NSC LM34919TL, LM34919 Datasheet

November 1, 2007

LM34919
40V, 600 mA Step Down COT Switching Regulator

General Description

The LM34919 Step Down Switching Regulator features all of
the functions needed to implement a low cost, efficient, buck
bias regulator capable of supplying 0.6A to the load. This buck
regulator contains an N-Channel Buck Switch, and is avail­able in a micro SMD package. The constant on-time feedback
regulation scheme requires no loop compensation, results in
fast load transient response, and simplifies circuit implemen­tation. The operating frequency remains constant with line
and load variations due to the inverse relationship between
the input voltage and the on-time. The valley current limit re­sults in a smooth transition from constant voltage to constant
current mode when current limit is detected, reducing the fre­quency and output voltage, without the use of foldback. Ad­ditional features include: VCC under-voltage lockout, thermal
shutdown, gate drive under-voltage lockout, and maximum
duty cycle limiter.

Features

■

Integrated N-Channel buck switch

■

Integrated start-up regulator

■

Input Voltage Range: 8V to 40V

■

No loop compensation required

■

Ultra-Fast transient response

■

Operating frequency remains constant with load current
and input voltage

■

Maximum switching frequency: 1.6 MHz

■

Maximum Duty Cycle Limited During Start-Up

■

Adjustable output voltage

■

Valley Current Limit At 0.64A

■

Precision internal reference

■

Low bias current

■

Highly efficient operation

■

Thermal shutdown

Typical Applications

■

High Efficiency Point-Of-Load (POL) Regulator

■

Non-Isolated Telecommunication Buck Regulator

■

Secondary High Voltage Post Regulator

Package

■

micro SMD

Basic Step Down Regulator

30004431

© 2007 National Semiconductor Corporation 300044 www.national.com

LM34919 40V, 600 mA Step Down COT Switching Regulator

Connection Diagrams

30004402

Bump Side

30004433

Top View

Ordering Information

Order Number Package Type NSC Package Drawing Junction Temperature Range Supplied As

LM34919TL 10-Bump micro SMD TLP10A1A −40°C to + 125°C 250 Units on Tape and

Reel

LM34919TLX 10-Bump micro SMD TLP10A1A −40°C to + 125°C 3000 Units on Tape and

Reel

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LM34919

Pin Descriptions

Pin Number Name Description Application Information

A1 RON/SD On-time control and shutdown An external resistor from VIN to this pin sets the buck switch

on-time. Grounding this pin shuts down the regulator.

A2 RTN Circuit Ground Ground for all internal circuitry other than the current limit

detection.

A3 FB Feedback input from the regulated

output

Internally connected to the regulation and over-voltage
comparators. The regulation level is 2.5V.

B1 SGND Sense Ground Re-circulating current flows into this pin to the current sense

resistor.

B3 SS Softstart An internal current source charges an external capacitor to

2.5V, providing the softstart function.

C1 ISEN Current sense The re-circulating current flows through the internal sense

resistor, and out of this pin to the free-wheeling diode.
Current limit is nominally set at 0.64A.

C3 VCC Output from the startup regulator Nominally regulates at 7.0V. An external voltage (7V-14V)

can be applied to this pin to reduce internal dissipation. An
internal diode connects VCC to VIN.

D1 VIN Input supply voltage Nominal input range is 8.0V to 40V.

D2 SW Switching Node Internally connected to the buck switch source. Connect to

the inductor, free-wheeling diode, and bootstrap capacitor.

D3 BST Boost pin for bootstrap capacitor Connect a 0.022 µF capacitor from SW to this pin. The

capacitor is charged from VCC via an internal diode during
each off-time.

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LM34919

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 RTN 44V
BST to RTN 52V
SW to RTN (Steady State) -1.5V
ESD Rating (Note 2)
Human Body Model 2kV
BST to VCC 44V
VIN to SW 44V

BST to SW 14V
VCC to RTN 14V
SGND to RTN -0.3V to +0.3V
SS to RTN -0.3V to 4V
All Other Inputs to RTN -0.3 to 7V
Storage Temperature Range -65°C to +150°C
JunctionTemperature 150°C

Operating Ratings (Note 1)

VIN 8.0V to 40V
Junction Temperature −40°C to + 125°C

Electrical Characteristics Specifications with standard type are for T

J

= 25°C only; limits in boldface type apply
over the full Operating Junction Temperature (TJ) range. 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. Unless otherwise stated the following conditions apply: VIN = 12V, RON = 200kΩ. See (Note 5).

Symbol Parameter Conditions Min Typ Max Units

Start-Up Regulator, V

CC

VCCReg VCC regulated output 6.6 7 7.4 V

VIN-VCC dropout voltage I

CC

= 0 mA,

VCC = UVLO

VCC

+ 250 mV

1.2 V

VCC output impedance

0 mA ≤ ICC ≤ 5 mA, VIN = 8V

175

Ω

VCC current limit (Note 3) VCC = 0V 9.5 mA

UVLO

VCCVCC

under-voltage lockout threshold VCC increasing 5.7 V

UVLO

VCC

hysteresis VCC decreasing 150 mV

UVLO

VCC

filter delay 100 mV overdrive 3 µs

I

Q

IIN operating current Non-switching, FB = 3V, SW = Open 0.5 0.8 mA

I

SD

IIN shutdown current RON/SD = 0V, SW = Open 75 150 µA

Switch Characteristics

Rds(on) Buck Switch Rds(on) I

TEST

= 200 mA 0.5 1.0

Ω

UVLO

GD

Gate Drive UVLO V

BST

- VSW Increasing 3.0 4.4 5.2 V

UVLOGD hysteresis 480 mV

Softstart Pin

V

SS

Pull-up voltage 2.5 V

Internal current source VSS = 1V 10.5 µA

Current Limit

I

LIM

Threshold Current out of ISEN 0.52 0.64 0.76 A

Resistance from ISEN to SGND 140

mΩ

Response time 150 ns

On Timer

tON - 1 On-time

VIN = 10V, RON = 200 kΩ

2.1 2.77 3.5 µs

tON - 2 On-time

VIN = 40V, RON = 200 kΩ

700 ns

Shutdown threshold Voltage at RON/SD rising 0.45 0.8 1.2 V

Threshold hysteresis Voltage at RON/SD 25 mV

Off Timer

t

OFF

Minimum Off-time 155 ns

Regulation and Over-Voltage Comparators (FB Pin)

V

REF

FB regulation threshold SS pin = steady state 2.440 2.5 2.550 V

FB over-voltage threshold 2.9 V

FB bias current FB = 3V 1 nA

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LM34919

Symbol Parameter Conditions Min Typ Max Units

Thermal Shutdown

T

SD

Thermal shutdown temperature 175 °C

Thermal shutdown hysteresis 20 °C

Thermal Resistance

θ

JA

Junction to Ambient
0 LFPM Air Flow

61 °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: VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading

Note 4: For detailed information on soldering micro SMD package, refer to the Application Note AN-1112.

Note 5: Typical specifications represent the most likely parametric norm at 25°C operation.

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LM34919

Typical Performance Characteristics

VCC vs V

IN

30004404

VCC vs I

CC

30004405

ICC vs Externally Applied V

CC

30004435

ON-TIME vs VIN and R

ON

30004436

Voltage at the R

ON/SD

Pin

30004437

Shutdown and Operating Current into V

IN

30004438

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LM34919