Texas Instruments LM2734 Manual de usuario

LM2734

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SNVS288I –SEPTEMBER 2004–REVISED APRIIL 2013

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

Check for Samples: LM2734

1FEATURES DESCRIPTION

The LM2734 regulator is a monolithic, high frequency,

234• Thin SOT-6 Package PWM step-down DC/DC converter in a 6-pin Thin

• 3.0V to 20V Input Voltage Range SOT package. It provides all the active functions to

• 0.8V to 18V Output Voltage Range provide local DC/DC conversion with fast transient

response and accurate regulation in the smallest

• 1A Output Current possible PCB area.

• 550kHz (LM2734Y) and 1.6MHz (LM2734X) With a minimum of external components and online

Switching Frequencies design support through WEBENCH®, the LM2734 is

• 300mΩNMOS Switch easy to use. The ability to drive 1A loads with an

• 30nA Shutdown Current internal 300mΩNMOS switch using state-of-the-art

• 0.8V, 2% Internal Voltage Reference 0.5µm BiCMOS technology results in the best power

density available. The world class control circuitry

• Internal Soft-Start allows for on-times as low as 13ns, thus supporting

• Current-Mode, PWM Operation exceptionally high frequency conversion over the

• WEBENCH®Online Design Tool entire 3V to 20V input operating range down to the

minimum output voltage of 0.8V. Switching frequency

• Thermal Shutdown is internally set to 550kHz (LM2734Y) or 1.6MHz

• LM2734XQ/LM2734YQ are AEC-Q100 Grade 1 (LM2734X), allowing the use of extremely small

Qualified and are Manufactured on an surface mount inductors and chip capacitors. Even

Automotive Grade Flow though the operating frequencies are very high,

efficiencies up to 90% are easy to achieve. External

APPLICATIONS shutdown is included, featuring an ultra-low stand-by

current of 30nA. The LM2734 utilizes current-mode

• Local Point of Load Regulation control and internal compensation to provide high-

• Core Power in HDDs performance regulation over a wide range of

• Set-Top Boxes operating conditions. Additional features include

internal soft-start circuitry to reduce inrush current,

• Battery Powered Devices pulse-by-pulse current limit, thermal shutdown, and

• USB Powered Devices output over-voltage protection.

• DSL Modems

• Notebook Computers

• Automotive

1Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of

Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.

2WEBENCH is a registered trademark of Texas Instruments, Inc..

3WEBENCH is a registered trademark of Texas Instruments.

4All other trademarks are the property of their respective owners.

Products conform to specifications per the terms of the Texas

Instruments standard warranty. Production processing does not

necessarily include testing of all parameters.

BOOST

GND

VIN

LM2734

VIN VIN

BOOST

GND

VOUT

C3 L1

OFF

LM2734

SNVS288I –SEPTEMBER 2004–REVISED APRIIL 2013

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Typical Application Circuit

Figure 1. Figure 2. Efficiency vs Load Current

VIN = 5V, VOUT = 3.3V

Connection Diagram

Figure 3. 6-Lead SOT Figure 4. Pin 1 Indentification

See Package Number DDC (R-PDSO-G6)

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 VIN + 0.3V.

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

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

capacitor.

These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam

during storage or handling to prevent electrostatic damage to the MOS gates.

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SNVS288I –SEPTEMBER 2004–REVISED APRIIL 2013

Absolute Maximum Ratings(1)(2)

VIN -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(3) 2kV

Storage Temp. Range -65°C to 150°C

Soldering Information Reflow Peak Pkg. Temp.(15sec) 260°C

(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 ensured. For specific specifications and the test conditions,

see Electrical Characteristics.

(2) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and

specifications.

(3) Human body model, 1.5kΩin series with 100pF.

Operating Ratings(1)

VIN 3V to 20V

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(2) 118°C/W

(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 ensured. For specific specifications and the test conditions,

see Electrical Characteristics.

(2) 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 2oz. copper on 4 layers in still air. For a 2 layer board using 1 oz. copper in still

air, θJA = 204°C/W.

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SNVS288I –SEPTEMBER 2004–REVISED APRIIL 2013

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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, VBOOST - VSW = 5V unless otherwise specified. Datasheet min/max

specification limits are ensured by design, test, or statistical analysis.

Symbol Parameter Conditions Min(1) Typ(2) Max(1) Units

VFB Feedback Voltage 0.784 0.800 0.816 V

ΔVFB/ΔVIN Feedback Voltage Line Regulation VIN = 3V to 20V 0.01 % / V

IFB Feedback Input Bias Current Sink/Source 10 250 nA

Undervoltage Lockout VIN Rising 2.74 2.90

UVLO Undervoltage Lockout VIN Falling 2.0 2.3 V

UVLO Hysteresis 0.30 0.44 0.62

LM2734X 1.2 1.6 1.9

FSW Switching Frequency MHz

LM2734Y 0.40 0.55 0.66

LM2734X 85 92

DMAX Maximum Duty Cycle %

LM2734Y 90 96

LM2734X 2

DMIN Minimum Duty Cycle %

LM2734Y 1

RDS(ON) Switch ON Resistance VBOOST - VSW = 3V 300 600 mΩ

ICL Switch Current Limit VBOOST - VSW = 3V 1.2 1.7 2.5 A

IQQuiescent Current Switching 1.5 2.5 mA

Quiescent Current (shutdown) VEN = 0V 30 nA

LM2734X (50% Duty Cycle) 2.5 3.5

IBOOST Boost Pin Current mA

LM2734Y (50% Duty Cycle) 1.0 1.8

Shutdown Threshold Voltage VEN Falling 0.4

VEN_TH V

Enable Threshold Voltage VEN Rising 1.8

IEN Enable Pin Current Sink/Source 10 nA

ISW Switch Leakage 40 nA

(1) Specified to Average Outgoing Quality Level (AOQL).

(2) Typicals represent the most likely parametric norm.

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SNVS288I –SEPTEMBER 2004–REVISED APRIIL 2013

Typical Performance Characteristics

All curves taken at VIN = 5V, VBOOST - VSW = 5V, L1 = 4.7 µH ("X"), L1 = 10 µH ("Y"), and TA= 25°C, unless specified

otherwise.

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

Figure 5. Figure 6.

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

Figure 7. Figure 8.

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

Figure 9. Figure 10.

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Typical Performance Characteristics (continued)

All curves taken at VIN = 5V, VBOOST - VSW = 5V, L1 = 4.7 µH ("X"), L1 = 10 µH ("Y"), and TA= 25°C, unless specified

otherwise. Oscillator Frequency vs Temperature - "X" Oscillator Frequency vs Temperature - "Y"

Figure 11. Figure 12.

Current Limit vs Temperature Current Limit vs Temperature

VIN = 5V VIN = 20V

Figure 13. Figure 14.

VFB vs Temperature RDSON vs Temperature

Figure 15. Figure 16.

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SNVS288I –SEPTEMBER 2004–REVISED APRIIL 2013

Typical Performance Characteristics (continued)

All curves taken at VIN = 5V, VBOOST - VSW = 5V, L1 = 4.7 µH ("X"), L1 = 10 µH ("Y"), and TA= 25°C, unless specified

otherwise. Line Regulation - "X"

IQSwitching vs Temperature VOUT = 1.5V, IOUT = 500mA

Figure 17. Figure 18.

Line Regulation - "Y" Line Regulation - "X"

VOUT = 1.5V, IOUT = 500mA VOUT = 3.3V, IOUT = 500mA

Figure 19. Figure 20.

Line Regulation - "Y"

VOUT = 3.3V, IOUT = 500mA

Figure 21.

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BOOST

Output

Control

Logic

Current

Limit

Thermal

Shutdown

Under

Voltage

Lockout

Corrective Ramp

Reset

Pulse

PWM

Comparator

Current-Sense Amplifier RSENSE

Internal

Regulator

and

Enable

Circuit

Oscillator

Driver 0.3:

Switch

Internal

Compensation

GND

Error Amplifier -

+VREF

0.8V

COUT

OFF

VBOOST

VSW

CBOOST

VOUT

CIN

VIN

ISENSE

-0.88V

OVP

Comparator

Error

Signal

LM2734

SNVS288I –SEPTEMBER 2004–REVISED APRIIL 2013

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Block Diagram

Figure 22.

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VIN

TON

Inductor

Current

D = TON/TSW

VSW

TOFF

TSW

IPK

Voltage

LM2734

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SNVS288I –SEPTEMBER 2004–REVISED APRIIL 2013

APPLICATION INFORMATION

THEORY OF OPERATION

The LM2734 is a constant frequency PWM buck regulator IC that delivers a 1A load current. The regulator has a

preset switching frequency of either 550kHz (LM2734Y) or 1.6MHz (LM2734X). These high frequencies allow the

LM2734 to operate with small surface mount capacitors and inductors, resulting in DC/DC converters that require

a minimum amount of board space. The LM2734 is internally compensated, so it is simple to use, and requires

few external components. The LM2734 uses current-mode control to regulate the output voltage.

The following operating description of the LM2734 will refer to the Simplified Block Diagram (Figure 22) and to

the waveforms in Figure 23. The LM2734 supplies a regulated output voltage by switching the internal NMOS

control switch at constant frequency and variable duty cycle. A switching cycle begins at the falling edge of the

reset pulse generated by the internal oscillator. When this pulse goes low, the output control logic turns on the

internal NMOS control switch. During this on-time, the SW pin voltage (VSW) swings up to approximately VIN, and

the inductor current (IL) increases with a linear slope. ILis 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 VREF. When the PWM comparator output goes high, the output 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.

Figure 23. LM2734 Waveforms of SW Pin Voltage and Inductor Current

BOOST FUNCTION

Capacitor CBOOST and diode D2 in Figure 24 are used to generate a voltage VBOOST. VBOOST - VSW is the gate

drive voltage to the internal NMOS control switch. To properly drive the internal NMOS switch during its on-time,

VBOOST needs to be at least 1.6V greater than VSW. Although the LM2734 will operate with this minimum voltage,

it may not have sufficient gate drive to supply large values of output current. Therefore, 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.

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

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LM2734

BOOST

GND

COUT

CBOOST

VOUT

CIN

VIN

VBOOST

LM2734

SNVS288I –SEPTEMBER 2004–REVISED APRIIL 2013

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Figure 24. VOUT Charges CBOOST

When the LM2734 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 (VIN)

2. From the output voltage (VOUT)

3. From an external distributed voltage rail (VEXT)

4. From a shunt or series zener diode

In the Simplifed Block Diagram of Figure 22, capacitor CBOOST and diode D2 supply the gate-drive current 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 23), 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 (1)

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

VSW = VIN – (RDSON x IL), (2)

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

VBOOST = 2VIN – (RDSON x IL) – VFD2 + VFD1 (3)

which is approximately

2VIN - 0.4V (4)

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

VIN - 0.2V (5)

An alternate method for charging CBOOST is to connect D2 to the output as shown in Figure 24. 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.

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 voltage by placing a zener diode D3 in series with D2, as shown in Figure 25. 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) > 1.6V

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