NSC LM25576MH, LM25576 Datasheet

March 2007

LM25576
SIMPLE SWITCHER® 42V, 3A Step-Down Switching
Regulator

General Description

The LM25576 is an easy to use SIMPLE SWITCHER® buck
regulator which allows design engineers to design and opti­mize a robust power supply using a minimum set of compo­nents. Operating with an input voltage range of 6 - 42V, the
LM25576 delivers 3A of continuous output current with an in­tegrated 170mΩ N-Channel MOSFET. The regulator utilizes
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 allow
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 LM25576 guarantees robustness with cy­cle-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 LM25576 is sup­ported by the full suite of WEBENCH® On-Line design tools.

Features

■

Integrated 42V, 170mΩ 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-20EP (Exposed Pad)

Simplified Application Schematic

20208701

WEBENCH® is a registered trademark of National Semiconductor Corporation.

© 2007 National Semiconductor Corporation 202087 www.national.com

LM25576 SIMPLE SWITCHER® 42V, 3A Step-Down Switching Regulator

Connection Diagram

20208702

Top View

20-Lead TSSOP

Ordering Information

Order Number Package Type NSC Package Drawing Supplied As

LM25576MH Exposed Pad TSSOP-20 MXA20A 73 Units in Rail

LM25576MHX Exposed Pad TSSOP-20 MXA20A 2500 Units on Tape and Reel

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LM25576

Pin Descriptions

Pin(s) Name Description Application Information

1 VCC 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.

2 SD 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 used to set a line
undervoltage shutdown threshold. If the SD pin is left open
circuit, a 5µA pull-up current source configures the regulator
fully operational.

3, 4 Vin Input supply voltage Nominal operating range: 6V to 42V

5 SYNC Oscillator synchronization input or output The internal oscillator can be synchronized to an external

clock with an external pull-down device. Multiple LM25576
devices can be synchronized together by connection of their
SYNC pins.

6 COMP Output of the internal error amplifier The loop compensation network should be connected

between this pin and the FB pin.

7 FB 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.

8 RT Internal oscillator frequency set input The internal oscillator is set with a single resistor, connected

between this pin and the AGND pin.

9 RAMP Ramp control signal An external capacitor connected between this pin and the

AGND pin sets the ramp slope used for current mode
control. Recommended capacitor range 50pF to 2000pF.

10 AGND Analog ground Internal reference for the regulator control functions

11 SS Soft-start An external capacitor and an internal 10µA current source

set the time constant for the rise of the error amp reference.
The SS pin is held low during standby, Vcc UVLO and
thermal shutdown.

12 OUT Output voltage connection Connect directly to the regulated output voltage.

13, 14 PGND Power ground Low side reference for the PRE switch and the IS sense

resistor.

15, 16 IS Current sense Current measurement connection for the re-circulating

diode. An internal sense resistor and a sample/hold circuit
sense the diode current near the conclusion of the off-time.
This current measurement provides the DC level of the
emulated current ramp.

17, 18 SW 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.

19 PRE 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 pre­charged before the LM25576 is enabled. An internal pre­charge MOSFET is turned on for 265ns each cycle just prior
to the on-time interval of the buck switch.

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LM25576

Pin(s) Name Description Application Information

20 BST 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.

NA EP Exposed Pad Exposed metal pad on the underside of the device. It is

recommended to connect this pad to the PWB ground plane,
in order to aid in heat dissipation.

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LM25576

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 45V

BST to GND 60V
PRE to GND 45V
SW to GND (Steady State) -1.5V
BST to V

CC

45V

SD, VCC 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 those with boldface type

apply over full Operating Junction Temperature range. VIN = 24V, 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 9 V

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 48 70 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) 170 340

mΩ

BOOST UVLO 3.8 V

BOOST UVLO Hysteresis 0.56 V

Pre-charge Switch Rds(on) 70

Ω

Pre-charge Switch on-time 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

RT = 11kΩ

425 485 545 kHz

SYNC Source Impedance 11

kΩ

SYNC Sink Impedance 110

Ω

SYNC Threshold (falling) 1.3 V

SYNC Frequency

RT = 11kΩ

550 kHz

SYNC Pulse Width Minimum 15 ns

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LM25576

Symbol Parameter Conditions Min Typ Max Units

RAMP GENERATOR

Ramp Current 1 Vin = 36V, Vout=10V 136 160 184 µA

Ramp Current 2 Vin = 10V, Vout=10V 18 25 32 µA

PWM COMPARATOR

Forced Off-time 416 500 575 ns

Min On-time 80 ns

COMP to PWM Comparator Offset 0.7 V

ERROR AMPLIFIER

Feedback Voltage Vfb = COMP 1.207 1.225 1.243 V

FB Bias Current 17 nA

DC Gain 70 dB

COMP Sink / Source Current 3 mA

Unity Gain Bandwidth 3 MHz

DIODE SENSE RESISTANCE

D

SENSE

42

mΩ

THERMAL SHUTDOWN

Tsd Thermal Shutdown Threshold 165 °C

Thermal Shutdown Hysteresis 25 °C

THERMAL RESISTANCE

θ

JC

Junction to Case 6 °C/W

θ

JA

Junction to Ambient 40 °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 R

T

20208720

Oscillator Frequency vs Temperature

F

OSC

= 200kHz

20208721

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LM25576

Soft Start Current vs Temperature

20208722

VCC vs I

CC

VIN = 12V

20208723

VCC vs V

IN

RL = 7kΩ

20208724

Error Amplifier Gain/Phase

A

VCL

= 101

20208725

Demoboard Efficiency vs I

OUT

and V

IN

20208726

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LM25576

Typical Application Circuit and Block Diagram

20208703

FIGURE 1.

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LM25576