National Semiconductor LM2738 Technical data

LM2738
550kHz/1.6MHz 1.5A Step-Down DC-DC Switching
Regulator

LM2738 550kHz/1.6MHz 1.5A Step-Down DC-DC Switching Regulator

April 10, 2008

General Description

The LM2738 regulator is a monolithic, high frequency, PWM
step-down DC/DC converter in an 8-pin LLP or 8-pin eMSOP
package. It provides all the active functions for local DC/DC
conversion with fast transient response and accurate regula­tion in the smallest possible PCB area.

With a minimum of external components, the LM2738 is easy
to use. The ability to drive 1.5A loads with an internal
250mΩ NMOS switch using state-of-the-art 0.5µm BiCMOS
technology results in the best power density available.
Switching frequency is internally set to 550kHz (LM2738Y) or

1.6MHz (LM2738X), 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 enable is included, featuring an
ultra-low stand-by current of 400nA. The LM2738 utilizes cur­rent-mode control and internal compensation to provide high­performance regulation over a wide range of operating
conditions. Additional features include internal soft-start cir­cuitry to reduce in-rush current, cycle-by-cycle current limit,
thermal shutdown, and output over-voltage protection.

Typical Application Circuit

Features

Space Saving LLP-8 and eMSOP-8 package

■

3.0V to 20V input voltage range

■

0.8V to 18V output voltage range

■

1.5A output current

■

550kHz (LM2738Y) and 1.6MHz (LM2738X)

■

switching frequencies
250mΩ NMOS switch

■

400nA shutdown current

■

0.8V, 2% internal voltage reference

■

Internal soft-start

■

Current-Mode, PWM operation

■

Thermal shutdown

■

Applications

Local Point of Load Regulation

■

Core Power in HDDs

■

Set-Top Boxes

■

Battery Powered Devices

■

USB Powered Devices

■

DSL Modems

■

Efficiency vs Load Current

VIN = 12V, V

30049101

© 2008 National Semiconductor Corporation 300491 www.national.com

OUT

= 3.3V

30049145

Connection Diagrams

LM2738

8-Pin LLP - TOP VIEW

NS Package Number SDA08A

30049161

8-Pin eMSOP - TOP VIEW

NS Package Number MUY08A

30049163

Ordering Information

Order Number Frequency

Option

LM2738XSD

LM2738XSDX 4500 Tape and Reel

LM2738YSD

LM2738YSDX 4500 Tape and Reel

LM2738XMY

LM2738XMYX 3500 Tape and Reel

LM2738YMY

LM2738YMYX 3500 Tape and Reel

* Contact the local sales office for the lead-free package.

1.6MHz

0.55MHz L174B

1.6MHz

0.55MHz SJBB

Package Type NSC Package

Drawing

8-Lead LLP SDA08A

8-Lead eMSOP MUY08A

Package Marking Supplied As

L237B

1000 Tape and Reel

1000 Tape and Reel

STDB

1000 Tape and Reel

1000 Tape and Reel

Pin Descriptions

Pin Name Function

1 BOOST

2

3

V

IN

V

CC

4 EN

5, 7 GND

6

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

8 SW

DAP GND Signal and power ground. Must be connected to GND on the PCB.

Boost voltage that drives the internal NMOS control switch. A
bootstrap capacitor is connected between the BOOST and SW pins.

Supply voltage for output power stage. Connect a bypass capacitor
to this pin. Must tie pins 2 and 3 together at package.

Input supply voltage of the IC. Connect a bypass capacitor to this pin.
Must tie pin 2 and 3 together at the package.

Enable control input. Logic high enables operation. Do not allow this
pin to float or be greater than V

+ 0.3V.

IN

Signal and power ground pins. Place the bottom resistor of the
feedback network as close as possible to these pins.

voltage.

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

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LM2738

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/

Soldering Information
Infrared/Convection Reflow (15sec) 220°C
Wave Soldering Lead Temp. (10sec) 260°C

Distributors for availability and specifications.

VIN, V

CC

-0.5V to 24V
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

Operating Ratings (Note 1)

VIN, V

CC

SW Voltage -0.5V to 20V
Boost Voltage -0.5V to 25.5V
Boost to SW Voltage 2.5V to 5.5V
Junction Temperature Range −40°C to +125°C

Thermal Resistance θJA for LLP/eMSOP(Note 3)

Thermal Shutdown (Note 3) 165°C

3V to 20V

60°C/W

Storage Temp. Range -65°C to 150°C

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 = 12V, V

are guaranteed by design, test, or statistical analysis.

Symbol Parameter Conditions

V

ΔVFB/ΔV

I

Feedback Voltage

FB

Feedback Voltage Line Regulation

IN

Feedback Input Bias Current

FB

Undervoltage Lockout

UVLO

Undervoltage Lockout

UVLO Hysteresis 0.4

F

D

D

R

DS(ON)

MAX

I

I

Switching Frequency

SW

Maximum Duty Cycle

Minimum Duty Cycle

MIN

Switch ON Resistance

Switch Current Limit

CL

Quiescent Current

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

The maximum allowable power dissipation at any ambient temperature is PD = (T
3” PC board with 2 oz. copper on 4 layers in still air in accordance to JEDEC standards. Thermal resistance varies greatly with layout, copper thicknes, number
of layers in PCB, power distribution, number of thermal vias, board size, ambient temperature, and air flow.

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

Enable Pin Current Sink/Source

Switch Leakage VIN = 20V

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

BOOST

VIN = 3V to 20V

Sink/Source

VIN Rising

VIN Falling

Min

(Note 4)

0.784 0.800 0.816 V

0.02 % / V

0.1 100 nA

2.7 2.90

2.0 2.3

Typ

(Note 5)

Max

(Note 4)

LM2738X 1.28 1.6 1.92

LM2738Y 0.364 0.55 0.676

LM2738X , Load=150mA 92

LM2738Y, Load=150mA 95

LM2738X 7.5

LM2738Y 2

V

- VSW = 3V,

BOOST

Load=400mA

V

- VSW = 3V, VIN = 3V

BOOST

250 500

2.0 2.9

Switching 1.9 3 mA

Non-Switching 1.9 mA

400 nA

LM2738X (27% Duty Cycle) 4.5

LM2738Y (27% Duty Cycle) 2.5

- 0.4

1.4 -

10 nA

100 nA

, θJA and TA .

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

J(MAX)

J(MAX)

Units

V

MHz

%

%

mΩ

A

mA

V

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Typical Performance Characteristics All curves taken at V

unless specified otherwise.

LM2738

Efficiency vs Load Current - "X" V

OUT

= 5V

= 12V, V

IN

- VSW = 5V, and TA = 25°C,

BOOST

Efficiency vs Load Current - "Y" V

OUT

= 5V

Efficiency vs Load Current - "X" V

Efficiency vs Load Current - "X" V

OUT

OUT

30049197

= 3.3V

30049151

= 1.5V

Efficiency vs Load Current - "Y" V

Efficiency vs Load Current - "Y" V

OUT

OUT

30049198

= 3.3V

30049152

= 1.5V

30049199

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30049131

LM2738

Typical Performance Characteristics All curves taken at V

unless specified otherwise.

Oscillator Frequency vs Temperature - "X"

30049127

Current Limit vs Temperature

VIN = 5V

Oscillator Frequency vs Temperature - "Y"

IQ Non-Switching vs Temperature

= 12V, V

IN

- VSW = 5V, and TA = 25°C,

BOOST

30049128

VFB vs Temperature

30049129

30049133

30049147

R

vs Temperature

DSON

30049130

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Typical Performance Characteristics All curves taken at V

unless specified otherwise.

= 12V, V

IN

- VSW = 5V, and TA = 25°C,

BOOST

Line Regulation - "X" (V

Line Regulation - "X" (V

= 1.5V, I

OUT

= 3.3V, I

OUT

= 750mA)

OUT

= 750mA)

OUT

30049156

Line Regulation - "Y" (V

Line Regulation - "Y" (V

= 1.5V, I

OUT

= 3.3V, I

OUT

= 750mA)

OUT

= 750mA)

OUT

30049154

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Load Regulation - "X" (V

OUT

= 1.5V)

30049155

30049176

Load Regulation - "Y" (V

OUT

30049153

= 1.5V)

30049175

LM2738

Typical Performance Characteristics All curves taken at V

unless specified otherwise.

Load Regulation - "X" (V

IQ Switching vs Temperature

OUT

= 3.3V)

Load Regulation - "Y" (V

30049177

Load Transient - "X" (V

= 12V, V

IN

- VSW = 5V, and TA = 25°C,

BOOST

= 3.3V)

OUT

= 3.3V, VIN = 12V)

OUT

30049178

(V

= 3.3V, VIN = 12, I

OUT

Startup - "X"

=1.5A (Resistive Load))

OUT

30049146

30049190

30049194

In-Rush Current - "X"

(V

= 3.3V, VIN = 12V, I

OUT

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=1.5A (Resistive Load) )

OUT

30049191

Block Diagram

LM2738

30049106

FIGURE 1. Simplified Internal Block Diagram

Application Information

THEORY OF OPERATION

The LM2738 is a constant frequency PWM buck regulator IC
that delivers a 1.5A load current. The regulator has a preset
switching frequency of either 550kHz (LM2738Y) or 1.6MHz
(LM2738X). These high frequencies allow the LM2738 to op­erate with small surface mount capacitors and inductors,
resulting in DC/DC converters that require a minimum amount
of board space. The LM2738 is internally compensated, so it
is simple to use, and requires few external components. The
LM2738 uses current-mode control to regulate the output
voltage.

The following operating description of the LM2738 will refer
to the Simplified Block Diagram (Figure 1) and to the wave­forms in Figure 2. The LM2738 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.
The regulator loop adjusts the duty cycle (D) to maintain a
constant output voltage.

30049107

FIGURE 2. LM2738 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 2.5V greater than VSW. It is recommended that V
be greater than 2.5V above VSW for best efficiency. V
VSW should not exceed the maximum operating limit of 5.5V.

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

BOOST

BOOST

–

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LM2738

5.5V > V

– VSW > 2.5V for best performance.

BOOST

When the LM2738 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 (3.0V < VIN < 5.5V)

2.

From the output voltage (2.5V < V

3.

From an external distributed voltage rail (2.5V < V

to a voltage sufficient to turn the

BOOST

:

BOOST

< 5.5V)

OUT

5.5V)

4.

From a shunt or series zener diode

BOOST

. This

<

EXT

In the Simplifed Block Diagram of Figure 1, capacitor
C

and diode D2 supply the gate-drive voltage 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

BOOST

BOOST

equals

is

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

FD1

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

BOOST

OUT

provides

.

(V

– VD3) > 2.5V

INMIN

30049109

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

30049148

FIGURE 5. Boost Voltage Supplied from the Shunt Zener

on V

IN

30049108

FIGURE 3. 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

Charges C

OUT

cannot be charged directly from

BOOST

OUT

BOOST

are greater than

OUT

are greater than 5.5V,

minus a zener volt-

OUT

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

(V

– VD3) < 5.5V

INMAX

BOOST

voltage.

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

9 www.national.com

ZENER

) / (1.4 x I

BOOST

+ I

ZENER

)