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PDF LM25010 Data sheet ( Hoja de datos )
| Número de pieza | LM25010 | |
| Descripción | Step-Down Switching Regulator | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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February 2006
LM25010
42V, 1.0A Step-Down Switching Regulator
General Description
The LM25010 features all the functions needed to implement
a low cost, efficient, buck regulator capable of supplying in
excess of 1A load current. This high voltage regulator inte-
grates an N-Channel Buck Switch, and is available in ther-
mally enhanced LLP-10 and TSSOP-14EP packages. The
constant on-time regulation scheme requires no loop com-
pensation resulting in fast load transient response and sim-
plified circuit implementation. The operating frequency re-
mains constant with line and load variations due to the
inverse relationship between the input voltage and the on-
time. The valley current limit detection is set at 1.25A. Addi-
tional features include: VCC under-voltage lock-out, thermal
shutdown, gate drive under-voltage lock-out, and maximum
duty cycle limiter.
Features
n Wide 6V to 42V Input Voltage Range
n Valley Current Limiting At 1.25A
n Programmable Switching Frequency Up To 1 MHz
n Integrated N-Channel Buck Switch
n Integrated High Voltage Bias Regulator
n No Loop Compensation Required
n Ultra-Fast Transient Response
n Nearly Constant Operating Frequency With Line and
Load Variations
n Adjustable Output Voltage
n 2.5V, ±2% Feedback Reference
n Programmable Soft-Start
n Thermal shutdown
Typical Applications
n Non-Isolated Telecommunications Regulator
n Secondary Side Post Regulator
n Power SUpply for Automotive Electronics
Package
n LLP-10 (4 mm x 4 mm)
n TSSOP-14EP
n Both Packages Have Exposed Thermal Pad For
Improved Heat Dissipation
Basic Step-Down Regulator
© 2006 National Semiconductor Corporation DS201727
20172743
www.national.com
1 page  
Electrical Charateristics Specifications with standard type are for TJ = 25˚C only; limits in boldface type apply
over the full Operating Junction Temperature (TJ) range. Minimum and Maximum limits are guaranteed through test, design,
or statistical correlation. Typical values represent the most likely parametric norm at TJ = 25˚C, and are provided for reference
purposes only. Unless otherwise stated the following conditions apply: VIN = 24V, RON = 200kΩ. See (Note 5). (Continued)
Symbol
Parameter
Conditions
Off Timer
tOFF Minimum Off-time
Regulation and Over-Voltage Comparators (FB Pin)
VREF
FB regulation threshold
FB over-voltage threshold
FB bias current
Thermal Shutdown
TSD Thermal shutdown temperature
Thermal shutdown hysteresis
Thermal Resistance
θJA Junction to Ambient, 0 LFPM SDC Package
Air Flow
MXA Package
θJC Junction to Case
SDC Package
MXA Package
Min Typ Max Units
260 ns
2.445
2.50
2.9
1
2.550
V
V
nA
175 ˚C
20 ˚C
40 ˚C/W
40
5.2 ˚C/W
5.2
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 100pF capacitor discharged through a 1.5kΩ resistor into each pin.
Note 3: VCC provides bias for the internal gate drive and control circuits. Device thermal limitations limit external loading.
Note 4: For detailed information on soldering plastic TSSOP and LLP packages refer to the Packaging Data Book available from National Semiconductor
Corporation.
Note 5: Typical specifications represent the most likely parametric norm at 25˚C operation.
5 www.national.com
5 Page 
Current Limit (Continued)
mode. The load current (IOCL) is equal to the current limit
threshold plus half the ripple current (∆I/2). The ripple ampli-
tude (∆I) is calculated from:
(9)
The current limit threshold can be increased by connecting
an external resistor (RCL) between SGND and ISEN. RCL
typically is less than 1Ω, and the calculation of its value is
explained in the Applications Information section. If the cur-
rent limit threshold is increased by adding RCL, the maximum
continuous load current should not exceed 1.5A, and the
peak current out of the SW pin should not exceed 2A.
FIGURE 4. Inductor Current - Current Limit Operation
20172720
N - Channel Buck Switch and
Driver
The LM25010 integrates an N-Channel buck switch and
associated floating high voltage gate driver. The peak cur-
rent through the buck switch should not exceed 2A, and the
load current should not exceed 1.5A. The gate driver circuit
is powered by the external bootstrap capacitor between BST
and SW (C4), which is recharged each off-time from VCC
through the internal high voltage diode. The minimum off-
time, nominally 260 ns, ensures sufficient time during each
cycle to recharge the bootstrap capacitor. A 0.022 µF ce-
ramic capacitor is recommended for C4.
Soft-Start
The soft-start feature allows the regulator to gradually reach
a steady state operating point, thereby reducing startup
stresses and current surges. At turn-on, while VCC is below
the under-voltage threshold (t1 in Figure 1), the SS pin is
internally grounded, and VOUT is held at 0V. When VCC
exceeds the under-voltage threshold (UVLO) an internal
11.5 µA current source charges the external capacitor (C6)
at the SS pin to 2.5V (t2 in Figure 1). The increasing SS
voltage at the non-inverting input of the regulation compara-
tor gradually increases the output voltage from zero to the
desired value. The soft-start feature keeps the load inductor
current from reaching the current limit threshold during
start-up, thereby reducing inrush currents.
An internal switch grounds the SS pin if VCC is below the
under-voltage lock-out threshold, if a thermal shutdown oc-
curs, or if the circuit is shutdown using the RON/SD pin.
Thermal Shutdown
The LM25010 should be operated below the Maximum Op-
erating Junction Temperature rating. If the junction tempera-
ture increases during a fault or abnormal operating condi-
tion, the internal Thermal Shutdown circuit activates typically
at 175˚C. The Thermal Shutdown circuit reduces power
dissipation by disabling the buck switch and the on-timer,
and grounding the SS pin. This feature helps prevent cata-
strophic failures from accidental device overheating. When
the junction temperature reduces below approximately
155˚C (20˚C typical hysteresis), the SS pin is released and
normal operation resumes.
Applications Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is
illustrated with a design example. Referring to the Block
Diagram, the circuit is to be configured for the following
specifications:
• VOUT = 5V
• VIN = 6V to 40V
• FS = 175 kHz
• Minimum load current = 200 mA
• Maximum load current = 1.0A
• Softstart time = 5 ms.
R1 and R2: These resistors set the output voltage, and their
ratio is calculated from:
R1/R2 = (VOUT/2.5V) - 1
(10)
R1/R2 calculates to 1.0. The resistors should be chosen
from standard value resistors in the range of 1.0 kΩ - 10 kΩ.
A value of 1.0 kΩ will be used for R1 and for R2.
RON, FS: RON can be chosen using Equation 7 to set the
nominal frequency, or from Equation 6 if the on-time at a
particular VIN is important. A higher frequency generally
means a smaller inductor and capacitors (value, size and
cost), but higher switching losses. A lower frequency means
11 www.national.com
11 Page | ||
| Páginas | Total 17 Páginas | |
| PDF Descargar | [ Datasheet LM25010.PDF ] | |
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