Datasheet LM2734Z, LM2734ZQ Datasheet (National Semiconductor)

Page 1

LM2734Z/LM2734ZQ Thin SOT23 1A Load Step-Down DC-DC Regulator

June 12, 2008

General Description

The LM2734Z regulator is a monolithic, high frequency, PWM step-down DC/DC converter assembled in a 6-pin Thin SOT23 and LLP non pull back package. It provides all the active functions to provide local DC/DC conversion with fast transient response and accurate regulation in the smallest possible PCB area.

With a minimum of external components and online design support through WEBENCH®™, the LM2734Z is easy to use. The ability to drive 1A loads with an internal 300mΩ NMOS switch using state-of-the-art 0.5µm BiCMOS technology results in the best power density available. The world class control circuitry allows for on-times as low as 13ns, thus supporting exceptionally high frequency conversion over the entire 3V to 20V input operating range down to the minimum output voltage of 0.8V. Switching frequency is internally set to 3MHz, allowing the use of extremely small surface mount inductors and chip capacitors. Even though the operating frequency is very high, efficiencies up to 85% are easy to achieve. External shutdown is included, featuring an ultra-low stand-by current of 30nA. The LM2734Z utilizes current-mode control and internal compensation to provide high-performance regulation over a wide range of operating conditions. Additional features include internal soft-start circuitry to reduce inrush current, pulse-by-pulse current limit, thermal shutdown, and output over-voltage protection.

Features

Thin SOT23-6 package, or 6 lead LLP package

■

3.0V to 20V input voltage range

■

0.8V to 18V output voltage range

■

1A output current

■

3MHz switching frequency

■

300mΩ NMOS switch

■

30nA shutdown current

■

0.8V, 2% internal voltage reference

■

Internal soft-start

■

Current-Mode, PWM operation

■

Thermal shutdown

■

LM2734ZQ is AEC-Q100 Grade 1 qualified and is

■

manufactured on an Automotive Grade Flow

Applications

DSL Modems

■

Local Point of Load Regulation

■

Battery Powered Devices

■

USB Powered Devices

■

Automotive

■

Typical Application Circuit

WEBENCH™ is a trademark of Transim.

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Efficiency vs Load Current

VIN = 5V, V

OUT

= 3.3V

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Connection Diagrams

LM2734Z/LM2734ZQ

NS Package Number MK06A

6-Lead TSOT

20130305

6-Lead LLP (3mm x 3mm)

NS Package Number SDE06A

20130360

Ordering Information

Order Number Package Type NSC Package

Drawing

LM2734ZMK

LM2734ZMKX SFTB 3000 Units on Tape and Reel

LM2734ZQMKE SVBB 250 Units on Tape and Reel AEC-Q100 Grade 1

TSOT-6 MK06A

LM2734ZQMK SVBB 1000 Units on Tape and Reel

LM2734ZQMKX SVBB 3000 Units on Tape and Reel

LM2734ZSD

LM2734ZSDX L163B 4500 Units on Tape and Reel

LM2734ZQSDE L238B 250 Units on Tape and Reel AEC-Q100 Grade 1

6-Lead LLP SDE06A

LM2734ZQSD L238B 1000 Units on Tape and Reel

LM2734ZQSDX L238B 4500 Units on Tape and Reel

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

Package

Supplied As Features

Marking

SFTB 1000 Units on Tape and Reel

Qualified. Automotive-Grade

Production Flow*

L163B 1000 Units on Tape and Reel

Qualified. Automotive-Grade

Production Flow*

Pin Descriptions

Pin Name Function

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

bootstrap capacitor is connected between the BOOST and SW pins.

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

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

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

voltage.

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

pin to float or be greater than V

5 V

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

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

capacitor.

DAP GND The Die Attach Pad is internally connected to GND

+ 0.3V.

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LM2734Z/LM2734ZQ

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.

-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)

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

Thermal Resistance θJA (Note 3)

TSOT23–6 118°C/W

3V to 20V

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 = 5V, V

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 0.30 0.44 0.62

DS(ON)

MAX

Switching Frequency

Maximum Duty Cycle

Minimum Duty Cycle

MIN

Switch ON Resistance

Switch Current Limit V

Quiescent Current Switching 1.5 2.5 mA

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

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

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

Note 5: Typicals represent the most likely parametric norm.

Boost Pin Current

Shutdown Threshold Voltage VEN Falling

Enable Threshold Voltage VEN Rising 1.8

Enable Pin Current Sink/Source

Switch Leakage

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

BOOST

VIN = 3V to 20V

Sink/Source

VIN Rising

VIN Falling

Min

(Note 4)

0.784 0.800 0.816 V

0.01 % / V

10 250 nA

2.74 2.90

2.0 2.3

2.2 3.0 3.6 MHz

78 85 %

Typ

(Note 5)

Max

(Note 4)

- VSW = 3V

BOOST

(TSOT Package)

- VSW = 3V

BOOST

(LLP Package)

- VSW = 3V 1.2 1.7 2.5 A

BOOST

(Switching)

300 600

340 650

4.25 6 mA

0.4

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

J(MAX)

, θJA and TA . The

J(MAX)

Units

mΩ

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Typical Performance Characteristics All curves taken at V

TA = 25°C, unless specified otherwise.

= 5V, V

- VSW = 5V, L1 = 2.2 µH and

BOOST

Efficiency vs Load Current

LM2734Z/LM2734ZQ

Efficiency vs Load Current

OUT

= 1.5V

= 5V

20130336

Efficiency vs Load Current

= 3.3V

OUT

20130351

Oscillator Frequency vs Temperature

20130337

Line Regulation

= 1.5V, I

OUT

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= 500mA

OUT

20130354

Line Regulation

= 3.3V, I

OUT

= 500mA

OUT

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Block Diagram

LM2734Z/LM2734ZQ

Application Information

THEORY OF OPERATION

The LM2734Z is a constant frequency PWM buck regulator IC that delivers a 1A load current. The regulator has a preset switching frequency of 3MHz. This high frequency allows the LM2734Z to operate with small surface mount capacitors and inductors, resulting in a DC/DC converter that requires a minimum amount of board space. The LM2734Z is internally compensated, so it is simple to use, and requires few external components. The LM2734Z uses current-mode control to regulate the output voltage.

The following operating description of the LM2734Z will refer to the Simplified Block Diagram (Figure 1) and to the waveforms in Figure 2. The LM2734Z 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.

20130306

FIGURE 1.

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.

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FIGURE 2. LM2734Z Waveforms of SW Pin Voltage and

Inductor Current

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BOOST FUNCTION

Capacitor C erate a voltage V to the internal NMOS control switch. To properly drive the internal NMOS switch during its on-time, V least 1.6V greater than VSW. Although the LM2734Z will op-

and diode D2 in Figure 3 are used to gen-

BOOST

. V

- VSW is the gate drive voltage

BOOST

needs to be at

BOOST

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

LM2734Z/LM2734ZQ

VSW for best efficiency. V maximum operating limit of 5.5V.

5.5V > V

– VSW > 2.5V for best performance.

BOOST

FIGURE 3. V

be greater than 2.5V above

BOOST

– VSW should not exceed the

BOOST

Charges C

OUT

BOOST

When the LM2734Z 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 (VIN)

From the output voltage (V

From an external distributed voltage rail (V

From a shunt or series zener diode

to a voltage sufficient to turn the

BOOST

)

OUT

In the Simplifed Block Diagram of Figure 1, capacitor C

and diode D2 supply the gate-drive current for the

BOOST

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

OFF

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

). Therefore the voltage stored across C

FD1

- VSW = VIN - V

BOOST

is charged via diode D2 by

BOOST

) (refer to Figure 2), V

), during which the

FD2

+ V

FD2

FD1

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

forcing V V

BOOST

VSW = VIN – (R

to rise thus reverse biasing D2. The voltage at

BOOST

is then

= 2VIN – (R

BOOST

DSON

x IL),

DSON

x IL) – V

FD2

+ V

which is approximately

2VIN - 0.4V

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

VIN - 0.2V

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

is to connect D2 to

BOOST

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

BOOST

)

EXT

BOOST

FD1

20130308

. This

equals

applied to the internal switch. In this circuit, C a gate drive voltage that is slightly less than V

In applications where both VIN and V

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

can be charged from VIN or V

BOOST

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

cannot be charged directly from

BOOST

OUT

are greater than

OUT

are greater than 5.5V,

minus a zener volt-

OUT

BOOST

OUT

provides

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

– VD3) > 1.6V

INMIN

FIGURE 4. Zener Reduces Boost Voltage from V

BOOST

voltage.

20130309

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

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

voltage is maintained during this time.

BOOST

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

into the BOOST pin supplies the gate current

BOOST

) is 1 mA. The

ZENER

the following formula:

= (D + 0.5) 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

is the voltage applied to the

ZENER

voltage across D2. Note that this formula for I ical current. For the worst case I by 25%. In that case, the worst case boost current will be

BOOST-MAX

= 1.25 x I

– VD2) mA

ZENER

and VD2 are in volts, and

gives typ-

BOOST

, increase the current

BOOST

R3 will then be given by

R3 = (VIN - V

For example, let VIN = 10V, V = 1mA, and duty cycle D = 50%. Then

= (0.5 + 0.5) x (5 - 0.7) mA = 4.3mA

BOOST

ZENER

) / (1.25 x I

ZENER

BOOST

+ I

ZENER

)

= 5V, VD2 = 0.7V, I

ZENER

R3 = (10V - 5V) / (1.25 x 4.3mA + 1mA) = 787Ω

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LM2734Z/LM2734ZQ

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 must be included to calculate a more accurate duty cycle. Calculate D by using the following formula:

FIGURE 5. Boost Voltage Supplied from the Shunt Zener

on V

ENABLE PIN / SHUTDOWN MODE

The LM2734Z 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 30nA. Switch leakage adds another 40nA from the input supply. 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 200µs. This forces the regulator output to ramp up in a more linear and controlled fashion, which helps reduce inrush current.

OUTPUT OVERVOLTAGE PROTECTION

The overvoltage comparator compares the FB pin voltage to a voltage that is 10% higher than the internal reference Vref. Once the FB pin voltage goes 10% 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 LM2734Z from operating until the input voltage exceeds 2.74V(typ).

The UVLO threshold has approximately 440mV 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 VIN is non-monotonic.

CURRENT LIMIT

The LM2734Z 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 1.7A (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.

VSW can be approximated by:

VSW = IO x R

DS(ON)

The diode forward drop (VD) can range from 0.3V to 0.7V depending on the quality of the diode. The lower VD is, 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. The ratio of ripple current (ΔiL) to output current (IO) is optimized when it is set between 0.3 and 0.4 at 1A. The ratio r is defined as:

One must also ensure that the minimum current limit (1.2A) is not exceeded, so the peak current in the inductor must be calculated. The peak current (I by:

LPK

) in the inductor is calculated

LPK

= IO + ΔIL/2

If r = 0.5 at an output of 1A, the peak current in the inductor will be 1.25A. The minimum guaranteed current limit over all operating conditions is 1.2A. One can either reduce r to 0.4 resulting in a 1.2A peak current, or make the engineering judgement that 50mA over will be safe enough with a 1.7A typical current limit and 6 sigma limits. When the designed maximum output current is reduced, the ratio r can be increased. At a current of 0.1A, r can be made as high as 0.9. The ripple ratio can be increased at lighter loads because the net ripple is actually quite low, and if r remains constant the inductor value can be made quite large. An equation empirically developed for the maximum ripple ratio at any current below 2A is:

r = 0.387 x I

OUT

-0.3667

Note that this is just a guideline. The LM2734Z 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.

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Now that the ripple current or ripple ratio is determined, the inductance is calculated by:

transient is provided mainly by the output capacitor. The output ripple of the converter is:

where fs is the switching frequency and IO is the output current. When selecting an inductor, make sure that it is capable

LM2734Z/LM2734ZQ

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 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 0.5A and the peak current is 0.7A, then the inductor should be specified with a saturation current limit of >0.7A. There is no need to specify the saturation or peak current of the inductor at the 1.7A typical switch current limit. The difference in inductor size is a factor of 5. Because of the operating frequency of the LM2734Z, 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 (DCR) will provide better operating efficiency. For recommended 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 10µF, although 4.7µF works well for input voltages below 6V. 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 greater than:

RMS-IN

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 LM2734Z, certain 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 or Cornell Dubilier ESR, 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 dielectrics. Consult capacitor manufacturer datasheet 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

) must be

When using MLCCs, the ESR is typically so low that the capacitive 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 LM2734Z, 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 at 10 µF of output capacitance. Capacitance can be increased significantly with little detriment to the regulator stability. Like the input capacitor, recommended multilayer ceramic capacitors are X7R or X5R. Again, verify actual capacitance at the desired operating voltage and temperature.

Check the RMS current rating of the capacitor. The RMS current rating of the capacitor chosen must also meet the following condition:

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 = IO x (1-D)

The reverse breakdown rating of the diode must be at least the maximum input voltage plus appropriate margin. To improve efficiency choose a Schottky diode with a low forward voltage drop.

BOOST DIODE

A standard diode such as the 1N4148 type is recommended. For V small-signal Schottky diode is recommended for greater effi-

circuits derived from voltages less than 3.3V, a

BOOST

ciency. A good choice is the BAT54 small signal diode.

BOOST CAPACITOR

A ceramic 0.01µF capacitor with a voltage rating of at least

6.3V is sufficient. The X7R and X5R MLCCs provide the best performance.

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 10kΩ.

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LM2734Z/LM2734ZQ

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 when completing the layout is the close coupling of the GND connections of the CIN 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 C connections of CIN and D1.

capacitor, which should be near the GND

OUT

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 R2 placed as close as possible to the GND of the IC. The V trace to R1 should be routed away from the inductor and any

OUT

other traces that are switching. High AC currents flow through the VIN, SW and V

so they should be as short and wide as possible. However,

OUT

traces,

making the traces wide increases radiated noise, so the designer must make this trade-off. Radiated noise can be decreased by choosing a 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 LM2734Z demo board as an example of a four-layer layout.

Calculating Efficiency, and Junction Temperature

The complete LM2734Z 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, where as 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).

) is the sum of two basic types of losses in

LOSS

VSW is the voltage drop across the internal NFET when it is on, and is equal to:

VSW = I

OUT

x R

DSON

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

) is accounted for,

DCR

This usually gives only a minor duty cycle change, and has been omitted in the examples for simplicity.

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

DIODE

= VD x I

OUT

(1-D)

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:

= I

OUT

x R

DCR

IND

The LM2734Z conduction loss is mainly associated with the internal NFET:

COND

= I

OUT

x R

DSON

x D

Switching losses are also associated with the internal NFET. 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 measuring the rise and fall times (10% to 90%) of the switch at the switch node:

SWF

SWR

= 1/2(VIN x I

PSW = P

OUT

SWF

x freq x T

+ P

SWR

FALL

RISE

)

Typical Rise and Fall Times vs Input Voltage

RISE

FALL

5V 8ns 4ns

10V 9ns 6ns

15V 10ns 7ns

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

1.5mA. The other operating power that needs to be calculated is that required to drive the internal NFET:

= I

BOOST

is normally between 3VDC and 5VDC. The I

BOOST

current is approximately 4.25mA. Total power losses are:

BOOST

x V

BOOST

rms

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Page 10

Design Example 1:

Operating Conditions

LM2734Z/LM2734ZQ

OUT

Freq 3MHz P

RISE

FALL

DSON

IND

DCR

5.0V P

2.5V P

1.0A P

0.35V P

1.5mA P

8ns P

330mΩ

75mΩ

D 0.568

η = 82%

Calculating the LM2734Z Junction Temperature

Thermal Definitions: TJ = Chip junction temperature TA = Ambient temperature

OUT

DIODE

IND

SWF

SWR

COND

BOOST

LOSS

θJC

θJA

2.5W

151mW

75mW

53mW

187mW

7.5mW

21mW

548mW

= Thermal resistance from chip junction to device case

= Thermal resistance from chip junction to ambient air

FIGURE 6. Cross-Sectional View of Integrated Circuit Mounted on a Printed Circuit Board.

Heat in the LM2734Z 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.

Thermal impedance is defined as:

Thermal impedance from the silicon junction to the ambient air is defined as:

www.national.com 10

20130373

This impedance can vary depending on the thermal properties of the PCB. This includes PCB size, weight of copper used to route traces and ground plane, and number of layers within the PCB. 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. If the Thin SOT23-6 package is used, place two to four thermal vias close to the ground pin of the device.

The datasheet specifies two different R Thin SOT23–6 package. The two numbers show the differ-

numbers for the

θJA

ence in thermal impedance for a four-layer board with 2oz. copper traces, vs. a four-layer board with 1oz. copper. R equals 120°C/W for 2oz. copper traces and GND plane, and

θJA

235°C/W for 1oz. copper traces and GND plane.

Page 11

LM2734Z/LM2734ZQ

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 case. (R SOT23-6 package. Knowing the internal dissipation from the

) is approximately 80°C/W for the Thin

θJC

efficiency calculation given previously, and the case temperature, which can be empirically measured on the bench we have:

Therefore:

TJ = (R

θJC

x P

LOSS

) + T

Design Example 2:

Operating Conditions

OUT

Freq 3MHz P

RISE

FALL

DSON

IND

DCR

5.0V P

2.5V P

1.0A P

0.35V P

1.5mA P

8ns P

330mΩ

75mΩ

OUT

DIODE

IND

SWF

SWR

COND

BOOST

LOSS

2.5W

151mW

75mW

53mW

187mW

7.5mW

21mW

548mW

D 0.568

Design Example 3:

Operating Conditions

Package SOT23-6

OUT

Freq 3MHz P

BOOST

RISE

FALL

DSON

IND

DCR

12.0V P

3.30V P

750mA P

0.35V P

1.5mA P

4mA P

5V P

8ns P

OUT

DIODE

IND

SWF

SWR

COND

BOOST

LOSS

8ns

400mΩ

75mΩ

2.475W

523mW

56.25mW

108mW

68.2mW

18mW

20mW

902mW

D 30.3%

Using a standard National Semiconductor Thin SOT23-6 demonstration board to determine the R four layer PCB is constructed using FR4 with 1/2oz copper

of the board. The

θJA

traces. The copper ground plane is on the bottom layer. The ground plane is accessed by two vias. The board measures

2.5cm x 3cm. It was placed in an oven with no forced airflow. The ambient temperature was raised to 94°C, and at that

temperature, the device went into thermal shutdown.

The second method can give a very accurate silicon junction temperature. The first step is to determine R cation. The LM2734Z has over-temperature protection cir-

of the appli-

θJA

cuitry. When the silicon temperature reaches 165°C, the device stops switching. The protection circuitry has a hysteresis of 15°C. Once the silicon temperature has decreased to approximately 150°C, the device will start to switch again. Knowing this, the R ing the early stages of the design by raising the ambient

for any PCB can be characterized dur-

θJA

temperature in the given 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 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.

If the junction temperature was to be kept below 125°C, then the ambient temperature cannot go above 54.2°C.

TJ - (R

θJA

x P

LOSS

) = T

The method described above to find the junction temperature in the Thin SOT23-6 package can also be used to calculate the junction temperature in the LLP package. The 6 pin LLP package has a R on the application. R as described in method #2 (see example 3).

11 www.national.com

= 20°C/W, and R

θJC

can be calculated in the same manner

θJA

can vary depending

θJA

Page 12

LLP Package

The LM2734Z is packaged in a Thin SOT23-6 package and the 6–pin LLP. The LLP package has the same footprint as the Thin SOT23-6, but is thermally superior due to the exposed ground paddle on the bottom of the package.

LM2734Z/LM2734ZQ

No Pullback LLP Configuration

of the LLP package is normally two to three times better

θJA

than that of the Thin SOT23-6 package for a similar PCB configuration (area, copper weight, thermal vias).

20130374

Design Example 4:

Operating Conditions

Package LLP-6

OUT

Freq 3MHz P

BOOST

RISE

FALL

DSON

IND

DCR

12.0V P

3.3V P

750mA P

0.35V P

1.5mA P

4mA P

5V P

8ns P

OUT

DIODE

IND

SWF

SWR

COND

BOOST

LOSS

8ns

400mΩ

75mΩ

2.475W

523mW

56.25mW

108mW

68.2mW

18mW

20mW

902mW

D 30.3%

This example follows example 2, but uses the LLP package. Using a standard National Semiconductor LLP-6 demonstration board, use Method 2 to determine R four layer PCB is constructed using FR4 with 1/2oz copper

of the board. The

θJA

traces. The copper ground plane is on the bottom layer. The ground plane is accessed by four vias. The board measures

2.5cm x 3cm. It was placed in an oven with no forced airflow. The ambient temperature was raised to 113°C, and at that

temperature, the device went into thermal shutdown.

20130370

FIGURE 7. Dog Bone

For certain high power applications, the PCB land may be modified to a "dog bone" shape (see Figure 7). By increasing the size of ground plane, and adding thermal vias, the R for the application can be reduced.

www.national.com 12

θJA

If the junction temperature is to be kept below 125°C, then the ambient temperature cannot go above 73.2°C.

TJ - (R

θJA

x P

LOSS

) = T

Page 13

Package Selection

To determine which package you should use for your specific application, variables need to be known before you can determine the appropriate package to use.

Maximum ambient system temperature

Internal LM2734Z power losses

Maximum junction temperature desired

of the specific application, or R

θJA

SOT23-6)

The junction temperature must be less than 125°C for the worst-case scenario.

(LLP or Thin

θJC

LM2734Z/LM2734ZQ

13 www.national.com

Page 14

LM2734Z Design Examples

LM2734Z/LM2734ZQ

20130342

FIGURE 8. V

Operating Conditions: 5V to 1.5V/1A

Derived from V

BOOST

Bill of Materials for Figure 8

Part ID Part Value Part Number Manufacturer

U1 1A Buck Regulator LM2734ZX National Semiconductor

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

C2, Output Cap 10µF, 6.3V, X5R C3216X5ROJ106M TDK

C3, Boost Cap 0.01uF, 16V, X7R C1005X7R1C103K TDK

D1, Catch Diode 0.3VF Schottky 1A, 10VR MBRM110L ON Semi

D2, Boost Diode 1VF @ 50mA Diode 1N4148W Diodes, Inc.

L1 2.2µH, 1.8A ME3220–222MX Coilcraft

8.87kΩ, 1%

10.2kΩ, 1%

100kΩ, 1%

CRCW06038871F Vishay

CRCW06031022F Vishay

CRCW06031003F Vishay

www.national.com 14

Page 15

20130343

LM2734Z/LM2734ZQ

FIGURE 9. V

Derived from V

BOOST

12V to 3.3V/1A

OUT

Bill of Materials for Figure 9

Part ID Part Value Part Number Manufacturer

U1 1A Buck Regulator LM2734ZX National Semiconductor

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

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

C3, Boost Cap 0.01µF, 16V, X7R C1005X7R1C103K TDK

D1, Catch Diode 0.34VF Schottky 1A, 30VR SS1P3L Vishay

D2, Boost Diode 0.6VF @ 30mA Diode BAT17 Vishay

L1 3.3µH, 1.3A ME3220–332MX Coilcraft

31.6kΩ, 1%

10.0 kΩ, 1%

100kΩ, 1%

CRCW06033162F Vishay

CRCW06031002F Vishay

CRCW06031003F Vishay

15 www.national.com

Page 16

LM2734Z/LM2734ZQ

20130344

FIGURE 10. V

Derived from V

BOOST

18V to 1.5V/1A

SHUNT

Bill of Materials for Figure 10

Part ID Part Value Part Number Manufacturer

U1 1A Buck Regulator LM2734ZX National Semiconductor

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

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

C3, Boost Cap 0.01µF, 16V, X7R C1005X7R1C103K TDK

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

D1, Catch Diode 0.4VF Schottky 1A, 30VR SS1P3L Vishay

D2, Boost Diode 1VF @ 50mA Diode 1N4148W Diodes, Inc.

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

L1 3.3µH, 1.3A ME3220–332MX Coilcraft

8.87kΩ, 1%

10.2kΩ, 1%

100kΩ, 1%

4.12kΩ, 1%

CRCW06038871F Vishay

CRCW06031022F Vishay

CRCW06031003F Vishay

CRCW06034121F Vishay

www.national.com 16

Page 17

20130349

LM2734Z/LM2734ZQ

FIGURE 11. V

Derived from Series Zener Diode (VIN)

BOOST

15V to 1.5V/1A

Bill of Materials for Figure 11

Part ID Part Value Part Number Manufacturer

U1 1A Buck Regulator LM2734ZX National Semiconductor

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

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

C3, Boost Cap 0.01µF, 16V, X7R C1005X7R1C103K TDK

D1, Catch Diode 0.4VF Schottky 1A, 30VR SS1P3L Vishay

D2, Boost Diode 1VF @ 50mA Diode 1N4148W Diodes, Inc.

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

L1 3.3µH, 1.3A ME3220–332MX Coilcraft

8.87kΩ, 1%

10.2kΩ, 1%

100kΩ, 1%

CRCW06038871F Vishay

CRCW06031022F Vishay

CRCW06031003F Vishay

17 www.national.com

Page 18

LM2734Z/LM2734ZQ

20130350

FIGURE 12. V

Derived from Series Zener Diode (V

BOOST

15V to 9V/1A

OUT

)

Bill of Materials for Figure 12

Part ID Part Value Part Number Manufacturer

U1 1A Buck Regulator LM2734ZX National Semiconductor

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

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

C3, Boost Cap 0.01µF, 16V, X7R C1005X7R1C103K TDK

D1, Catch Diode 0.4VF Schottky 1A, 30VR SS1P3L Vishay

D2, Boost Diode 1VF @ 50mA Diode 1N4148W Diodes, Inc.

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

L1 2.2µH, 1.8A ME3220–222MX Coilcraft

102kΩ, 1%

10.2kΩ, 1%

100kΩ, 1%

CRCW06031023F Vishay

CRCW06031022F Vishay

CRCW06031003F Vishay

www.national.com 18

Page 19

Physical Dimensions inches (millimeters) unless otherwise noted

LM2734Z/LM2734ZQ

6-Lead SOT23 Package

NS Package Number MK06A

6-Lead LLP Package

NS Package Number SDE06A

19 www.national.com

Page 20

Notes

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

Products Design Support

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Ethernet www.national.com/ethernet Packaging www.national.com/packaging

Interface www.national.com/interface Quality and Reliability www.national.com/quality

LVDS www.national.com/lvds Reference Designs www.national.com/refdesigns

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Switching Regulators www.national.com/switchers

LDOs www.national.com/ldo

LED Lighting www.national.com/led

PowerWise www.national.com/powerwise

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Wireless (PLL/VCO) www.national.com/wireless

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LIFE SUPPORT POLICY

LM2734Z/LM2734ZQ Thin SOT23 1A Load Step-Down DC-DC Regulator

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Datasheet LM2734Z, LM2734ZQ Datasheet (National Semiconductor)

Specifications and Main Features

Frequently Asked Questions

User Manual

General Description

Features

Applications

Typical Application Circuit

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

Block Diagram

Application Information

THEORY OF OPERATION

FIGURE 1.

BOOST FUNCTION

Design Guide

INDUCTOR SELECTION

ENABLE PIN / SHUTDOWN MODE

SOFT-START

OUTPUT OVERVOLTAGE PROTECTION

UNDERVOLTAGE LOCKOUT

CURRENT LIMIT

THERMAL SHUTDOWN

INPUT CAPACITOR

OUTPUT CAPACITOR

CATCH DIODE

BOOST DIODE

BOOST CAPACITOR

OUTPUT VOLTAGE

PCB Layout Considerations

Calculating Efficiency, and Junction Temperature

Typical Rise and Fall Times vs Input Voltage

Calculating the LM2734Z Junction Temperature

FIGURE 6. Cross-Sectional View of Integrated Circuit Mounted on a Printed Circuit Board.

LLP Package

FIGURE 7. Dog Bone

Package Selection

LM2734Z Design Examples

Bill of Materials for Figure 8

Bill of Materials for Figure 9

Bill of Materials for Figure 10

Bill of Materials for Figure 11

Bill of Materials for Figure 12

Physical Dimensions inches (millimeters) unless otherwise noted