NSC LM20242, LM20242MH Datasheet

November 1, 2007

LM20242
36V, 2A PowerWise® Adjustable Frequency Synchronous
Buck Regulator

General Description

The LM20242 is a full featured 1MHz capable synchronous
buck regulator capable of delivering up to 2A of load current.
The current mode control loop is externally compensated with
only two external components, offering both high perfor­mance and ease of use. The device is optimized to work over
the input voltage range of 4.5V to 36V making it well suited
for high voltage systems.

The device features internal Over Voltage Protection (OVP)
and Over Current Protection (OCP) circuits for increased sys­tem reliability. A precision Enable pin and integrated UVLO
allows the turn on of the device to be tightly controlled and
sequenced. Startup inrush currents are limited by both an in­ternally fixed and externally adjustable soft-start circuit. Fault
detection and supply sequencing are possible with the inte­grated PGOOD circuit.

The LM20242 is designed to work well in multi-rail power
supply architectures. The output voltage of the device can be
configured to track a higher voltage rail using the SS/TRK pin.
If the output of the LM20242 is pre-biased at startup it will not
sink current to pull the output low until the internal soft-start
ramp exceeds the voltage at the feedback pin.

The LM20242 is offered in an exposed pad 20 pin TSSOP
package that can be soldered to the PCB, eliminating the
need for bulky heatsinks.

Features

■

2A Output Current, 3.7A peak current

■

130 mΩ/110 mΩ integrated power MOSFETs

■

1.5% output voltage accuracy

■

Current Mode Control, selectable compensation

■

Resistor programmed, 1MHz capable oscillator

■

Synchronous rectifier with diode emulation

■

Adjustable output voltage down to 0.8V

■

Compatible with pre-biased loads

■

Programmable soft-start with external capacitor

■

Precision enable pin with hysteresis

■

OVP, UVLO inputs and PGOOD output

■

Internally protected with peak current limit, thermal
shutdown and restart

■

Accurate current limit with frequency foldback

■

Non-linear current mode slope compensation

■

eTSSOP-20 exposed pad package

Applications

■

Simple to design, high efficiency point of load regulation
from a 4.5V to 36V bus

■

High Performance DSPs, FPGAs, ASICs and
Microprocessors

■

Communications Infrastructure, Automotive

Simplified Application Circuit

30031401

PowerWise® is a registered trademark of National Semiconductor Corporation.

© 2007 National Semiconductor Corporation 300314 www.national.com

LM20242 36V, 2A PowerWise

®

Adjustable Frequency Synchronous Buck Regulator

Connection Diagram

30031402

Top View

eTSSOP-20 Package

Ordering Information

Order Number Package Type NSC Package Drawing Package Marking Supplied As

LM20242MH eTSSOP-20 MXA20A 20242MH 73 Units per Rail

LM20242MHE 250 Units per Tape and Reel

LM20242MHX 2500 Units per Tape and Reel

Pin Descriptions

Pin(s) Name Description Application Information

1 SS/TRK Soft-Start or Tracking control input An internal 5 µA current source charges an external capacitor to set

the soft-start rate. The PWM can Track to an external voltage ramp
with a low impedance source. If left open, an internal 1 ms SS ramp
is activated.

2 FB Feedback input to the error amplifier

from the regulated output

This pin is connected to the inverting input of the internal
transconductance error amplifier. An 800 mV reference is internally
connected to the non-inverting input of the error amplifier.

3 PGOOD Power good output signal Open drain output indicating the output voltage is regulating within

tolerance. A pull-up resistor of 10 kΩ to 100 kΩ is recommended if this
function is used.

4 COMP Output of the internal error amplifier and

input to the Pulse Width Modulator

The loop compensation network should be connected between the
COMP pin and the AGND pin.

5,6,15,16 VIN Input supply voltage Nominal operating range: 4.5V to 36V.

7,8,13,14 SW Switch pin The drain terminal of the internal Synchronous Rectifier power

NMOSFET and the source terminal of the internal Control power
NMOSFET.

9,10,11 GND Ground Internal reference for the power MOSFETs.

12 AGND Analog ground Internal reference for the regulator control functions.

17 BOOT Boost input for bootstrap capacitor An internal diode from VCC to BOOT charges an external capacitor

required from SW to BOOT to power the Control MOSFET gate driver.

18 VCC Output of the high voltage linear

regulator. The VCC voltage is regulated
to approximately 5.5V.

VCC tracks VIN up to about 7.2V. Above VIN = 7.2V, VCC is regulated
to approximately 5.5 Volts. A 0.1 µF to 1 µF ceramic decoupling
capacitor is required. The VCC pin is an output only.

19 EN Enable or UVLO input An external voltage divider can be used to set the line undervoltage

lockout threshold. If the EN pin is left unconnected, a 2 µA pull-up
current source pulls the EN pin high to enable the regulator.

20 RT Internal oscillator frequency adjust input Normally biased at 550 mV. An external resistor connected between

RT and AGND sets the internal oscillator frequency.

EP Exposed

Pad

Exposed pad Exposed metal pad on the underside of the package with a weak

electrical connection to GND. Connect this pad to the PC board ground
plane in order to improve heat dissipation.

www.national.com 2

LM20242

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 -0.3V to +38V
BOOT to GND -0.3V to +43V
BOOT to SW -0.3V to +7V
SW to GND -0.5V to +38V
SW to GND (Transient) -1.5V (< 20 ns)
FB, EN, SS/TRK, PGOOD to

GND

-0.3V to +6V

VCC to GND -0.3V to +8V
Storage Temperature -65°C to 150°C
ESD Rating
Human Body Model (Note 2) 2kV

Operating Ratings

VIN to GND +4.5V to +36V
Junction Temperature −40°C to + 125°C

Electrical Characteristics Unless otherwise stated, the following conditions apply: V

VIN

= 12V. Limits in standard
type are for TJ = 25°C only, limits in bold face type apply over the junction temperature (TJ) range of -40°C to +125°C. 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.

Symbol Parameter Conditions Min Typ Max Units

V

FB

Feedback pin voltage V

VIN

= 4.5V to 36V

V

COMP

= 500 mV to 700 mV

0.788 0.8 0.812 V

R

HSW-DS(ON)

High-Side MOSFET On-Resistance ISW = 200 mA 130 225

mΩ

R

LSW-DS(ON)

Low-Side MOSFET On-Resistance ISW = 200 mA 110 190

mΩ

I

Q

Operating Quiescent Current V

VIN

= 4.5V to 36V 2 3 mA

I

SD

Shutdown Quiescent current VEN = 0V 150 180 µA

V

UVLO

VIN Under Voltage Lockout Rising V

VIN

3.65 3.9 4.2 V

V

UVLO(HYS)

VIN Under Voltage Lockout Hysteresis 200 400 mV

V

VCC

VCC Voltage I

VCC

= -5 mA, VEN = 5V 5.5 V

I

SS

Soft-Start Pin Source Current VSS = 0V 3 5 7 µA

I

BOOT

BOOT Diode Leakage V

BOOT

= 4V 10 nA

V

F-BOOT

BOOT Diode Forward Voltage I

BOOT

= -100 mA 0.9 1.1 V

Powergood

V

FB(OVP)

Over Voltage Protection Rising Threshold V

FB(OVP)

/ V

FB

107 110 112 %

V

FB(OVP-HYS)

Over Voltage Protection Hysteresis

Δ

VFB(OVP)

/ V

FB

2 3 %

V

FB(PG)

PGOOD Rising Threshold V

FB(PG)

/ V

FB

93 95 97 %

V

FB(PG-HYS)

PGOOD Hysteresis

Δ

VFB(PG)

/ V

FB

2 3 %

T

PGOOD

PGOOD delay 20 µs

I

PGOOD(SNK)

PGOOD Low Sink Current V

PGOOD

= 0.5V 0.6 1 mA

I

PGOOD(SRC)

PGOOD High Leakage Current V

PGOOD

= 5V 5 200 nA

Oscillator

F

SW1

Switching Frequency 1

RRT = 49.9 kΩ

675 750 825 kHz

F

SW2

Switching Frequency 2

RRT = 249 kΩ

225 250 325 kHz

T

OFF-TIME

Minimum Off Time 50 ns

V

RT

RT pin voltage

RRT = 250 kΩ

550 mV

Error Amplifier

I

FB

Feedback pin bias current VFB = 1V 50 nA

I

COMP(SRC)

COMP Output Source Current VFB = 0V

V

COMP

= 0V

200 400 µA

I

COMP(SNK)

COMP Output Sink Current VFB = 1V

V

COMP

= 0.5V

200 350 µA

g

m

Error Amplifier DC Transconductance I

COMP

= -50 µA to +50 µA 400 515 600 µmho

A

VOL

Error Amplifier Voltage Gain COMP pin open 2000 V/V

GBW Error Amplifier Gain-Bandwidth Product COMP pin open 7 MHz

3 www.national.com

LM20242

Symbol Parameter Conditions Min Typ Max Units

Current Limit

I

LIM

Cycle By Cycle Current Limit 3.1 3.7 4.65 A

T

ILIM

Cycle By Cycle Current Limit Delay 150 ns

Enable

V

EN(RISING)

EN Pin Rising Threshold 1.2 1.25 1.3 V

V

EN(HYS)

EN Pin Hysteresis 50 mV

I

EN

EN Source Current VEN = 0V, V

VIN

= 12V 2 µA

Thermal Shutdown

T

SD

Thermal Shutdown 170 °C

T

SD(HYS)

Thermal Shutdown Hysteresis 20 °C

Thermal Resistance

θ

JC

Junction to Case 5.6 °C/W

θ

JA

Junction to Ambient 0 LFM airflow 30 °C/W

Note 1: Absolute Maximum Ratings indicate limits beyond witch damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor to 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 Unless otherwise specified: T

J

= 25°C, V

VIN

= 12V

Efficiency vs. Load Current

fSW = 350 kHz

30031403

Efficiency vs. Load Current

fSW = 500 kHz

30031404

www.national.com 4

LM20242

Error Amplifier Gain

30031405

Error Amplifier Phase

30031406

Line Regulation

30031407

VCC vs. V

IN

30031408

Non-Switching IQ vs. V

IN

30031409

Shutdown IQ vs. V

IN

30031410

5 www.national.com

LM20242

PGOOD VOL vs. I

PGOOD

30031419

EN Threshold and Hysteresis vs. Temperature

30031421

EN Current vs. Temperature

30031422

Oscillator Frequency vs. R

RT

30031453

Oscillator Frequency vs. V

IN

30031454

High-Side FET Resistance vs. Temperature

30031455

www.national.com 6

LM20242