LM2738YMYEVAL - NATIONAL SEMICONDUCTOR

April 10, 2008

LM2738

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

Regulator

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 regula-

tion 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 sur-

face 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 cur-

rent-mode control and internal compensation to provide high-

performance regulation over a wide range of operating

conditions. Additional features include internal soft-start cir-

cuitry to reduce in-rush current, cycle-by-cycle current limit,

thermal shutdown, and output over-voltage protection.

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

Typical Application Circuit

30049101

Efficiency vs Load Current

VIN = 12V, VOUT = 3.3V

30049145

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

Connection Diagrams

30049161

8-Pin LLP - TOP VIEW

NS Package Number SDA08A

30049163

8-Pin eMSOP - TOP VIEW

NS Package Number MUY08A

Ordering Information

Order Number Frequency

Option

Package Type NSC Package

Drawing

Package Marking Supplied As

LM2738XSD 1.6MHz

8-Lead LLP SDA08A

L237B 1000 Tape and Reel

LM2738XSDX 4500 Tape and Reel

LM2738YSD 0.55MHz L174B 1000 Tape and Reel

LM2738YSDX 4500 Tape and Reel

LM2738XMY 1.6MHz

8-Lead eMSOP MUY08A

STDB 1000 Tape and Reel

LM2738XMYX 3500 Tape and Reel

LM2738YMY 0.55MHz SJBB 1000 Tape and Reel

LM2738YMYX 3500 Tape and Reel

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

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.

2VIN

Supply voltage for output power stage. Connect a bypass capacitor

to this pin. Must tie pins 2 and 3 together at package.

3VCC

Input supply voltage of the IC. Connect a bypass capacitor to this pin.

Must tie pin 2 and 3 together at the package.

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

pin to float or be greater than VIN + 0.3V.

5, 7 GND Signal and power ground pins. Place the bottom resistor of the

feedback network as close as possible to these pins.

6FB Feedback pin. Connect FB to the external resistor divider to set output

voltage.

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

capacitor.

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

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LM2738

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, VCC -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

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)

VIN, VCC 3V to 20V

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) 60°C/W

Thermal Shutdown (Note 3) 165°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, VBOOST - VSW = 5V unless otherwise specified. Datasheet min/max specification limits

are guaranteed by design, test, or statistical analysis.

Symbol Parameter Conditions Min

(Note 4)

Typ

(Note 5)

Max

(Note 4) Units

VFB Feedback Voltage 0.784 0.800 0.816 V

ΔVFB/ΔVIN Feedback Voltage Line Regulation VIN = 3V to 20V 0.02 % / V

IFB Feedback Input Bias Current Sink/Source 0.1 100 nA

UVLO

Undervoltage Lockout VIN Rising 2.7 2.90

Undervoltage Lockout VIN Falling 2.0 2.3

UVLO Hysteresis 0.4

FSW Switching Frequency LM2738X 1.28 1.6 1.92 MHz

LM2738Y 0.364 0.55 0.676

DMAX Maximum Duty Cycle LM2738X , Load=150mA 92 %

LM2738Y, Load=150mA 95

DMIN Minimum Duty Cycle LM2738X 7.5 %

LM2738Y 2

RDS(ON) Switch ON Resistance VBOOST - VSW = 3V,

Load=400mA

250 500 mΩ

ICL Switch Current Limit VBOOST - VSW = 3V, VIN = 3V 2.0 2.9 A

Quiescent Current Switching 1.9 3mA

Non-Switching 1.9 mA

Quiescent Current (shutdown) VEN = 0V 400 nA

IBOOST Boost Pin Current LM2738X (27% Duty Cycle) 4.5 mA

LM2738Y (27% Duty Cycle) 2.5

VEN_TH

Shutdown Threshold Voltage VEN Falling - 0.4 V

Enable Threshold Voltage VEN Rising 1.4 -

IEN Enable Pin Current Sink/Source 10 nA

ISW Switch Leakage VIN = 20V 100 nA

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 TJ(MAX) , θJA and TA .

The maximum allowable power dissipation at any ambient temperature is PD = (TJ(MAX) – TA)/θJA . All numbers apply for packages soldered directly onto a 3” x

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.

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LM2738

Typical Performance Characteristics All curves taken at VIN = 12V, VBOOST - VSW = 5V, and TA = 25°C,

unless specified otherwise.

Efficiency vs Load Current - "X" VOUT = 5V

30049197

Efficiency vs Load Current - "Y" VOUT = 5V

30049198

Efficiency vs Load Current - "X" VOUT = 3.3V

30049151

Efficiency vs Load Current - "Y" VOUT = 3.3V

30049152

Efficiency vs Load Current - "X" VOUT = 1.5V

30049199

Efficiency vs Load Current - "Y" VOUT = 1.5V

30049131

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LM2738

Typical Performance Characteristics All curves taken at VIN = 12V, VBOOST - VSW = 5V, and TA = 25°C,

unless specified otherwise.

Oscillator Frequency vs Temperature - "X"

30049127

Oscillator Frequency vs Temperature - "Y"

30049128

Current Limit vs Temperature

VIN = 5V

30049129

IQ Non-Switching vs Temperature

30049147

VFB vs Temperature

30049133

RDSON vs Temperature

30049130

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LM2738

Typical Performance Characteristics All curves taken at VIN = 12V, VBOOST - VSW = 5V, and TA = 25°C,

unless specified otherwise.

Line Regulation - "X" (VOUT = 1.5V, IOUT = 750mA)

30049156

Line Regulation - "Y" (VOUT = 1.5V, IOUT = 750mA)

30049154

Line Regulation - "X" (VOUT = 3.3V, IOUT = 750mA)

30049155

Line Regulation - "Y" (VOUT = 3.3V, IOUT = 750mA)

30049153

Load Regulation - "X" (VOUT = 1.5V)

30049176

Load Regulation - "Y" (VOUT = 1.5V)

30049175

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LM2738

Typical Performance Characteristics All curves taken at VIN = 12V, VBOOST - VSW = 5V, and TA = 25°C,

unless specified otherwise.

Load Regulation - "X" (VOUT = 3.3V)

30049177

Load Regulation - "Y" (VOUT = 3.3V)

30049178

IQ Switching vs Temperature

30049146

Load Transient - "X" (VOUT = 3.3V, VIN = 12V)

30049194

Startup - "X"

(VOUT = 3.3V, VIN = 12, IOUT=1.5A (Resistive Load))

30049190

In-Rush Current - "X"

(VOUT = 3.3V, VIN = 12V, IOUT=1.5A (Resistive Load) )

30049191

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LM2738

Block Diagram

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 op-

erate 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 wave-

forms in Figure 2. The LM2738 supplies a regulated output

voltage by switching the internal NMOS control switch at con-

stant 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 con-

trol logic turns on the internal NMOS control switch. During

this on-time, the SW pin voltage (VSW) swings up to approxi-

mately 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 propor-

tional to the difference between the feedback voltage and

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

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 CBOOST and diode D2 in Figure 3 are used to gen-

erate a voltage VBOOST. VBOOST - VSW is the gate drive voltage

to the internal NMOS control switch. To properly drive the in-

ternal NMOS switch during its on-time, VBOOST needs to be at

least 2.5V greater than VSW. It is recommended that VBOOST

be greater than 2.5V above VSW for best efficiency. VBOOST –

VSW should not exceed the maximum operating limit of 5.5V.

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LM2738

5.5V > VBOOST – VSW > 2.5V for best performance.

When the LM2738 starts up, internal circuitry from the

BOOST pin supplies a maximum of 20mA to CBOOST. This

current charges CBOOST to a voltage sufficient to turn the

switch on. The BOOST pin will continue to source current to

CBOOST until the voltage at the feedback pin is greater than

0.76V.

There are various methods to derive VBOOST:

1. From the input voltage (3.0V < VIN < 5.5V)

2. From the output voltage (2.5V < VOUT < 5.5V)

3. From an external distributed voltage rail (2.5V < VEXT <

5.5V)

4. From a shunt or series zener diode

In the Simplifed Block Diagram of Figure 1, capacitor

CBOOST and diode D2 supply the gate-drive voltage for the

NMOS switch. Capacitor CBOOST is charged via diode D2 by

VIN. During a normal switching cycle, when the internal NMOS

control switch is off (TOFF) (refer to Figure 2), VBOOST equals

VIN minus the forward voltage of D2 (VFD2), during which the

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

D1 (VFD1). Therefore the voltage stored across CBOOST is

VBOOST - VSW = VIN - VFD2 + VFD1

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

VSW = VIN – (RDSON x IL),

forcing VBOOST to rise thus reverse biasing D2. The voltage at

VBOOST is then

VBOOST = 2VIN – (RDSON x IL) – VFD2 + VFD1

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 CBOOST is to connect D2 to

the output as shown in Figure 3. The output voltage should

be between 2.5V and 5.5V, so that proper gate voltage will be

applied to the internal switch. In this circuit, CBOOST provides

a gate drive voltage that is slightly less than VOUT.

30049108

FIGURE 3. VOUT Charges CBOOST

In applications where both VIN and VOUT are greater than

5.5V, or less than 3V, CBOOST cannot be charged directly from

these voltages. If VIN and VOUT are greater than 5.5V,

CBOOST can be charged from VIN or VOUT minus a zener volt-

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

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 VBOOST voltage.

(VINMAX – VD3) < 5.5V

(VINMIN – VD3) > 2.5V

30049109

FIGURE 4. Zener Reduces Boost Voltage from VIN

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 capac-

itance. The 0.1 µF parallel shunt capacitor ensures that the

VBOOST voltage is maintained during this time.

30049148

FIGURE 5. Boost Voltage Supplied from the Shunt Zener

on VIN

Resistor R3 should be chosen to provide enough RMS current

to the zener diode (D3) and to the BOOST pin. A recom-

mended choice for the zener current (IZENER) is 1 mA. The

current IBOOST into the BOOST pin supplies the gate current

of the NMOS control switch and varies typically according to

the following formula for the X version:

IBOOST = 0.56 x (D + 0.54) x (VZENER – VD2) mA

IBOOST can be calculated for the Y version using the following:

IBOOST = 0.22 x (D + 0.54) x (VZENER - VD2) µA

where D is the duty cycle, VZENER and VD2 are in volts, and

IBOOST is in milliamps. VZENER is the voltage applied to the

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

voltage across D2. Note that this formula for IBOOST gives typ-

ical current. For the worst case IBOOST, increase the current

by 40%. In that case, the worst case boost current will be

IBOOST-MAX = 1.4 x IBOOST

R3 will then be given by

R3 = (VIN - VZENER) / (1.4 x IBOOST + IZENER)

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LM2738

For example, using the X-version let VIN = 10V, VZENER = 5V,

VD2 = 0.7V, IZENER = 1mA, and duty cycle D = 50%. Then

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

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

ENABLE PIN / SHUTDOWN MODE

The LM2738 has a shutdown mode that is controlled by the

enable pin (EN). When a logic low voltage is applied to EN,

the part is in shutdown mode and its quiescent current drops

to typically 400nA. The voltage at this pin should never ex-

ceed VIN + 0.3V.

SOFT-START

This function forces VOUT to increase at a controlled rate dur-

ing start up. During soft-start, the error amplifier’s reference

voltage ramps from 0V to its nominal value of 0.8V in approx-

imately 600µs. This forces the regulator output to ramp up in

a more linear and controlled fashion, which helps reduce in

rush current.

OUTPUT OVERVOLTAGE PROTECTION

The overvoltage comparator compares the FB pin voltage to

a voltage that is 16% higher than the internal reference Vref.

Once the FB pin voltage goes 16% above the internal refer-

ence, the internal NMOS control switch is turned off, which

allows the output voltage to decrease toward regulation.

UNDERVOLTAGE LOCKOUT

Undervoltage lockout (UVLO) prevents the LM2738 from op-

erating until the input voltage exceeds 2.7V (typ).

The UVLO threshold has approximately 400mV of hysteresis,

so the part will operate until VIN drops below 2.3V (typ). Hys-

teresis prevents the part from turning off during power up if

the VIN ramp-up is non-monotonic.

CURRENT LIMIT

The LM2738 uses cycle-by-cycle current limiting to protect

the output switch. During each switching cycle, a current limit

comparator detects if the output switch current exceeds 2.9A

(typ), and turns off the switch until the next switching cycle

begins.

THERMAL SHUTDOWN

Thermal shutdown limits total power dissipation by turning off

the output switch when the IC junction temperature exceeds

165°C. After thermal shutdown occurs, the output switch

doesn’t turn on until the junction temperature drops to ap-

proximately 150°C.

Design Guide

INDUCTOR SELECTION

The Duty Cycle (D) can be approximated quickly using the

ratio of output voltage (VO) to input voltage (VIN):

The catch diode (D1) forward voltage drop and the voltage

drop across the internal NMOS switch must be included to

calculate a more accurate duty cycle. Calculate D by using

the following formula:

VSW can be approximated by:

VSW = IOUT x RDSON

The diode forward drop (VD) can range from 0.3V to 0.7V de-

pending on the quality of the diode. The lower the VD, the

higher the operating efficiency of the converter. The inductor

value determines the output ripple current. Lower inductor

values decrease the size of the inductor, but increase the

output ripple current. An increase in the inductor value will

decrease the output ripple current.

One must ensure that the minimum current limit (2.0A) is not

exceeded, so the peak current in the inductor must be calcu-

lated. The peak current (ILPK) in the inductor is calculated by:

ILPK = IOUT + ΔiL

30049180

FIGURE 6. Inductor Current

In general,

ΔiL = 0.1 x (IOUT) → 0.2 x (IOUT)

If ΔiL = 33.3% of 1.50A, the peak current in the inductor will

be 2.0A. The minimum guaranteed current limit over all op-

erating conditions is 2.0A. One can either reduce ΔiL, or make

the engineering judgment that zero margin will be safe

enough. The typical current limit is 2.9A.

The LM2738 operates at frequencies allowing the use of ce-

ramic output capacitors without compromising transient re-

sponse. Ceramic capacitors allow higher inductor ripple

without significantly increasing output ripple. See the output

capacitor section for more details on calculating output volt-

age ripple. Now that the ripple current is determined, the

inductance is calculated by:

Where

When selecting an inductor, make sure that it is capable of

supporting the peak output current without saturating. Induc-

tor saturation will result in a sudden reduction in inductance

and prevent the regulator from operating correctly. Because

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LM2738

of the speed of the internal current limit, the peak current of

the inductor need only be specified for the required maximum

output current. For example, if the designed maximum output

current is 1.0A and the peak current is 1.25A, then the induc-

tor should be specified with a saturation current limit of >

1.25A. There is no need to specify the saturation or peak cur-

rent of the inductor at the 2.9A typical switch current limit.

Because of the operating frequency of the LM2738, ferrite

based inductors are preferred to minimize core losses. This

presents little restriction since the variety of ferrite-based in-

ductors is huge. Lastly, inductors with lower series resistance

(RDCR) will provide better operating efficiency. For recom-

mended inductors see Example Circuits.

INPUT CAPACITOR

An input capacitor is necessary to ensure that VIN does not

drop excessively during switching transients. The primary

specifications of the input capacitor are capacitance, voltage,

RMS current rating, and ESL (Equivalent Series Inductance).

The recommended input capacitance is 10 µF.The input volt-

age rating is specifically stated by the capacitor manufacturer.

Make sure to check any recommended deratings and also

verify if there is any significant change in capacitance at the

operating input voltage and the operating temperature. The

input capacitor maximum RMS input current rating (IRMS-IN)

must be greater than:

Neglecting inductor ripple simplifies the above equation to:

It can be shown from the above equation that maximum RMS

capacitor current occurs when D = 0.5. Always calculate the

RMS at the point where the duty cycle D is closest to 0.5. The

ESL of an input capacitor is usually determined by the effec-

tive cross sectional area of the current path. A large leaded

capacitor will have high ESL and a 0805 ceramic chip capac-

itor will have very low ESL. At the operating frequencies of the

LM2738, leaded capacitors may have an ESL so large that

the resulting impedance (2πfL) will be higher than that re-

quired to provide stable operation. As a result, surface mount

capacitors are strongly recommended.

Sanyo POSCAP, Tantalum or Niobium, Panasonic SP, and

multilayer ceramic capacitors (MLCC) are all good choices for

both input and output capacitors and have very low ESL. For

MLCCs it is recommended to use X7R or X5R type capacitors

due to their tolerance and temperature characteristics. Con-

sult capacitor manufacturer datasheets to see how rated

capacitance varies over operating conditions.

OUTPUT CAPACITOR

The output capacitor is selected based upon the desired out-

put ripple and transient response. The initial current of a load

transient is provided mainly by the output capacitor. The out-

put ripple of the converter is:

When using MLCCs, the ESR is typically so low that the ca-

pacitive ripple may dominate. When this occurs, the output

ripple will be approximately sinusoidal and 90° phase shifted

from the switching action. Given the availability and quality of

MLCCs and the expected output voltage of designs using the

LM2738, there is really no need to review any other capacitor

technologies. Another benefit of ceramic capacitors is their

ability to bypass high frequency noise. A certain amount of

switching edge noise will couple through parasitic capaci-

tances in the inductor to the output. A ceramic capacitor will

bypass this noise while a tantalum will not. Since the output

capacitor is one of the two external components that control

the stability of the regulator control loop, most applications will

require a minimum of 22 µF of output capacitance. Capaci-

tance, in general, is often increased when operating at lower

duty cycles. Refer to the circuit examples at the end of the

datasheet for suggested output capacitances of common ap-

plications. Like the input capacitor, recommended multilayer

ceramic capacitors are X7R or X5R types.

CATCH DIODE

The catch diode (D1) conducts during the switch off-time. A

Schottky diode is recommended for its fast switching times

and low forward voltage drop. The catch diode should be

chosen so that its current rating is greater than:

ID1 = IOUT x (1-D)

The reverse breakdown rating of the diode must be at least

the maximum input voltage plus appropriate margin. To im-

prove efficiency, choose a Schottky diode with a low forward

voltage drop.

OUTPUT VOLTAGE

The output voltage is set using the following equation where

R2 is connected between the FB pin and GND, and R1 is

connected between VO and the FB pin. A good value for R2

is 10k. When designing a unity gain converter (Vo = 0.8V), R1

should be between 0Ω and 100Ω, and R2 should not be load-

ed.

VREF = 0.80V

PCB LAYOUT CONSIDERATIONS

When planning layout there are a few things to consider when

trying to achieve a clean, regulated output. The most impor-

tant consideration is the close coupling of the GND connec-

tions of the input capacitor and the catch diode D1. These

ground ends should be close to one another and be connect-

ed to the GND plane with at least two through-holes. Place

these components as close to the IC as possible. Next in im-

portance is the location of the GND connection of the output

capacitor, which should be near the GND connections of

CIN and D1. There should be a continuous ground plane on

the bottom layer of a two-layer board except under the switch-

ing node island. The FB pin is a high impedance node and

care should be taken to make the FB trace short to avoid noise

pickup and inaccurate regulation. The feedback resistors

should be placed as close as possible to the IC, with the GND

of R1 placed as close as possible to the GND of the IC. The

VOUT trace to R2 should be routed away from the inductor and

any other traces that are switching. High AC currents flow

through the VIN, SW and VOUT traces, so they should be as

short and wide as possible. However, making the traces wide

increases radiated noise, so the designer must make this

trade-off. Radiated noise can be decreased by choosing a

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LM2738

shielded inductor. The remaining components should also be

placed as close as possible to the IC. Please see Application

Note AN-1229 for further considerations and the LM2738 de-

mo board as an example of a four-layer layout.

RECOMMENED OPERATING AREA DUE TO MINIMUM

ON TIME

The LM2738 operates over a wide range of conditions, which

is limited by the ON time of the device. A graph is provided to

show the recommended operating area for the "X" at the full

load (1.5A) and at 25°C ambient. The "Y" version of the

LM2738 operates at a lower frequency and therefore oper-

ates over the entire range of operating voltages.

30049187

FIGURE 7. LM2738X - 1.6MHz (25°C, LOAD=1.5A)

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LM2738

Calculating Efficiency, and Junction

Temperature

The complete LM2738 DC/DC converter efficiency can be

calculated in the following manner.

Calculations for determining the most significant power loss-

es are shown below. Other losses totaling less than 2% are

not discussed.

Power loss (PLOSS) is the sum of two basic types of losses in

the converter: switching and conduction. Conduction losses

usually dominate at higher output loads, whereas switching

losses remain relatively fixed and dominate at lower output

loads. The first step in determining the losses is to calculate

the duty cycle (D):

VSW is the voltage drop across the internal NFET when it is

on, and is equal to:

VSW = IOUT x RDSON

VD is the forward voltage drop across the Schottky catch

diode. It can be obtained from the diode manufactures Elec-

trical Characteristics section. If the voltage drop across the

inductor (VDCR) is accounted for, the equation becomes:

The conduction losses in the free-wheeling Schottky diode

are calculated as follows:

PDIODE = VD x IOUT x (1-D)

Often this is the single most significant power loss in the cir-

cuit. Care should be taken to choose a Schottky diode that

has a low forward voltage drop.

Another significant external power loss is the conduction loss

in the output inductor. The equation can be simplified to:

PIND = IOUT2 x RDCR

The LM2738 conduction loss is mainly associated with the

internal NFET switch:

If the inductor ripple current is fairly small, the conduction

losses can be simplified to:

PCOND = IOUT2 x RDSON x D

Switching losses are also associated with the internal NFET

switch. They occur during the switch on and off transition pe-

riods, where voltages and currents overlap resulting in power

loss. The simplest means to determine this loss is to empiri-

cally measure the rise and fall times (10% to 90%) of the

switch at the switch node.

Switching Power Loss is calculated as follows:

PSWR = 1/2(VIN x IOUT x FSW x TRISE)

PSWF = 1/2(VIN x IOUT x FSW x TFALL)

PSW = PSWR + PSWF

Another loss is the power required for operation of the internal

circuitry:

PQ = IQ x VIN

IQ is the quiescent operating current, and is typically around

1.9mA for the 0.55MHz frequency option.

Typical Application power losses are:

Power Loss Tabulation

VIN 12.0V

VOUT 3.3V POUT 4.125W

IOUT 1.25A

VD0.34V PDIODE 317mW

FSW 550kHz

IQ1.9mA PQ22.8mW

TRISE 8nS PSWR 33mW

TFALL 8nS PSWF 33mW

RDS(ON) 275mΩPCOND 118mW

INDDCR 70mΩPIND 110mW

D 0.275 PLOSS 634mW

η86.7% PINTERNAL 207mW

ΣPCOND + PSW + PDIODE + PIND + PQ = PLOSS

ΣPCOND + PSWF + PSWR + PQ = PINTERNAL

PINTERNAL = 207mW

Thermal Definitions

TJ = Chip junction temperature

TA = Ambient temperature

RθJC = Thermal resistance from chip junction to device case

RθJA = Thermal resistance from chip junction to ambient air

Heat in the LM2738 due to internal power dissipation is re-

moved through conduction and/or convection.

Conduction: Heat transfer occurs through cross sectional ar-

eas of material. Depending on the material, the transfer of

heat can be considered to have poor to good thermal con-

ductivity properties (insulator vs. conductor).

Heat Transfer goes as:

Silicon → package → lead frame → PCB

Convection: Heat transfer is by means of airflow. This could

be from a fan or natural convection. Natural convection occurs

when air currents rise from the hot device to cooler air.

13 www.national.com

LM2738

Thermal impedance is defined as:

Thermal impedance from the silicon junction to the ambient

air is defined as:

The PCB size, weight of copper used to route traces and

ground plane, and number of layers within the PCB can great-

ly effect RθJA. The type and number of thermal vias can also

make a large difference in the thermal impedance. Thermal

vias are necessary in most applications. They conduct heat

from the surface of the PCB to the ground plane. Four to six

thermal vias should be placed under the exposed pad to the

ground plane if the LLP package is used.

Thermal impedance also depends on the thermal properties

due to the application's operating conditions (Vin, Vo, Io etc),

and the surrounding circuitry.

Silicon Junction Temperature Determination Method 1:

To accurately measure the silicon temperature for a given

application, two methods can be used. The first method re-

quires the user to know the thermal impedance of the silicon

junction to top case temperature.

Some clarification needs to be made before we go any further.

RθJC is the thermal impedance from all six sides of an IC

package to silicon junction.

RΦJC is the thermal impedance from top case to the silicon

junction.

In this data sheet we will use RΦJC so that it allows the user

to measure top case temperature with a small thermocouple

attached to the top case.

RΦJC is approximately 30°C/Watt for the 8-pin LLP package

with the exposed pad. Knowing the internal dissipation from

the efficiency calculation given previously, and the case tem-

perature, which can be empirically measured on the bench

we have:

Therefore:

Tj = (RΦJC x PLOSS) + TC

From the previous example:

Tj = (RΦJC x PINTERNAL) + TC

Tj = 30°C/W x 0.207W + TC

The second method can give a very accurate silicon junction

temperature.

The first step is to determine RθJA of the application. The

LM2738 has over-temperature protection circuitry. When the

silicon temperature reaches 165°C, the device stops switch-

ing. The protection circuitry has a hysteresis of about 15°C.

Once the silicon temperature has decreased to approximately

150°C, the device will start to switch again. Knowing this, the

RθJA for any application can be characterized during the early

stages of the design one may calculate the RθJA by placing

the PCB circuit into a thermal chamber. Raise the ambient

temperature in the given working application until the circuit

enters thermal shutdown. If the SW-pin is monitored, it will be

obvious when the internal NFET stops switching, indicating a

junction temperature of 165°C. Knowing the internal power

dissipation from the above methods, the junction tempera-

ture, and the ambient temperature RθJA can be determined.

Once this is determined, the maximum ambient temperature

allowed for a desired junction temperature can be found.

An example of calculating RθJA for an application using the

National Semiconductor LM2738 LLP demonstration board is

shown below.

The four layer PCB is constructed using FR4 with ½ oz copper

traces. The copper ground plane is on the bottom layer. The

ground plane is accessed by two vias. The board measures

3.0cm x 3.0cm. It was placed in an oven with no forced airflow.

The ambient temperature was raised to 144°C, and at that

temperature, the device went into thermal shutdown.

From the previous example:

PINTERNAL = 207mW

If the junction temperature was to be kept below 125°C, then

the ambient temperature could not go above 109°C

Tj - (RθJA x PLOSS) = TA

125°C - (102°C/W x 207mW) = 104°C

LLP Package

30049174

FIGURE 8. Internal LLP Connection

For certain high power applications, the PCB land may be

modified to a "dog bone" shape (see Figure 9). By increasing

the size of ground plane, and adding thermal vias, the RθJA

for the application can be reduced.

www.national.com 14

LM2738

30049179

FIGURE 9. 8-Lead LLP PCB Dog Bone Layout

15 www.national.com

LM2738

LM2738X Circuit Example 1

30049142

FIGURE 10. LM2738X (1.6MHz)

VBOOST Derived from VIN

5V to 1.5V/1.5A

Bill of Materials for Figure 10

Part ID Part Value Part Number Manufacturer

U1 1.5A Buck Regulator LM2738X National Semiconductor

C1, Input Cap 10µF, 6.3V, X5R C3216X5ROJ106M TDK

C2, Output Cap 22µF, 6.3V, X5R C3216X5ROJ226M TDK

C3, Boost Cap 0.1uF, 16V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34VF Schottky 1.5A, 30V CRS08 Toshiba

D2, Boost Diode 1VF @ 100mA Diode BAT54WS Diodes, Inc.

L1 2.2µH, 1.9A, MSS5131-222ML Coilcraft

R1 8.87kΩ, 1% CRCW06038871F Vishay

R2 10.2kΩ, 1% CRCW06031022F Vishay

R3 100kΩ, 1% CRCW06031003F Vishay

www.national.com 16

LM2738

LM2738X Circuit Example 2

30049193

FIGURE 11. LM2738X (1.6MHz)

VBOOST Derived from VOUT

12V to 3.3V/1.5A

Bill of Materials for

Part ID Part Value Part Number Manufacturer

U1 1.5A Buck Regulator NSC LM2738X

C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK

C2, Output Cap 33µF, 6.3V, X5R C3216X5ROJ336M TDK

C3, Boost Cap 0.1µF, 16V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34VF Schottky 1.5A, 30V CRS08 Toshiba

D2, Boost Diode 1VF @ 100mA Diode BAT54WS Diodes, Inc.

L1 5µH, 2.9A MSS7341- 502NL Coilcraft

R1 31.6kΩ, 1% CRCW06033162F Vishay

R2 10kΩ, 1% CRCW06031002F Vishay

R3 100kΩ, 1% CRCW06031003F Vishay

17 www.national.com

LM2738

LM2738X Circuit Example 3

30049144

FIGURE 12. LM2738X (1.6MHz)

VBOOST Derived from VSHUNT

18V to 1.5V/1.5A

Bill of Materials for

Part ID Part Value Part Number Manufacturer

U1 1.5A Buck Regulator LM2738X National Semiconductor

C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK

C2, Output Cap 47µF, 6.3V, X5R C3216X5ROJ476M TDK

C3, Boost Cap 0.1µF, 16V, X7R C1005X7R1C104K TDK

C4, Shunt Cap 0.1µF, 6.3V, X5R C1005X5R0J104K TDK

D1, Catch Diode 0.34VF Schottky 1.5A, 30V CRS08 Toshiba

D2, Boost Diode 1VF @ 100mA Diode BAT54WS Diodes, Inc.

D3, Zener Diode 5.1V 250Mw SOT-23 BZX84C5V1 Vishay

L1 2.7µH, 1.76A VLCF5020T-2R7N1R7 TDK

R1 8.87kΩ, 1% CRCW06038871F Vishay

R2 10.2kΩ, 1% CRCW06031022F Vishay

R3 100kΩ, 1% CRCW06031003F Vishay

R4 4.12kΩ, 1% CRCW06034121F Vishay

www.national.com 18

LM2738

LM2738X Circuit Example 4

30049149

FIGURE 13. LM2738X (1.6MHz)

VBOOST Derived from Series Zener Diode (VIN)

15V to 1.5V/1.5A

Bill of Materials for

Part ID Part Value Part Number Manufacturer

U1 1.5A Buck Regulator LM2738X National Semiconductor

C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK

C2, Output Cap 47µF, 6.3V, X5R C3216X5ROJ476M TDK

C3, Boost Cap 0.1µF, 16V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34VF Schottky 1.5A, 30V CRS08 Toshiba

D2, Boost Diode 1VF @ 100mA Diode BAT54WS Diodes, Inc.

D3, Zener Diode 11V 350Mw SOT-23 BZX84C11T Diodes, Inc.

L1 3.3µH, 3.5A MSS7341-332NL Coilcraft

R1 8.87kΩ, 1% CRCW06038871F Vishay

R2 10.2kΩ, 1% CRCW06031022F Vishay

R3 100kΩ, 1% CRCW06031003F Vishay

19 www.national.com

LM2738

LM2738X Circuit Example 5

30049150

FIGURE 14. LM2738X (1.6MHz)

VBOOST Derived from Series Zener Diode (VOUT)

15V to 9V/1.5A

Bill of Materials for

Part ID Part Value Part Number Manufacturer

U1 1.5A Buck Regulator LM2738X National Semiconductor

C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK

C2, Output Cap 22µF, 16V, X5R C3216X5R1C226M TDK

C3, Boost Cap 0.1µF, 16V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34VF Schottky 1.5A, 30V CRS08 Toshiba

D2, Boost Diode 1VF @ 100mA Diode BAT54WS Diodes, Inc.

D3, Zener Diode 4.3V 350mw SOT-23 BZX84C4V3 Diodes, Inc.

L1 6.2µH, 2.5A MSS7341-622NL Coilcraft

R1 102kΩ, 1% CRCW06031023F Vishay

R2 10.2kΩ, 1% CRCW06031022F Vishay

R3 100kΩ, 1% CRCW06031003F Vishay

www.national.com 20

LM2738

LM2738Y Circuit Example 6

30049142

FIGURE 15. LM2738Y (550kHz)

VBOOST Derived from VIN

5V to 1.5V/1.5A

Bill of Materials for Figure 15

Part ID Part Value Part Number Manufacturer

U1 1.5A Buck Regulator LM2738Y National Semiconductor

C1, Input Cap 10µF, 6.3V, X5R C3216X5ROJ106M TDK

C2, Output Cap 47µF, 6.3V, X5R C3216X5ROJ476M TDK

C3, Boost Cap 0.1µF, 16V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34VF Schottky 1.5A, 30V CRS08 Toshiba

D2, Boost Diode 1VF @ 100mA Diode BAT54WS Diodes, Inc.

L1 6.2µH, 2.5A, MSS7341-622NL Coilcraft

R1 8.87kΩ, 1% CRCW06038871F Vishay

R2 10.2kΩ, 1% CRCW06031022F Vishay

R3 100kΩ, 1% CRCW06031003F Vishay

21 www.national.com

LM2738

LM2738Y Circuit Example 7

30049193

FIGURE 16. LM2738Y (550kHz)

VBOOST Derived from VOUT

12V to 3.3V/1.5A

Bill of Materials for

Part ID Part Value Part Number Manufacturer

U1 1.5A Buck Regulator LM2738Y National Semiconductor

C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK

C2, Output Cap 47µF, 6.3V, X5R C3216X5ROJ476M TDK

C3, Boost Cap 0.1µF, 16V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34VF Schottky 1.5A, 30V CRS08 Toshiba

D2, Boost Diode 1VF @ 100mA Diode BAT54WS Vishay

L1 12µH, 1.7A, MSS7341-123NL Coilcraft

R1 31.6kΩ, 1% CRCW06033162F Vishay

R2 10.0 kΩ, 1% CRCW06031002F Vishay

R3 100kΩ, 1% CRCW06031003F Vishay

www.national.com 22

LM2738

LM2738Y Circuit Example 8

30049144

FIGURE 17. LM2738Y (550kHz)

VBOOST Derived from VSHUNT

18V to 1.5V/1.5A

Bill of Materials for

Part ID Part Value Part Number Manufacturer

U1 1.5A Buck Regulator LM2738Y National Semiconductor

C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK

C2, Output Cap (47µF, 6.3V, X5R) x 2 = 94µF C3216X5ROJ476M TDK

C3, Boost Cap 0.1µF, 16V, X7R C1005X7R1C104K TDK

C4, Shunt Cap 0.1µF, 6.3V, X5R C1005X5R0J104K TDK

D1, Catch Diode 0.34VF Schottky 1.5A, 30V CRS08 Toshiba

D2, Boost Diode 1VF @ 100mA Diode BAT54WS Diodes, Inc.

D3, Zener Diode 5.1V 250Mw SOT-23 BZX84C5V1 Vishay

L1 8.7µH, 2.2A MSS7341-872NL Coilcraft

R1 8.87kΩ, 1% CRCW06038871F Vishay

R2 10.2kΩ, 1% CRCW06031022F Vishay

R3 100kΩ, 1% CRCW06031003F Vishay

R4 4.12kΩ, 1% CRCW06034121F Vishay

23 www.national.com

LM2738

LM2738Y Circuit Example 9

30049149

FIGURE 18. LM2738Y (550kHz)

VBOOST Derived from Series Zener Diode (VIN)

15V to 1.5V/1.5A

Bill of Materials for

Part ID Part Value Part Number Manufacturer

U1 1.5A Buck Regulator LM2738Y National Semiconductor

C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK

C2, Output Cap (47µF, 6.3V, X5R) x 2 = 94µF C3216X5ROJ476M TDK

C3, Boost Cap 0.1µF, 16V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34VF Schottky 1.5A, 30V CRS08 Toshiba

D2, Boost Diode 1VF @ 100mA Diode BAT54WS Diodes, Inc.

D3, Zener Diode 11V 350Mw SOT-23 BZX84C11T Diodes, Inc.

L1 8.7µH, 2.2A MSS7341-872NL Coilcraft

R1 8.87kΩ, 1% CRCW06038871F Vishay

R2 10.2kΩ, 1% CRCW06031022F Vishay

R3 100kΩ, 1% CRCW06031003F Vishay

www.national.com 24

LM2738

LM2738Y Circuit Example 10

30049150

FIGURE 19. LM2738Y (550kHz)

VBOOST Derived from Series Zener Diode (VOUT)

15V to 9V/1.5A

Bill of Materials for

Part ID Part Value Part Number Manufacturer

U1 1.5A Buck Regulator LM2738Y National Semiconductor

C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK

C2, Output Cap 22µF, 16V, X5R C3216X5R1C226M TDK

C3, Boost Cap 0.1µF, 16V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34VF Schottky 1.5A, 30V CRS08 Toshiba

D2, Boost Diode 1VF @ 100mA Diode BAT54WS Diodes, Inc.

D3, Zener Diode 4.3V 350mw SOT-23 BZX84C4V3 Diodes, Inc.

L1 15µH, 2.1A SLF7055T150M2R1-3PF TDK

R1 102kΩ, 1% CRCW06031023F Vishay

R2 10.2kΩ, 1% CRCW06031022F Vishay

R3 100kΩ, 1% CRCW06031003F Vishay

25 www.national.com

LM2738

Physical Dimensions inches (millimeters) unless otherwise noted

8-Lead LLP Package

NS Package Number SDA08A

8-Lead eMSOP Package

NS Package Number MUY08A

www.national.com 26

LM2738

Notes

27 www.national.com

LM2738

Notes

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

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