National Semiconductor LM2742 Technical data

October 31, 2007

LM2742 N-Channel FET Synchronous Buck Regulator Controller for Low Output Voltages

General Description

The LM2742 is a high-speed, synchronous, switching regulator controller. It is intended to control currents of 0.7A to 20A with up to 95% conversion efficiencies. Power up and down sequencing is achieved with the power-good flag, adjustable soft-start and output enable features. The LM2742 operates from a low-current 5V bias and can convert from a 1V to 16V power rail. The part utilizes a fixed-frequency, voltage-mode, PWM control architecture and the switching frequency is adjustable from 50kHz to 2MHz by setting the value of an external resistor. Current limit is achieved by monitoring the voltage drop across the on-resistance of the low-side MOSFET, which enables on-times on the order of 40ns, one of the best in the industry. The wide range of operating frequencies gives the power supply designer the flexibility to fine-tune component size, cost, noise and efficiency. The adaptive, non-overlapping MOSFET gate-drivers and high-side bootstrap structure helps to further maximize efficiency. The highside power FET drain voltage can be from 1V to 16V and the output voltage is adjustable down to 0.6V.

Typical Application

Features

Input power from 1V to 16V

■

Output voltage adjustable down to 0.6V

■

Power Good flag, adjustable soft-start and output enable

■

for easy power sequencing Reference Accuracy: 1.5% (0°C - 125°C)

■

Current limit without sense resistor

■

Soft start

■

Switching frequency from 50 kHz to 2 MHz

■

40ns typical minimum on-time

■

TSSOP-14 package

■

Applications

POL power supply modules

■

Cable Modems

■

Set-Top Boxes/ Home Gateways

■

DDR Core Power

■

High-Efficiency Distributed Power

■

Local Regulation of Core Power

■

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Connection Diagram

LM2742

14-Lead Plastic TSSOP

θJA = 155°C/W

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Ordering Information

Order Number Package Type NSC Package Drawing Supplied As

LM2742MTC TSSOP-14 MTC14 94 Units, Raill

LM2742MTCX TSSOP-14 MTC14 2500 Units on Tape and Reel

Pin Descriptions

BOOT (Pin 1) - Supply rail for the N-channel MOSFET gate

drive. The voltage should be at least one gate threshold above the regulator input voltage to properly turn on the high-side NFET.

LG (Pin 2) - Gate drive for the low-side N-channel MOSFET. This signal is interlocked with HG to avoid shoot-through problems.

PGND (Pins 3, 13) - Ground for FET drive circuitry. It should be connected to system ground.

SGND (Pin 4) - Ground for signal level circuitry. It should be connected to system ground.

VCC (Pin 5) - Supply rail for the controller. PWGD (Pin 6) - Power Good. This is an open drain output.

The pin is pulled low when the chip is in UVP, OVP, or UVLO mode. During normal operation, this pin is connected to V or other voltage source through a pull-up resistor.

ISEN (Pin 7) - Current limit threshold setting. This sources a fixed 50µA current. A resistor of appropriate value should be connected between this pin and the drain of the low-side FET.

EAO (Pin 8) - Output of the error amplifier. The voltage level on this pin is compared with an internally generated ramp

signal to determine the duty cycle. This pin is necessary for compensating the control loop.

SS (Pin 9) - Soft start pin. A capacitor connected between this pin and ground sets the speed at which the output voltage ramps up. Larger capacitor value results in slower output voltage ramp but also lower inrush current.

FB (Pin 10) - This is the inverting input of the error amplifier, which is used for sensing the output voltage and compensating the control loop.

FREQ (Pin 11) - The switching frequency is set by connecting a resistor between this pin and ground.

SD (Pin 12) - IC Logic Shutdown. When this pin is pulled low the chip turns off both the high side and low side switches. While this pin is low, the IC will not start up. An internal 20µA pull-up connects this pin to VCC. For a device which turns on the low side switch during shutdown, see the pin compatible LM2737.

HG (Pin 14) - Gate drive for the high-side N-channel MOSFET. This signal is interlocked with LG to avoid shoot-through problems.

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Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications.

BOOTV 21V LG and HG to GND (Note 3) -2V to 21V Junction Temperature 150°C Storage Temperature −65°C to 150°C

Lead Temperature (soldering, 10sec) 260°C

Infrared or Convection (20sec) 235°C ESD Rating 2 kV

Operating Ratings

Supply Voltage (VCC)

Junction Temperature Range −40°C to +125°C

Thermal Resistance (θJA)

4.5V to 5.5V

155°C/W

Soldering Information

Electrical Characteristics

VCC = 5V unless otherwise indicated. Typicals and limits appearing in plain type apply for TA=TJ=+25°C. Limits appearing in boldface type apply over full Operating Temperature Range. Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.

Symbol Parameter Conditions Min Typ Max Units

FB_ADJ

Q-V5

PWGD1

PWGD2

SS-ON

SS-OC

VCC = 4.5V, 0°C to +125°C

VCC = 5V, 0°C to +125°C

FB Pin Voltage

VCC = 5.5V, 0°C to +125°C

VCC = 4.5V, −40°C to +125°C

VCC = 5V, −40°C to +125°C

VCC = 5.5V, −40°C to +125°C

UVLO Thresholds Rising

Falling

SD = 5V, FB = 0.55V Fsw = 600kHz

Operating VCC Current

SD = 5V, FB = 0.65V Fsw = 600kHz 0.8 1.7

Shutdown VCC Current SD = 0V 0.15 0.4 0.7 mA

PWGD Pin Response Time FB Voltage Going Up 6 µs

PWGD Pin Response Time FB Voltage Going Down 6 µs

SD Pin Internal Pull-up Current 20 µA

SS Pin Source Current SS Voltage = 2.5V

0°C to +125°C

-40°C to +125°C

SS Pin Sink Current During Over

SS Voltage = 2.5V

Current

0.591 0.6 0.609

0.589 0.6 0.609

4.2

3.6

1 1.5 2

2.2

8 5

11 11

15 15

µA

Symbol Parameter Conditions Min Typ Max Units

LM2742

GATE DRIVE

Q-BOOT

DS1

DS2

DS3

DS4

BOOT Pin Quiescent Current BOOT = 12V, EN = 0

0°C to +125°C

-40°C to +125°C

Top FET Driver Pull-Up ON resistance

Top FET Driver Pull-Down ON resistance

Bottom FET Driver Pull-Up ON resistance

Bottom FET Driver Pull-Down ON resistance

BOOT-SW = 5V@350mA

95 95

OSCILLATOR

300

500 600 700

490 600 700

1400

2000

OSC

PWM Frequency

D Max Duty Cycle f

on-min

Minimum on-time 40 ns

= 590kΩ

FADJ

= 88.7kΩ

FADJ

= 42.2kΩ, 0°C to +125°C

FADJ

= 42.2kΩ, -40°C to +125°C

FADJ

= 17.4kΩ

FADJ

= 11.3kΩ

FADJ

= 300kHz

PWM

= 600kHz

PWM

LOGIC INPUTS AND OUTPUTS

SD-IH

SD-IL

PWGD-TH-LO

PWGD-TH-HI

PWGD-HYS

SD Pin Logic High Trip Point 2.6 3.5 V

SD Pin Logic Low Trip Point 0°C to +125°C

-40°C to +125°C

1.3

1.25

1.6

PWGD Pin Trip Points FB Voltage Going Down

0°C to +125°C

-40°C to +125°C

0.413

0.410

0.430

0.446

PWGD Pin Trip Points FB Voltage Going Up

0°C to +125°C

-40°C to +125°C

PWGD Hysteresis FB Voltage Going Down FB Voltage

Going Up

0.691

0.688

0.710

110

0.734

160 215

µA

Ω

kHz

Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Operating ratings indicate conditions for which the device operates correctly. Operating Ratings do not imply guaranteed performance limits.

Note 2: The human body model is a 100pF capacitor discharged through a 1.5k resistor into each pin.

Note 3: The LG and HG pin can have -2V to -0.5V applied for a maximum duty cycle of 10% with a maximum period of 1 second. There is no duty cycle or

maximum period limitation for a LG and HG pin voltage range of -0.5V to 21V.

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Typical Performance Characteristics

LM2742

Efficiency (VO = 1.5V)

FSW = 300kHz, TA = 25°C

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VCC Operating Current vs Temperature

FSW = 600kHz, No-Load

Efficiency (VO = 3.3V)

FSW = 300kHz, TA = 25°C

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Bootpin Current vs Temperature for BOOTV = 12V

FSW = 600kHz, Si4826DY FET, No-Load

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Bootpin Current vs Temperature with 5V Bootstrap

FSW = 600kHz, Si4826DY FET, No-Load

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PWM Frequency vs Temperature

for R

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FADJ

= 43.2kΩ

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LM2742

vs PWM Frequency

FADJ

(in 100 to 800kHz range), TA = 25°C

vs PWM Frequency

FADJ

(in 900 to 2000kHz range), TA = 25°C

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VCC Operating Current Plus Boot Current vs

PWM Frequency (Si4826DY FET, TA = 25°C)

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Switch Waveforms (HG Rising)

VIN = 5V, VO = 1.8V

IO = 3A, FSW = 600kHz

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Switch Waveforms (HG Falling)

VIN = 5V, VO = 1.8V

IO = 3A, CSS = 10nF

FSW = 600kHz

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Start-Up (No-Load)

VIN = 10V, VO = 1.2V

CSS = 10nF, FSW = 300kHz

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LM2742

Start-Up (Full-Load) VIN = 10V, VO = 1.2V IO = 10A, CSS = 10nF

FSW = 300kHz

Start Up (Full Load, 10x CSS)

VIN = 10V, VO = 1.2V

IO = 10A, CSS = 100nF

FSW = 300kHz

Start Up (No-Load, 10x CSS)

VIN = 10V, VO = 1.2V

CSS = 100nF, FSW = 300kHz

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Start Up (Into 1.2V Pre-Bias)

VIN = 12V, VO = 2.5V

No Load, No Soft Start Capacitor

FSW = 300kHz

Start Up (Into 1.2V Pre-Bias)

VIN = 12V, VO = 2.5V

No Load, CSS = 10nF

FSW = 300kHz

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Shutdown

VIN = 12V, VO = 1.2V

IO = 10A, CSS = 10nF

FSW = 300kHz

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LM2742

Shutdown (No Load)

VIN = 12V, VO = 1.2V

IO = 10A, CSS = 10nF

FSW = 300kHz

Load Transient Response (IO = 0 to 4A)

VIN = 12V, VO = 1.2V

FSW = 300kHz

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Load Transient Response (IO = 4 to 0A)

VIN = 12V, VO = 1.2V

FSW = 300kHz

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Line Transient Response (VIN =12V to 5V)

VO = 1.2V, IO = 5A

FSW = 300kHz

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Line Transient Response (VIN =5V to 12V)

VO = 1.2V, IO = 5A

FSW = 300kHz

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Line Transient Response

VO = 1.2V, IO = 5A

FSW = 300kHz

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