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PDF LM5576 Data sheet ( Hoja de datos )
| Número de pieza | LM5576 | |
| Descripción | Step-Down Switching | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LM5576 (archivo pdf) en la parte inferior de esta página. Total 24 Páginas | ||
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March 2007
LM5576
SIMPLE SWITCHER® 75V, 3A Step-Down Switching
Regulator
General Description
The LM5576 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 - 75V, the
LM5576 delivers 3A of continuous output current with an in-
tegrated 170mΩ N-Channel MOSFET. The regulator utilizes
an Emulated Current Mode architecture which provides in-
herent 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 operat-
ing frequency is adjustable from 50kHz to 500kHz to allow
optimization of size and efficiency. To reduce EMI, a frequen-
cy synchronization pin allows multiple IC’s from the
LM(2)557x family to self-synchronize or to synchronize to an
external clock. The LM5576 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 power enhanced TSSOP-20 package featuring an exposed
die attach pad for thermal dissipation. The LM5576 is sup-
ported by the full suite of WEBENCH® On-Line design tools.
Features
■ Integrated 75V, 170mΩ N-channel MOSFET
■ Ultra-wide input voltage range from 6V to 75V
■ Adjustable output voltage as low as 1.225V
■ 1.5% feedback reference accuracy
■ Operating frequency adjustable between 50kHz and
500kHz 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-20EP (Exposed Pad)
Simplified Application Schematic
WEBENCH® is a registered trademark of National Semiconductor Corporation.
© 2007 National Semiconductor Corporation 201899
20189901
www.national.com
1 page  
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN to GND
BST to GND
PRE to GND
SW to GND (Steady State)
BST to VCC
SD, VCC to GND
76V
90V
76V
-1.5V
76V
14V
BST to SW
OUT to GND
SYNC, SS, FB, RAMP to GND
ESD Rating (Note 2)
Human Body Model
Storage Temperature Range
14V
Limited to Vin
7V
2kV
-65°C to +150°C
Operating Ratings (Note 1)
VIN
Operation Junction Temperature
6V to 75V
−40°C to + 125°C
Electrical Characteristics Specifications with standard typeface are for TJ = 25°C, and those with boldface type
apply over full Operating Junction Temperature range. VIN = 48V, RT = 32.4kΩ unless otherwise stated. (Note 3)
Symbol
Parameter
Conditions
Min Typ Max Units
STARTUP REGULATOR
VccReg
Vcc Regulator Output
6.85 7.15 7.45
V
Vcc LDO Mode turn-off
9V
Vcc Current Limit
Vcc = 0V
25 mA
VCC SUPPLY
Vcc UVLO Threshold
(Vcc increasing)
5.03 5.35 5.67
V
Vcc Undervoltage Hysteresis
0.25 V
Bias Current (Iin)
FB = 1.3V
3.4 4.5
mA
Shutdown Current (Iin)
SD = 0V
57 85
µA
SHUTDOWN THRESHOLDS
Shutdown Threshold
(SD Increasing)
0.47 0.7
0.9
V
Shutdown Hysteresis
0.1 V
Standby Threshold
(Standby Increasing)
1.17 1.225
1.28
V
Standby Hysteresis
0.1 V
SD Pull-up Current Source
5 µA
SWITCH CHARACTERSICS
Buck Switch Rds(on)
BOOST UVLO
170 340
3.8
mΩ
V
BOOST UVLO Hysteresis
0.56 V
Pre-charge Switch Rds(on)
Pre-charge Switch on-time
70 Ω
265 ns
CURRENT LIMIT
Cycle by Cycle Current Limit
RAMP = 0V
3.6 4.2
5.1
A
Cycle by Cycle Current Limit Delay
RAMP = 2.5V
100 ns
SOFT-START
SS Current Source
7 10 14
µA
OSCILLATOR
Frequency1
180 200
220
kHz
Frequency2
SYNC Source Impedance
RT = 11kΩ
425 485
11
545
kHz
kΩ
SYNC Sink Impedance
SYNC Threshold (falling)
110 Ω
1.3 V
SYNC Frequency
SYNC Pulse Width Minimum
RT = 11kΩ
550
15
kHz
ns
5 www.national.com
5 Page 
20189907
FIGURE 5. Simplified Oscillator Block Diagram and SYNC I/O Circuit
Error Amplifier and PWM
Comparator
The internal high gain error amplifier generates an error signal
proportional to the difference between the regulated output
voltage and an internal precision reference (1.225V). The
output of the error amplifier is connected to the COMP pin
allowing the user to provide loop compensation components,
generally a type II network, as illustrated in Figure 1. This
network creates a pole at DC, a zero and a noise reducing
high frequency pole. The PWM comparator compares the
emulated current sense signal from the RAMP generator to
the error amplifier output voltage at the COMP pin.
RAMP Generator
The ramp signal used in the pulse width modulator for current
mode control is typically derived directly from the buck switch
current. This switch current corresponds to the positive slope
portion of the output inductor current. Using this signal for the
PWM ramp simplifies the control loop transfer function to a
single pole response and provides inherent input voltage
feed-forward compensation. The disadvantage of using the
buck switch current signal for PWM control is the large leading
edge spike due to circuit parasitics that must be filtered or
blanked. Also, the current measurement may introduce sig-
nificant propagation delays. The filtering, blanking time and
propagation delay limit the minimum achievable pulsewidth.
In applications where the input voltage may be relatively large
in comparison to the output voltage, controlling small
pulsewidths and duty cycles is necessary for regulation. The
LM5576 utilizes a unique ramp generator, which does not ac-
tually measure the buck switch current but rather reconstructs
the signal. Reconstructing or emulating the inductor current
provides a ramp signal to the PWM comparator that is free of
leading edge spikes and measurement or filtering delays. The
current reconstruction is comprised of two elements; a sam-
ple & hold DC level and an emulated current ramp.
11 www.national.com
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LM5576.PDF ] | |
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