March 2007 LM25574
SIMPLE SWITCHER® 42V, 0.5A Step-Down Switching Regulator
General Description
The LM25574 is an easy to u SIMPLE SWITCHER® buck regulator which allows design engineers to design and opti-mize a robust power supply using a minimum t 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 respon, and ea 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 lf-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学习 英语
■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
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WEBENCH® is a registered trademark of National Semiconductor Corporation.
© 2007 National Semiconductor LM25574 SIMPLE SWITCHER® 42V, 0.5A Step-Down Switching Regulator
Connection Diagram
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Top View 16-Lead TSSOP
Ordering Information
Order Number Package Type NSC Package Drawing Supplied As LM25574MT TSSOP-16MTC1692 Units in Rail
LM25574MTX
TSSOP-16
cerberusMTC16
2500 Units on Tape and Reel
2
L M 25574
Pin Descriptions
Pin(s)Name Description Application Information 1VCC Output of the bias regulator Vcc tracks Vin up to 9V. Beyond 9V, Vcc is regulated to 7
Volts. A 0.1uF to 1uF ceramic decoupling capacitor is
required. An external voltage (7.5V – 14V) can be applied
to this pin to reduce internal power dissipation.
2SD Shutdown or UVLO input If the SD pin voltage is below 0.7V the regulator will be in a
low power state. If the SD pin voltage is between 0.7V and
1.225V the regulator will be in standby mode. If the SD pin
voltage is above 1.225V the regulator will be operational. An
external voltage divider can be ud to t a line
undervoltage shutdown threshold. If the SD pin is left open
circuit, a 5µA pull-up current source configures the regulator
fully operational.
3Vin Input supply voltage Nominal operating range: 6V to 42V
4SYNC Oscillator synchronization input or output The internal oscillator can be synchronized to an external
clock with an external pull-down device. Multiple LM25574
devices can be synchronized together by connection of their
SYNC pins.
5COMP Output of the internal error amplifier The loop compensation network should be connected
between this pin and the FB pin.
6FB Feedback signal from the regulated
output This pin is connected to the inverting input of the internal error amplifier. The regulation threshold is 1.225V.
7RT Internal oscillator frequency t input The internal oscillator is t with a single resistor, connected
between this pin and the AGND pin.
8RAMP Ramp control signal An external capacitor connected between this pin and the
AGND pin ts the ramp slope ud for current mode
control. Recommended capacitor range 50pF to 2000pF. 9AGND Analog ground Internal reference for the regulator control functions
10SS Soft-start An external capacitor and an internal 10µA current source
t the time constant for the ri of the error amp reference.
The SS pin is held low during standby, Vcc UVLO and
thermal shutdown.
11OUT Output voltage connection Connect directly to the regulated output voltage.
12PGND Power ground Low side reference for the PRE switch and the IS n
resistor.
13IS Current n Current measurement connection for the re-circulating
diode. An internal n resistor and a sample/hold circuit
n the diode current near the conclusion of the off-time.剑桥商务英语
This current measurement provides the DC level of the
emulated current ramp.
14SW Switching node The source terminal of the internal buck switch. The SW pin
should be connected to the external Schottky diode and to
the buck inductor.
15PRE Pre-charge assist for the bootstrap
capacitor This open drain output can be connected to SW pin to aid charging the bootstrap capacitor during very light load conditions or in applications where the output may be pre-charged before the LM25574 is enabled. An internal pre-charge MOSFET is turned on for 250ns each cycle just prior to the on-time interval of the buck switch.
16BST Boost input for bootstrap capacitor An external capacitor is required between the BST and the
SW pins. A 0.022µF ceramic capacitor is recommended.
The capacitor is charged from Vcc via an internal diode
during the off-time of the buck switch.
LM25574
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,plea contact the National Semiconductor Sales Office/Distributors for availability and specifications.V IN to GND 45V BST to GND 60V PRE to GND
45V SW to GND (Steady State)-1.5V BST to V CC
45V SD, V CC to GND
14V
BST to SW 14V
OUT to GND
Limited to Vin SYNC, SS, FB, RAMP to GND 7V ESD Rating (Note 2) Human Body Model
2kV
Storage Temperature Range -65°C to +150°C
Operating Ratings
(Note 1)
V IN
6V to 42V
Operation Junction Temperature
−40°C to + 125°C
Electrical Characteristics
Specifications with standard typeface are for T J = 25°C, and tho with boldface type
apply over full Operating Junction Temperature range . V IN = 24V, R T = 32.4k Ω unless otherwi stated. (Note 3)
Symbol
Parameter
Conditions
Min Typ Max Units STARTUP REGULATOR
VccReg
Vcc Regulator Output 6.857.157.45V Vcc LDO Mode turn-off 9 V
Vcc Current Limit
Vcc = 0V 25 mA VCC SUPPLY
Vcc UVLO Threshold (Vcc increasing) 5.03 5.35 5.67V Vcc Undervoltage Hysteresis
0.35 V Bias Current (Iin)FB = 1.3V 3.7 4.5mA Shutdown Current (Iin)SD = 0V 4870µA SHUTDOWN THRESHOLDS
Shutdown Threshold (SD Increasing)0.470.70.9V Shutdown Hysteresis
0.1 V Standby Threshold (Standby Increasing) 1.17 1.225 1.28V Standby Hysteresis 0.1 V SD Pull-up Current Source 5 µA SWITCH CHARACTERSICS
Buck Switch Rds(on) 7501500m Ω BOOST UVLO
4 V BOOST UVLO Hysteresis 0.56 V Pre-charge Switch Rds(on) 70 Ω
Pre-charge Switch on-time
250 ns CURRENT LIMIT
星期二的英语单词Cycle by Cycle Current Limit RAMP = 0V 0.60.70.8A
Cycle by Cycle Current Limit Delay
RAMP = 2.5V 75 ns SOFT-START
SS Current Source 71014µA OSCILLATOR
Frequency1
180200220kHz Frequency2
R T = 11k Ω425485545kHz SYNC Source Impedance 11 k Ω SYNC Sink Impedance 110 Ω SYNC Threshold (falling)
1.3 V SYNC Frequency R T = 11k Ω550 kHz
SYNC Pul Width Minimum
15
ns
4
L M 25574
Symbol Parameter Conditions Min Typ Max Units RAMP GENERATOR
Ramp Current 1Vin = 36V, Vout=10V272310368µA
Ramp Current 2Vin = 10V, Vout=10V365064µA PWM COMPARATOR
Forced Off-time416500575ns
Min On-time80ns
COMP to PWM Comparator Offt0.7V ERROR AMPLIFIER
nobu
Feedback Voltage Vfb = COMP 1.207 1.225 1.243V
FB Bias Current17nA
DC Gain70dB
COMP Sink / Source Current3mA
Unity Gain Bandwidth3MHz DIODE SENSE RESISTANCE
D
SENSE
vanilla是什么意思250mΩTHERMAL SHUTDOWN长春翻译
Tsd Thermal Shutdown Threshold165°C Thermal Shutdown Hysteresis25°C THERMAL RESISTANCE
θJC Junction to Ca30°C/W θJA Junction to Ambient90°C/W
bebop
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, e 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 ud to calculate National’s Average Outgoing Quality Level (AOQL).
Typical Performance Characteristics
Oscillator Frequency vs Rget stupid
T
volturi20214120
Oscillator Frequency vs Temperature
F
OSC
= 200kHz
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LM25574
