National Semiconductor LM2734 Technical data

LM2734
Thin SOT23 1A Load Step-Down DC-DC Regulator

LM2734 Thin SOT23 1A Load Step-Down DC-DC Regulator

June 4, 2008

General Description

The LM2734 regulator is a monolithic, high frequency, PWM
step-down DC/DC converter in a 6-pin Thin SOT23 package.
It provides all the active functions to provide local DC/DC
conversion with fast transient response and accurate regula­tion in the smallest possible PCB area.

With a minimum of external components and online design
support through WEBENCH®™, the LM2734 is easy to use.
The ability to drive 1A loads with an internal 300mΩ NMOS
switch using state-of-the-art 0.5µm BiCMOS technology re­sults in the best power density available. The world class
control circuitry allows for on-times as low as 13ns, thus sup­porting exceptionally high frequency conversion over the en­tire 3V to 20V input operating range down to the minimum
output voltage of 0.8V. Switching frequency is internally set
to 550kHz (LM2734Y) or 1.6MHz (LM2734X), allowing the
use of extremely small surface mount inductors and chip ca­pacitors. Even though the operating frequencies are very
high, efficiencies up to 90% are easy to achieve. External
shutdown is included, featuring an ultra-low stand-by current
of 30nA. The LM2734 utilizes current-mode control and inter­nal compensation to provide high-performance regulation
over a wide range of operating conditions. Additional features
include internal soft-start circuitry to reduce inrush current,
pulse-by-pulse current limit, thermal shutdown, and output
over-voltage protection.

Features

Thin SOT23-6 package

■

3.0V to 20V input voltage range

■

0.8V to 18V output voltage range

■

1A output current

■

550kHz (LM2734Y) and 1.6MHz (LM2734X)

■

switching frequencies
300mΩ NMOS switch

■

30nA shutdown current

■

0.8V, 2% internal voltage reference

■

Internal soft-start

■

Current-Mode, PWM operation

■

WEBENCH® online design tool

■

Thermal shutdown

■

LM2734XQ/LM2734YQ are AEC-Q100 Grade 1 qualified

■

and are manufactured on an Automotive Grade Flow

Applications

Local Point of Load Regulation

■

Core Power in HDDs

■

Set-Top Boxes

■

Battery Powered Devices

■

USB Powered Devices

■

DSL Modems

■

Notebook Computers

■

Automotive

■

Typical Application Circuit

Efficiency vs Load Current

VIN = 5V, V

20102301

WEBENCH™ is a trademark of Transim.

© 2008 National Semiconductor Corporation 201023 www.national.com

OUT

= 3.3V

20102345

Connection Diagrams

LM2734

6-Lead TSOT

20102305

Pin 1 Indentification

20102360

NS Package Number MK06A

Ordering Information

Order Number Package Type NSC Package

Drawing

LM2734XMK

LM2734XMKX SFDB 3000 Units on Tape and Reel

LM2734XQMKE SUKB 250 Units on Tape and Reel AEC-Q100 Grade 1

LM2734XQMK SUKB 1000 Units on Tape and Reel

LM2734XQMKX SUKB 3000 Units on Tape and Reel

LM2734YMK SFEB 1000 Units on Tape and Reel

TSOT-6 MK06A

LM2734YMKX SFEB 3000 Units on Tape and Reel

LM2734YQMKE SVCB 250 Units on Tape and Reel AEC-Q10-0 Grade 1

LM2734YQMK SVCB 1000 Units on Tape and Reel

LM2734YQMKX SVCB 3000 Units on Tape and Reel

*Automotive Grade (Q) product incorporates enhanced manufacturing and support processes for the automotive market, including defect detection methodologies.
Reliability qualification is compliant with the requirements and temperature grades defined in the AEC-Q100 standard. Automotive grade products are identified
with the letter Q. For more information go to http://www.national.com/automotive.

Package

Supplied As Features

Marking

SFDB 1000 Units on Tape and Reel

Qualified. Automotive

Grade Production Flow*

Qualified. Automotive

Grade Production Flow*

Pin Descriptions

Pin Name Function

1 BOOST Boost voltage that drives the internal NMOS control switch. A

bootstrap capacitor is connected between the BOOST and SW pins.

2 GND Signal and Power ground pin. Place the bottom resistor of the

feedback network as close as possible to this pin for accurate
regulation.

3 FB Feedback pin. Connect FB to the external resistor divider to set output

voltage.

4 EN Enable control input. Logic high enables operation. Do not allow this

pin to float or be greater than V

5 V

IN

Input supply voltage. Connect a bypass capacitor to this pin.

6 SW Output switch. Connects to the inductor, catch diode, and bootstrap

capacitor.

+ 0.3V.

IN

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LM2734

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.

V

IN

SW Voltage -0.5V to 24V
Boost Voltage -0.5V to 30V
Boost to SW Voltage -0.5V to 6.0V
FB Voltage -0.5V to 3.0V
EN Voltage -0.5V to (VIN + 0.3V)

Junction Temperature 150°C
ESD Susceptibility (Note 2) 2kV

-0.5V to 24V

Storage Temp. Range -65°C to 150°C
Soldering Information
Infrared/Convection Reflow (15sec) 220°C
Wave Soldering Lead Temp. (10sec) 260°C

Operating Ratings (Note 1)

V

IN

SW Voltage -0.5V to 20V
Boost Voltage -0.5V to 25V
Boost to SW Voltage 1.6V to 5.5V
Junction Temperature Range −40°C to +125°C

Thermal Resistance θJA (Note 3)

3V to 20V

118°C/W

Electrical Characteristics

Specifications with standard typeface are for TJ = 25°C, and those in boldface type apply over the full Operating Temperature
Range (TJ = -40°C to 125°C). VIN = 5V, V

guaranteed by design, test, or statistical analysis.

Symbol Parameter Conditions

V

ΔVFB/ΔV

I

FB

Feedback Voltage

FB

Feedback Voltage Line Regulation

IN

Feedback Input Bias Current

Undervoltage Lockout

UVLO

Undervoltage Lockout

UVLO Hysteresis 0.30 0.44 0.62

F

D

D

R

DS(ON)

SW

MAX

MIN

I

CL

I

Switching Frequency

Maximum Duty Cycle

Minimum Duty Cycle

Switch ON Resistance V

Switch Current Limit V

Quiescent Current Switching 1.5 2.5 mA

Q

Quiescent Current (shutdown) VEN = 0V

I

BOOST

V

EN_TH

I

EN

I

SW

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

Note 2: Human body model, 1.5kΩ in series with 100pF.

Note 3: Thermal shutdown will occur if the junction temperature exceeds 165°C. The maximum power dissipation is a function of T

maximum allowable power dissipation at any ambient temperature is PD = (T
board with 2oz. copper on 4 layers in still air. For a 2 layer board using 1 oz. copper in still air, θJA = 204°C/W.

Note 4: Guaranteed to National’s Average Outgoing Quality Level (AOQL).

Note 5: Typicals represent the most likely parametric norm.

Boost Pin Current

Shutdown Threshold Voltage VEN Falling

Enable Threshold Voltage VEN Rising 1.8

Enable Pin Current Sink/Source

Switch Leakage

- VSW = 5V unless otherwise specified. Datasheet min/max specification limits are

BOOST

VIN = 3V to 20V

Sink/Source

VIN Rising

VIN Falling

Min

(Note 4)

0.784 0.800 0.816 V

0.01 % / V

10 250 nA

2.74 2.90

2.0 2.3

Typ

(Note 5)

Max

(Note 4)

LM2734X 1.2 1.6 1.9

LM2734Y 0.40 0.55 0.66

LM2734X 85 92

LM2734Y 90 96

LM2734X 2

LM2734Y 1

- VSW = 3V 300 600

BOOST

- VSW = 3V 1.2 1.7 2.5 A

BOOST

30

LM2734X (50% Duty Cycle) 2.5 3.5

LM2734Y (50% Duty Cycle) 1.0 1.8

0.4

– TA)/θJA . All numbers apply for packages soldered directly onto a 3” x 3” PC

J(MAX)

10

40

, θJA and TA . The

J(MAX)

Units

V

MHz

%

%

mΩ

nA

mA

V

nA

nA

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Typical Performance Characteristics All curves taken at V

L1 = 10 µH ("Y"), and TA = 25°C, unless specified otherwise.

LM2734

Efficiency vs Load Current - "X" V

OUT

= 5V

= 5V, V

IN

- VSW = 5V, L1 = 4.7 µH ("X"),

BOOST

Efficiency vs Load Current - "Y" V

OUT

= 5V

Efficiency vs Load Current - "X" V

Efficiency vs Load Current - "X" V

OUT

OUT

20102336

= 3.3V

20102351

= 1.5V

Efficiency vs Load Current - "Y" V

Efficiency vs Load Current - "Y" V

OUT

OUT

20102334

= 3.3V

20102352

= 1.5V

20102337

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20102335

LM2734

Oscillator Frequency vs Temperature - "X"

20102327

Current Limit vs Temperature

VIN = 5V

Oscillator Frequency vs Temperature - "Y"

20102328

Current Limit vs Temperature

VIN = 20V

VFB vs Temperature

20102329

20102333

20102347

R

vs Temperature

DSON

20102330

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LM2734

IQ Switching vs Temperature

Line Regulation - "X"

V

OUT

= 1.5V, I

= 500mA

OUT

Line Regulation - "Y"

V

OUT

= 1.5V, I

= 500mA

OUT

Line Regulation - "Y"

V

OUT

= 3.3V, I

= 500mA

OUT

20102346

20102354

Line Regulation - "X"

V

OUT

= 3.3V, I

= 500mA

OUT

20102356

20102355

20102353

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Block Diagram

LM2734

Application Information

THEORY OF OPERATION

The LM2734 is a constant frequency PWM buck regulator IC
that delivers a 1A load current. The regulator has a preset
switching frequency of either 550kHz (LM2734Y) or 1.6MHz
(LM2734X). These high frequencies allow the LM2734 to op­erate with small surface mount capacitors and inductors,
resulting in DC/DC converters that require a minimum amount
of board space. The LM2734 is internally compensated, so it
is simple to use, and requires few external components. The
LM2734 uses current-mode control to regulate the output
voltage.

The following operating description of the LM2734 will refer
to the Simplified Block Diagram (Figure 1) and to the wave­forms in Figure 2. The LM2734 supplies a regulated output
voltage by switching the internal NMOS control switch at con­stant frequency and variable duty cycle. A switching cycle
begins at the falling edge of the reset pulse generated by the
internal oscillator. When this pulse goes low, the output con­trol logic turns on the internal NMOS control switch. During
this on-time, the SW pin voltage (VSW) swings up to approxi­mately VIN, and the inductor current (IL) increases with a linear
slope. IL is measured by the current-sense amplifier, which
generates an output proportional to the switch current. The
sense signal is summed with the regulator’s corrective ramp
and compared to the error amplifier’s output, which is propor­tional to the difference between the feedback voltage and
V

. When the PWM comparator output goes high, the out-

REF

put switch turns off until the next switching cycle begins.
During the switch off-time, inductor current discharges
through Schottky diode D1, which forces the SW pin to swing
below ground by the forward voltage (VD) of the catch diode.

FIGURE 1.

20102306

The regulator loop adjusts the duty cycle (D) to maintain a
constant output voltage.

20102307

FIGURE 2. LM2734 Waveforms of SW Pin Voltage and

Inductor Current

BOOST FUNCTION

Capacitor C
erate a voltage V
to the internal NMOS control switch. To properly drive the in­ternal NMOS switch during its on-time, V
least 1.6V greater than VSW. Although the LM2734 will oper-

and diode D2 in Figure 3 are used to gen-

BOOST

BOOST

. V

- VSW is the gate drive voltage

BOOST

needs to be at

BOOST

ate with this minimum voltage, it may not have sufficient gate
drive to supply large values of output current. Therefore, it is
recommended that V

be greater than 2.5V above V

BOOST

SW

7 www.national.com

for best efficiency. V
imum operating limit of 5.5V.

LM2734

5.5V > V

– VSW > 2.5V for best performance.

BOOST

FIGURE 3. V

– VSW should not exceed the max-

BOOST

Charges C

OUT

BOOST

When the LM2734 starts up, internal circuitry from the
BOOST pin supplies a maximum of 20mA to C
current charges C
switch on. The BOOST pin will continue to source current to
C

until the voltage at the feedback pin is greater than

BOOST

0.76V.
There are various methods to derive V

1.

From the input voltage (VIN)

2.

From the output voltage (V

3.

From an external distributed voltage rail (V

4.

From a shunt or series zener diode

to a voltage sufficient to turn the

BOOST

:

BOOST

)

OUT

In the Simplifed Block Diagram of Figure 1, capacitor
C

and diode D2 supply the gate-drive current for the

BOOST

NMOS switch. Capacitor C
VIN. During a normal switching cycle, when the internal NMOS
control switch is off (T
VIN minus the forward voltage of D2 (V

OFF

current in the inductor (L) forward biases the Schottky diode
D1 (V

). Therefore the voltage stored across C

FD1

V

- VSW = VIN - V

BOOST

is charged via diode D2 by

BOOST

) (refer to Figure 2), V

), during which the

FD2

+ V

FD2

FD1

When the NMOS switch turns on (TON), the switch pin rises
to

forcing V
V

BOOST

VSW = VIN – (R

to rise thus reverse biasing D2. The voltage at

BOOST

is then

V

= 2VIN – (R

BOOST

DSON

x IL),

DSON

x IL) – V

FD2

+ V

which is approximately

2VIN - 0.4V

for many applications. Thus the gate-drive voltage of the
NMOS switch is approximately

VIN - 0.2V

An alternate method for charging C
the output as shown in Figure 3. The output voltage should

is to connect D2 to

BOOST

be between 2.5V and 5.5V, so that proper gate voltage will be
applied to the internal switch. In this circuit, C
a gate drive voltage that is slightly less than V

In applications where both VIN and V

5.5V, or less than 3V, C
these voltages. If VIN and V
C

can be charged from VIN or V

BOOST

age by placing a zener diode D3 in series with D2, as shown

cannot be charged directly from

BOOST

are greater than 5.5V,

OUT

OUT

BOOST

OUT

are greater than

OUT

minus a zener volt-

in Figure 4. When using a series zener diode from the input,
ensure that the regulation of the input supply doesn’t create
a voltage that falls outside the recommended V

BOOST

BOOST

)

EXT

BOOST

BOOST

FD1

.

20102308

. This

equals

is

provides

voltage.

(V

– VD3) < 5.5V

INMAX

(V

– VD3) > 1.6V

INMIN

20102309

FIGURE 4. Zener Reduces Boost Voltage from V

IN

An alternative method is to place the zener diode D3 in a
shunt configuration as shown in Figure 5. A small 350mW to
500mW 5.1V zener in a SOT-23 or SOD package can be used
for this purpose. A small ceramic capacitor such as a 6.3V,

0.1µF capacitor (C4) should be placed in parallel with the
zener diode. When the internal NMOS switch turns on, a pulse
of current is drawn to charge the internal NMOS gate capac­itance. The 0.1 µF parallel shunt capacitor ensures that the
V

voltage is maintained during this time.

BOOST

Resistor R3 should be chosen to provide enough RMS current
to the zener diode (D3) and to the BOOST pin. A recom­mended choice for the zener current (I
current I
of the NMOS control switch and varies typically according to

into the BOOST pin supplies the gate current

BOOST

) is 1 mA. The

ZENER

the following formula for the X version:

I

= 0.56 x (D + 0.54) x (V

BOOST

I

can be calculated for the Y version using the following:

BOOST

I

= 0.22 x (D + 0.54) x (V

BOOST

where D is the duty cycle, V
I

is in milliamps. V

BOOST

anode of the boost diode (D2), and VD2 is the average forward

ZENER

and VD2 are in volts, and

ZENER

is the voltage applied to the

voltage across D2. Note that this formula for I
ical current. For the worst case I
by 40%. In that case, the worst case boost current will be

I

BOOST-MAX

BOOST

= 1.4 x I

– VD2) mA

ZENER

- VD2) µA

ZENER

gives typ-

BOOST

, increase the current

BOOST

R3 will then be given by

R3 = (VIN - V

For example, using the X-version let VIN = 10V, V
VD2 = 0.7V, I

I

BOOST

= 1mA, and duty cycle D = 50%. Then

ZENER

= 0.56 x (0.5 + 0.54) x (5 - 0.7) mA = 2.5mA

ZENER

) / (1.4 x I

BOOST

+ I

ZENER

ZENER

)

= 5V,

R3 = (10V - 5V) / (1.4 x 2.5mA + 1mA) = 1.11kΩ

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