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PDF LM25574 Data sheet ( Hoja de datos )
| Número de pieza | LM25574 | |
| Descripción | Step-Down Switching Regulator | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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January 2007
LM25574
42V, 0.5A Step-Down Switching Regulator
General Description
The LM25574 is an easy to use SIMPLE SWITCHER® buck
regulator which allows design engineers to design and opti-
mize a robust power supply using a minimum set of compo-
nents. Operating with an input voltage range of 6 - 42V, the
LM25574 delivers 0.5A of continuous output current with an
integrated 750mΩ N-Channel MOSFET. The regulator uti-
lizes an Emulated Current Mode architecture which provides
inherent line regulation, tight load transient response, and
ease of loop compensation without the usual limitation of low-
duty cycles associated with current mode regulators. The
operating frequency is adjustable from 50kHz to 1MHz to al-
low optimization of size and efficiency. To reduce EMI, a
frequency synchronization pin allows multiple IC’s from the
LM(2)557x family to self-synchronize or to synchronize to an
external clock. The LM25574 guarantees robustness with cy-
cle-by-cycle current limit, short-circuit protection, thermal
shut-down, and remote shut-down. The device is available in
a TSSOP-16 package. The LM25574 is supported by the full
suite of WEBENCH® On-Line design tools.
Features
■ Integrated 42V, 750mΩ N-channel MOSFET
■ Ultra-wide input voltage range from 6V to 42V
■ Adjustable output voltage as low as 1.225V
■ 1.5% feedback reference accuracy
■ Operating frequency adjustable between 50kHz and
1MHz with single resistor
■ Master or slave frequency synchronization
■ Adjustable soft-start
■ Emulated current mode control architecture
■ Wide bandwidth error amplifier
■ Built-in protection
Package
■ TSSOP-16
Simplified Application Schematic
WEBENCH® is a registered trademark of National Semiconductor Corporation.
© 2007 National Semiconductor Corporation 202141
20214101
www.national.com
1 page  
Symbol
Parameter
RAMP GENERATOR
Ramp Current 1
Ramp Current 2
PWM COMPARATOR
Forced Off-time
Min On-time
COMP to PWM Comparator Offset
ERROR AMPLIFIER
Feedback Voltage
FB Bias Current
DC Gain
COMP Sink / Source Current
Unity Gain Bandwidth
THERMAL SHUTDOWN
Tsd Thermal Shutdown Threshold
Thermal Shutdown Hysteresis
THERMAL RESISTANCE
θJC Junction to Case
θJA Junction to Ambient
Conditions
Vin = 36V, Vout=10V
Vin = 10V, Vout=10V
Vfb = COMP
Min Typ Max
272 310
36 50
368
64
416 500
80
0.7
575
1.207
3
1.225
17
70
3
1.243
165
25
30
90
Units
µA
µA
ns
ns
V
V
nA
dB
mA
MHz
°C
°C
°C/W
°C/W
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: Min and Max limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical
Quality Control (SQC) methods. Limits are used to calculate National’s Average Outgoing Quality Level (AOQL).
Typical Performance Characteristics
Oscillator Frequency vs RT
Oscillator Frequency vs Temperature
FOSC = 200kHz
20214120
5
20214121
www.national.com
5 Page 
FIGURE 6. Composition of Current Sense Signal
20214108
The sample & hold DC level illustrated in Figure 6 is derived
from a measurement of the re-circulating Schottky diode an-
ode current. The re-circulating diode anode should be con-
nected to the IS pin. The diode current flows through an
internal current sense resistor between the IS and PGND
pins. The voltage level across the sense resistor is sampled
and held just prior to the onset of the next conduction interval
of the buck switch. The diode current sensing and sample &
hold provide the DC level of the reconstructed current signal.
The positive slope inductor current ramp is emulated by an
external capacitor connected from the RAMP pin to AGND
and an internal voltage controlled current source. The ramp
current source that emulates the inductor current is a function
of the Vin and Vout voltages per the following equation:
IRAMP = (10µ x (Vin – Vout)) + 50µA
Proper selection of the RAMP capacitor depends upon the
selected value of the output inductor. The value of CRAMP can
be selected from: CRAMP = L x 5 x 10-6, where L is the value
of the output inductor in Henrys. With this value, the scale
factor of the emulated current ramp will be approximately
equal to the scale factor of the DC level sample and hold
(2.0V / A). The CRAMP capacitor should be located very close
to the device and connected directly to the pins of the IC
(RAMP and AGND).
For duty cycles greater than 50 percent, current mode control
circuits are subject to sub-harmonic oscillation. Adding a fixed
slope voltage ramp signal (slope compensation) to the current
sense signal prevents this oscillation. The 50µA of offset cur-
rent provided from the emulated current source adds some
fixed slope to the ramp signal. In some very high duty cycle
applications, additional slope may be required. In these ap-
plications, the ramp capacitor value can be decreased to
increase the ramp slope compensation.
11 www.national.com
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| Páginas | Total 22 Páginas | |
| PDF Descargar | [ Datasheet LM25574.PDF ] | |
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