmonic Oscillation
10135511
The compensation ramp has been added internally in
LM3478. The slope of this compensation ramp has been
selected to satisfy most of the applications. The slope of the
internal compensation ramp depends on the frequency. This
slope can be calculated using the formula:
MC = VSL.FS Volts/second
In the above equation, VSL is the amplitude of the internal
compensation ramp. Limits for VSL have been specified in
the electrical characteristics.
In order to provide the user additional flexibility, a patented
scheme has been implemented inside the IC to increase the
slope of the compensation ramp externally, if the need
arises. Adding a single external resistor, RSL(as shown in
Figure 4) increases the slope of the compensation ramp, MC
by :
In this equation, ∆VSL is equal to 40.10-6RSL. Hence,
∆VSL versus RSL has been plotted in Figure 5 for different
frequencies.
11 www.national.com
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| Páginas | Total 21 Páginas | |
| PDF Descargar | [ Datasheet LM3478MM.PDF ] | |
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