NSC LM34917ATL, LM34917A Datasheet

May 8, 2008

LM34917A Ultra Small 33V, 1.25A Constant On-Time Buck Switching Regulator with Intelligent Current Limit

General Description

The LM34917A Step-Down Switching Regulator features all the functions needed to implement a low cost, efficient, buck bias regulator capable of supplying at least 1.25A to the load. To reduce excessive switch current due to the possibility of a saturating inductor the valley current limit threshold changes with input and output voltages, and the on-time is reduced when current limit is detected. This buck regulator contains an N-Channel Buck Switch, and is available in the 12 pin 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 lock-out, input over-voltage shutdown, thermal shutdown, gate drive under-voltage lock-out, and maximum duty cycle limit.

Features

■

Functional Input Voltage Range: 8V to 33V

■

Micro SMD package

■

Input Over-Voltage Shutdown at ≊35V

■

Transient Capability to 50V

■

Integrated N-Channel buck switch

■

Valley current limit varies with VIN and V

OUT

to reduce

excessive inductor current

■

On-time is reduced when in current limit

■

Integrated start-up regulator

■

No loop compensation required

■

Ultra-Fast transient response

■

Maximum switching frequency: 2 MHz

■

Operating frequency remains nearly constant with load current and input voltage variations

■

Programmable soft-start

■

Precision internal reference

■

Adjustable output voltage

■

Thermal shutdown

Typical Applications

■

High Efficiency Point-Of-Load (POL) Regulator

■

Non-Isolated Buck Regulator

■

Secondary High Voltage Post Regulator

Package

■

micro SMD Package

Basic Step Down Regulator

20216601

LM34917A Ultra Small 33V, 1.25A Constant On-Time Buck Switching Regulator with Intelligent

Current Limit

Connection Diagrams

20216629

Bump Side

20216630

Top View

Ordering Information

Order Number Package Type NSC Package

Drawing

Junction Temperature Range Supplied As

LM34917ATL 12-Bump micro

SMD

TLA12UNA −40°C to + 125°C 250 Units Tape and Reel

LM34917ATLX 3k Units Tape and Reel

Pin Descriptions

Pin Number Name Description Application Information

A1 SGND Sense Ground Re-circulating current flows into this pin to the current sense

resistor.

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 ISEN Current sense The re-circulating current flows out of this pin to the free-

wheeling diode.

B2 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.

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

2.5V, providing the softstart function.

C1,C2 VIN Input supply voltage Operating input range is 8.0V to 33V, with over-voltage

shutdown internally set at ≊35V. Transient capability is 50V.

C3 VCC Output from the startup regulator Nominally regulated at 7.0V. Connect a 0.1 µF capacitor

from this pin to RTN. An external voltage (8V to 14V) can be applied to this pin to reduce internal dissipation. An internal diode connects VCC to VIN.

D1,D2 SW Switching Node Internally connected to the buck switch source. Connect to

the inductor, 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 each off-time via an internal diode.

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LM34917A

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 50V BST to RTN 64V SW to RTN (Steady State) -1.5V BST to VCC 50V VIN to SW 50V BST to SW 14V VCC to RTN 14V SGND to RTN -0.3V to +0.3V

Current out of ISEN See text SS to RTN -0.3V to 4V All Other Inputs to RTN -0.3 to 7V ESD Rating (Note 2) Human Body Model 2kV Storage Temperature Range -65°C to +150°C Junction Temperature 150°C

Operating Ratings (Note 1)

VIN Voltage

8.0V to 33V

Junction Temperature −40°C to + 125°C

Electrical Characteristics Limits in standard type are for T

= 25°C only; limits in boldface 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. Unless otherwise stated the following conditions apply: VIN = 12V, RON = 200kΩ. See (Note 4) and (Note 5).

Symbol Parameter Conditions Min Typ Max Units

Start-Up Regulator, V

VCCReg VCC regulated output Vin > 9V 6.6 7.0 7.4 V

VIN-VCC dropout voltage I

= 0 mA,

VCC = UVLO

VCC

+ 250 mV

1.3 V

VCC output impedance

(0 mA ≤ ICC ≤ 5 mA)

VIN = 8V 150

Ω

VIN = 12V 0.75

VCC current limit (Note 3) VCC = 0V 11 mA

UVLO

VCC

VCC under-voltage lockout threshold

VCC increasing 5.45 V

UVLO

VCC

hysteresis VCC decreasing 145 mV

UVLO

VCC

filter delay 100 mV overdrive 3 µs

IIN operating current Non-switching, FB = 3V 0.68 0.95 mA

IIN shutdown current RON/SD = 0V 85 160 µA

Switch Characteristics

Rds(on) Buck Switch Rds(on) I

TEST

= 200 mA 0.33 0.7

Ω

UVLO

Gate Drive UVLO V

BST

- VSW Increasing 2.65 4 4.62 V

UVLOGD hysteresis 450 mV

Softstart Pin

Pull-up voltage 2.5 V

Internal current source 11.6 µA

RES

Restart threshold after OVP shutdown

0.18 V

Current Limit

LIM

Threshold VIN = 8V, VFB = 2.4V 1.15 1.35 1.55 A

VIN = 30V, VFB = 2.4V 1.05 1.2 1.45

VIN = 30V, VFB = 1.0V 0.95 1.15 1.35

Response time 150 ns

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LM34917A

Symbol Parameter Conditions Min Typ Max Units

On Timer

tON - 1 On-time (normal operation)

VIN = 10V, RON = 200 kΩ

2.1 2.8 3.5 µs

tON - 2 On-time (normal operation)

VIN = 32V, RON = 200 kΩ

860 ns

tON - 3 On-time (current limit)

VIN = 10V, RON = 200 kΩ

1.13 µs

Shutdown threshold at RON/SDVoltage at RON/SD rising 0.3 0.65 1.0 V

Shutdown Threshold

hysteresis

Voltage at RON/SD falling 40 mV

Off Timer

OFF

Minimum Off-time 90 ns

Regulation and Over-Voltage Comparators (FB Pin)

REF

FB regulation threshold SS pin = steady state 2.445 2.50 2.550 V

FB over-voltage threshold 2.9 V

FB bias current FB = 3V 10 nA

Input Over-Voltage Shutdown

IN(OV)

Shutdown voltage threshold at VIN

VIN increasing 33.0 34.8 36.9 V

Thermal Shutdown

Thermal shutdown temperature Junction temperature rising 175 °C

Thermal shutdown hysteresis 20 °C

Thermal Resistance

Junction to Ambient 0 LFPM Air Flow

58 °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 packages, refer to AN-1112 available from National Semiconductor Corporation.

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

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LM34917A

Typical Performance Characteristics

Unless otherwise specified the following conditions apply: TJ = 25°C

Efficiency at 1.5 MHz

20216645

Efficiency at 2 MHz

20216646

VCC vs. V

20216603

ON-Time vs. VIN and R

20216604

Valley Current Limit Threshold vs. VFB and V

20216606

Voltage at the RON/SD Pin

20216608

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