National Semiconductor LM2738 Technical data

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

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

STDB

1000 Tape and Reel

Pin Descriptions

Pin Name Function

1 BOOST

4 EN

5, 7 GND

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.

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

-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

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

ΔVFB/ΔV

Feedback Voltage

Feedback Voltage Line Regulation

Feedback Input Bias Current

Undervoltage Lockout

UVLO

Undervoltage Lockout

UVLO Hysteresis 0.4

DS(ON)

MAX

Switching Frequency

Maximum Duty Cycle

Minimum Duty Cycle

MIN

Switch ON Resistance

Switch Current Limit

Quiescent Current

Quiescent Current (shutdown) VEN = 0V

BOOST

EN_TH

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

- VSW = 3V,

BOOST

Load=400mA

- 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)

Units

MHz

mΩ

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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

- VSW = 5V, and TA = 25°C,

BOOST

Efficiency vs Load Current - "Y" V

OUT

= 5V

Efficiency vs Load Current - "X" V

OUT

30049197

= 3.3V

30049151

= 1.5V

Efficiency vs Load Current - "Y" V

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

- VSW = 5V, and TA = 25°C,

BOOST

30049128

VFB vs Temperature

30049129

30049133

30049147

vs Temperature

DSON

30049130

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Typical Performance Characteristics All curves taken at V

unless specified otherwise.

= 12V, V

- VSW = 5V, and TA = 25°C,

BOOST

Line Regulation - "X" (V

= 1.5V, I

OUT

= 3.3V, I

OUT

= 750mA)

OUT

= 750mA)

OUT

30049156

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

- VSW = 5V, and TA = 25°C,

BOOST

= 3.3V)

OUT

= 3.3V, VIN = 12V)

OUT

30049178

= 3.3V, VIN = 12, I

OUT

Startup - "X"

=1.5A (Resistive Load))

OUT

30049146

30049190

30049194

In-Rush Current - "X"

= 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

. V

- VSW is the gate drive voltage

BOOST

needs to be at

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

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

From the output voltage (2.5V < V

From an external distributed voltage rail (2.5V < V

to a voltage sufficient to turn the

BOOST

< 5.5V)

OUT

5.5V)

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

- VSW = VIN - V

BOOST

is charged via diode D2 by

BOOST

) (refer to Figure 2), V

), during which the

FD2

+ V

FD2

FD1

BOOST

equals

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

= 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

– VD3) > 2.5V

INMIN

30049109

FIGURE 4. Zener Reduces Boost Voltage from V

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

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

– 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:

= 0.56 x (D + 0.54) x (V

BOOST

can be calculated for the Y version using the following:

BOOST

= 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

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

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ZENER

) / (1.4 x I

BOOST

+ I

ZENER

)

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

LM2738

BOOST

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

ZENER

= 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 exceed VIN + 0.3V.

SOFT-START

This function forces V ing start up. During soft-start, the error amplifier’s reference

to increase at a controlled rate dur-

OUT

voltage ramps from 0V to its nominal value of 0.8V in approximately 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 reference, 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 operating 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). Hysteresis prevents the part from turning off during power up if the VIN ramp-up is non-monotonic.

ZENER

= 5V,

VSW can be approximated by:

VSW = I

OUT

x R

DSON

The diode forward drop (VD) can range from 0.3V to 0.7V depending 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 calculated. The peak current (I

) in the inductor is calculated by:

LPK

= I

OUT

+ Δi

LPK

30049180

FIGURE 6. Inductor Current

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 approximately 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:

In general,

ΔiL = 0.1 x (I

) → 0.2 x (I

OUT

)

If ΔiL = 33.3% of 1.50A, the peak current in the inductor will be 2.0A. The minimum guaranteed current limit over all operating 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 ceramic output capacitors without compromising transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple. See the output capacitor section for more details on calculating output voltage 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. Inductor 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 inductor should be specified with a saturation current limit of >

1.25A. There is no need to specify the saturation or peak current 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 inductors is huge. Lastly, inductors with lower series resistance (R

) will provide better operating efficiency. For recom-

DCR

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 voltage 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 (I must be greater than:

RMS-IN

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 capacitances 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. Capacitance, 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 applications. 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:

)

The reverse breakdown rating of the diode must be at least

ID1 = I

OUT

x (1-D)

the maximum input voltage plus appropriate margin. To improve efficiency, choose a Schottky diode with a low forward voltage drop.

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 effective cross sectional area of the current path. A large leaded capacitor will have high ESL and a 0805 ceramic chip capacitor 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 required 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. Consult capacitor manufacturer datasheets to see how rated capacitance varies over operating conditions.

OUTPUT CAPACITOR

The output capacitor is selected based upon the desired output ripple and transient response. The initial current of a load transient is provided mainly by the output capacitor. The output ripple of the converter is:

When using MLCCs, the ESR is typically so low that the capacitive ripple may dominate. When this occurs, the output

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 loaded.

= 0.80V

REF

PCB LAYOUT CONSIDERATIONS

When planning layout there are a few things to consider when trying to achieve a clean, regulated output. The most important consideration is the close coupling of the GND connections of the input capacitor and the catch diode D1. These ground ends should be close to one another and be connected to the GND plane with at least two through-holes. Place these components as close to the IC as possible. Next in importance 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 switching 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 V

trace to R2 should be routed away from the inductor and

OUT

any other traces that are switching. High AC currents flow through the VIN, SW and V short and wide as possible. However, making the traces wide

traces, so they should be as

OUT

increases radiated noise, so the designer must make this trade-off. Radiated noise can be decreased by choosing a

11 www.national.com

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-

LM2738

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 operates 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 losses are shown below. Other losses totaling less than 2% are not discussed.

Power loss (P 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:

VD is the forward voltage drop across the Schottky catch diode. It can be obtained from the diode manufactures Electrical Characteristics section. If the voltage drop across the inductor (V

The conduction losses in the free-wheeling Schottky diode are calculated as follows:

Often this is the single most significant power loss in the circuit. 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:

The LM2738 conduction loss is mainly associated with the internal NFET switch:

) is the sum of two basic types of losses in

LOSS

VSW = I

) is accounted for, the equation becomes:

DCR

DIODE

IND

OUT

= VD x I

= I

OUT

x R

OUT

x R

DSON

x (1-D)

DCR

If the inductor ripple current is fairly small, the conduction losses can be simplified to:

COND

= I

OUT

x R

DSON

x D

Switching losses are also associated with the internal NFET switch. They occur during the switch on and off transition periods, where voltages and currents overlap resulting in power loss. The simplest means to determine this loss is to empirically measure the rise and fall times (10% to 90%) of the switch at the switch node.

Switching Power Loss is calculated as follows:

= 1/2(VIN x I

SWR

= 1/2(VIN x I

SWF

PSW = P

OUT

SWR

x FSW x T

+ P

SWF

RISE

FALL

)

Another loss is the power required for operation of the internal circuitry:

PQ = IQ x V

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

OUT

RISE

FALL

DS(ON)

IND

DCR

D 0.275 P

ΣP

COND

ΣP

COND

12.0V

3.3V P

OUT

1.25A

0.34V P

DIODE

550kHz

1.9mA P

8nS P

275mΩ

70mΩ

86.7% P

+ PSW + P

+ P

SWF

INTERNAL

DIODE

+ P

SWR

SWF

COND

IND

LOSS

INTERNAL

+ P

IND

+ PQ = P

= 207mW

+ PQ = P

INTERNAL

4.125W

317mW

22.8mW

33mW

118mW

110mW

634mW

207mW

LOSS

Thermal Definitions

TJ = Chip junction temperature TA = Ambient temperature R

= Thermal resistance from chip junction to device case

θJC

= Thermal resistance from chip junction to ambient air

θJA

Heat in the LM2738 due to internal power dissipation is removed through conduction and/or convection.

Conduction: Heat transfer occurs through cross sectional areas of material. Depending on the material, the transfer of heat can be considered to have poor to good thermal conductivity 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

Thermal impedance is defined as:

LM2738

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 greatly effect R 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 requires 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. R

is the thermal impedance from all six sides of an IC

θJC

package to silicon junction. R

is the thermal impedance from top case to the silicon

ΦJC

junction. In this data sheet we will use R

to measure top case temperature with a small thermocouple attached to the top case.

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

ΦJC

with the exposed pad. Knowing the internal dissipation from the efficiency calculation given previously, and the case temperature, which can be empirically measured on the bench we have:

. The type and number of thermal vias can also

θJA

so that it allows the user

ΦJC

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 temperature, and the ambient temperature R

can be determined.

θJA

Once this is determined, the maximum ambient temperature allowed for a desired junction temperature can be found.

An example of calculating R National Semiconductor LM2738 LLP demonstration board is

for an application using the

θJA

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:

INTERNAL

= 207mW

If the junction temperature was to be kept below 125°C, then the ambient temperature could not go above 109°C

Tj - (R

θJA

x P

LOSS

) = T

125°C - (102°C/W x 207mW) = 104°C

LLP Package

Therefore:

Tj = (R

ΦJC

x P

LOSS

) + T

From the previous example:

Tj = (R

Tj = 30°C/W x 0.207W + T

ΦJC

x P

INTERNAL

) + T

The second method can give a very accurate silicon junction temperature.

The first step is to determine R LM2738 has over-temperature protection circuitry. When the

of the application. The

θJA

silicon temperature reaches 165°C, the device stops switching. 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 R

for any application can be characterized during the early

θJA

stages of the design one may calculate the R the PCB circuit into a thermal chamber. Raise the ambient

by placing

θJA

temperature in the given working application until the circuit

www.national.com 14

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 R for the application can be reduced.

θJA

30049179

FIGURE 9. 8-Lead LLP PCB Dog Bone Layout

LM2738

15 www.national.com

LM2738X Circuit Example 1

LM2738

FIGURE 10. LM2738X (1.6MHz)

Derived from V

BOOST

5V to 1.5V/1.5A

Bill of Materials for Figure 10

30049142

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

8.87kΩ, 1%

10.2kΩ, 1%

100kΩ, 1%

CRCW06038871F Vishay

CRCW06031022F Vishay

CRCW06031003F Vishay

www.national.com 16

LM2738X Circuit Example 2

FIGURE 11. LM2738X (1.6MHz)

Derived from V

BOOST

12V to 3.3V/1.5A

OUT

Bill of Materials for

LM2738

30049193

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

31.6kΩ, 1%

10kΩ, 1%

100kΩ, 1%

CRCW06033162F Vishay

CRCW06031002F Vishay

CRCW06031003F Vishay

17 www.national.com

LM2738X Circuit Example 3

LM2738

FIGURE 12. LM2738X (1.6MHz)

Derived from V

BOOST

18V to 1.5V/1.5A

SHUNT

Bill of Materials for

30049144

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

8.87kΩ, 1%

10.2kΩ, 1%

100kΩ, 1%

4.12kΩ, 1%

CRCW06038871F Vishay

CRCW06031022F Vishay

CRCW06031003F Vishay

CRCW06034121F Vishay

www.national.com 18

LM2738X Circuit Example 4

BOOST

LM2738

30049149

FIGURE 13. LM2738X (1.6MHz)

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

8.87kΩ, 1%

10.2kΩ, 1%

100kΩ, 1%

CRCW06038871F Vishay

CRCW06031022F Vishay

CRCW06031003F Vishay

19 www.national.com

LM2738X Circuit Example 5

LM2738

30049150

Derived from Series Zener Diode (V

BOOST

15V to 9V/1.5A

OUT

)

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

FIGURE 14. LM2738X (1.6MHz)

102kΩ, 1%

10.2kΩ, 1%

100kΩ, 1%

CRCW06031023F Vishay

CRCW06031022F Vishay

CRCW06031003F Vishay

www.national.com 20

LM2738Y Circuit Example 6

FIGURE 15. LM2738Y (550kHz)

Derived from V

BOOST

5V to 1.5V/1.5A

LM2738

30049142

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

8.87kΩ, 1%

10.2kΩ, 1%

100kΩ, 1%

CRCW06038871F Vishay

CRCW06031022F Vishay

CRCW06031003F Vishay

21 www.national.com

LM2738Y Circuit Example 7

LM2738

30049193

FIGURE 16. LM2738Y (550kHz)

Derived from V

BOOST

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

31.6kΩ, 1%

10.0 kΩ, 1%

100kΩ, 1%

CRCW06033162F Vishay

CRCW06031002F Vishay

CRCW06031003F Vishay

OUT

www.national.com 22

LM2738Y Circuit Example 8

FIGURE 17. LM2738Y (550kHz)

Derived from V

BOOST

18V to 1.5V/1.5A

SHUNT

LM2738

30049144

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

8.87kΩ, 1%

10.2kΩ, 1%

100kΩ, 1%

4.12kΩ, 1%

CRCW06038871F Vishay

CRCW06031022F Vishay

CRCW06031003F Vishay

CRCW06034121F Vishay

23 www.national.com

LM2738Y Circuit Example 9

LM2738

30049149

FIGURE 18. LM2738Y (550kHz)

Derived from Series Zener Diode (VIN)

BOOST

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

8.87kΩ, 1%

10.2kΩ, 1%

100kΩ, 1%

15V to 1.5V/1.5A

Bill of Materials for

CRCW06038871F Vishay

CRCW06031022F Vishay

CRCW06031003F Vishay

www.national.com 24

LM2738Y Circuit Example 10

LM2738

30049150

Derived from Series Zener Diode (V

BOOST

15V to 9V/1.5A

OUT

)

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

FIGURE 19. LM2738Y (550kHz)

102kΩ, 1%

10.2kΩ, 1%

100kΩ, 1%

CRCW06031023F Vishay

CRCW06031022F Vishay

CRCW06031003F Vishay

25 www.national.com

Physical Dimensions inches (millimeters) unless otherwise noted

LM2738

8-Lead LLP Package

NS Package Number SDA08A

8-Lead eMSOP Package

NS Package Number MUY08A

www.national.com 26

Notes

LM2738

27 www.national.com

Notes

For more National Semiconductor product information and proven design tools, visit the following Web sites at:

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

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National Semiconductor LM2738 Technical data

Specifications and Main Features

Frequently Asked Questions

User Manual

General Description

Typical Application Circuit

Features

Applications

Connection Diagrams

Ordering Information

Pin Descriptions

Absolute Maximum Ratings (Note 1)

Operating Ratings (Note 1)

Electrical Characteristics

Typical Performance Characteristics All curves taken at V

Typical Performance Characteristics All curves taken at V

Typical Performance Characteristics All curves taken at V

Typical Performance Characteristics All curves taken at V

Block Diagram

FIGURE 1. Simplified Internal Block Diagram

Application Information

THEORY OF OPERATION

BOOST FUNCTION

ENABLE PIN / SHUTDOWN MODE

SOFT-START

OUTPUT OVERVOLTAGE PROTECTION

UNDERVOLTAGE LOCKOUT

FIGURE 6. Inductor Current

CURRENT LIMIT

THERMAL SHUTDOWN

Design Guide

INDUCTOR SELECTION

INPUT CAPACITOR

CATCH DIODE

OUTPUT CAPACITOR

OUTPUT VOLTAGE

PCB LAYOUT CONSIDERATIONS

RECOMMENED OPERATING AREA DUE TO MINIMUM ON TIME

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

Calculating Efficiency, and Junction Temperature

Power Loss Tabulation

Thermal Definitions

LLP Package

FIGURE 8. Internal LLP Connection

FIGURE 9. 8-Lead LLP PCB Dog Bone Layout

LM2738X Circuit Example 1

Bill of Materials for Figure 10

LM2738X Circuit Example 2

Bill of Materials for

LM2738X Circuit Example 3

Bill of Materials for

LM2738X Circuit Example 4

Bill of Materials for

LM2738X Circuit Example 5

Bill of Materials for

LM2738Y Circuit Example 6

Bill of Materials for Figure 15

LM2738Y Circuit Example 7

Bill of Materials for

LM2738Y Circuit Example 8

Bill of Materials for

LM2738Y Circuit Example 9

Bill of Materials for

LM2738Y Circuit Example 10

Bill of Materials for

Physical Dimensions inches (millimeters) unless otherwise noted