National Semiconductor LM2747 Technical data

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LM2747
Synchronous Buck Controller with Pre-bias Startup, and
Optional Clock Synchronization

LM2747 Synchronous Buck Controller with Pre-bias Startup, and Optional Clock Synchronization

March 2006

General Description

The LM2747 is a high-speed synchronous buck regulator
controller with a feedback voltage accuracy of
provide simple down conversion to output voltages as low as

0.6V. Though the control section of the IC is rated for 3 to 6V,
the driver section is designed to accept input supply rails as
high as 14V. The use of adaptive non-overlapping MOSFET
gate drivers helps avoid potential shoot-through problems
while maintaining high efficiency. The IC is designed for the
more cost-effective option of driving only N-channel MOS­FETs in both the high-side and low-side positions. It senses
the low-side switch voltage drop for providing a simple,
adjustable current limit.

The LM2747 features a fixed-frequency voltage-mode PWM
control architecture which is adjustable from 50 kHz to 1
MHz with one external resistor. In addition, the LM2747 also
allows the switching frequency to be synchronized to an
external clock signal over the range of 250 kHz to 1 MHz.
This wide range of switching frequency gives the power
supply designer the flexibility to make better tradeoffs be­tween component size, cost and efficiency.

Features include the ability to startup with a pre-biased load
on the output, soft-start, input undervoltage lockout (UVLO)
and Power Good (based on both undervoltage and overvolt­age detection). In addition, the shutdown pin of the IC can be
used for providing startup delay, and the soft-start pin can be
used for implementing precise tracking, for the purpose of
sequencing with respect to an external rail.

±

1%. It can

Features

n±1% feedback voltage accuracy over temperature
n Switching frequency from 50 kHz to 1 MHz
n Switching frequency synchronize range 250 kHz to 1

MHz

n Startup with a pre-biased output load
n Power stage input voltage from 1V to 14V
n Control stage input voltage from 3V to 6V
n Output voltage adjustable down to 0.6V
n Power Good flag and shutdown
n Output overvoltage and undervoltage detection
n Low-side adjustable current sensing
n Adjustable soft-start
n Tracking and sequencing with shutdown and soft start

pins

n TSSOP-14 package

Applications

n Down Conversion from 3.3V
n Cable Modem, DSL and ADSL
n Laser Jet and Ink Jet Printers
n Low Voltage Power Modules
n DSP, ASIC, Core and I/O

Typical Application

20150901

© 2006 National Semiconductor Corporation DS201509 www.national.com

Connection Diagram

LM2747

14-Lead Plastic TSSOP

20150990

= 155˚C/W

θ

JA

NS Package Number MTC14

Ordering Information

Order Number Package Type NSC Package Drawing Supplied As

LM2747MTC

LM2747MTCX 2500 Units on Tape and Reel

Pin Description

BOOT (Pin 1) - Bootstrap pin. This is the supply rail for the

high-side gate driver. When the high-side MOSFET turns on,
the voltage on this pin should be at least one gate threshold
above the regulator input voltage V
MOSFET. See MOSFET Gate Drivers in the Application
Information section for more details on how to select MOS­FETs.

LG (Pin 2) - Low-gate drive pin. This is the gate drive for the
low-side N-channel MOSFET. This signal is interlocked with
the high-side gate drive HG (Pin 14), so as to avoid shoot­through.

PGND (Pins 3, 13) - Power ground. This is also the ground
for the low-side MOSFET driver. Both the pins must be
connected together on the PCB and form a ground plane,
which is usually also the system ground.

SGND (Pin 4) - Signal ground. It should be connected
appropriately to the ground plane with due regard to good
layout practices in switching power regulator circuits.

(Pin 5) Supply rail for the control sections of the IC.

V

CC

PWGD (Pin 6) - Power Good pin. This is an open drain

output, which is typically meant to be connected to V
any other low voltage source through a pull-up resistor.
Choose the pull-up resistor so that the current going into this
pin is kept below 1 mA. A recommended value for the pull-up
resistor is 100 kΩ for most applications. The voltage on this
pin is thus pulled low under output undervoltage or overvolt­age fault conditions and also under input UVLO.

(Pin 7) - Current limit threshold setting pin. This sources

I

SEN

a fixed 40 µA current. A resistor of appropriate value should
be connected between this pin and the drain of the low-side

TSSOP-14 MTC14

to properly turn on the

IN

or

CC

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/TRACK (Pin 9) - Soft-start and tracking pin. This pin is
internally connected to the non-inverting input of the error
amplifier during soft-start, and in fact any time the SS/
TRACK pin voltage happens to be below the internal refer­ence voltage. For the basic soft-start function, a capacitor of
minimum value 1 nF is connected from this pin to ground. To
track the rising ramp of another power supply’s output, con­nect a resistor divider from the output of that supply to this
pin as described in Application Information.

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

FREQ/SYNC (Pin 11) - Frequency adjust pin. The switching
frequency is set by connecting a resistor of suitable value
between this pin and ground. Some typical values (rounded
up to the nearest standard values) are 150 kΩ for 200 kHz,
100 kΩ for 300 kHz, 51.1 kΩ for 500 kHz, 18.7 kΩ for 1 MHz.
This pin is also used to synchronize to an external clock
within the range of 250kHz to 1MHz.

SD (Pin 12) - IC shutdown pin. Pull this pin to V
the IC is enabled. Connect to ground to disable the IC. Under
shutdown, both high-side and low-side drives are off. This
pin also features a precision threshold for power supply
sequencing purposes, as well as a low threshold to ensure
minimal quiescent current.

HG (Pin 14) - High-gate drive pin. This is the gate drive for
the high-side N-channel MOSFET. This signal is interlocked

with LG (Pin 2) to avoid shoot-through.
MOSFET (switch node). The minimum value for this resistor
is1kΩ.

94 Units on Rail

to ensure

CC

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LM2747

Absolute Maximum Ratings (Note 1)

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

V

CC

BOOT Voltage -0.3 to 18V

I

SEN

FREQ/SYNC Voltage -0.5 to V

All other pins -0.3 to V

Junction Temperature 150˚C

Storage Temperature −65˚C to 150˚C

-0.3 to 7V

-0.3 to 14V

+ 0.3V

CC

+ 0.3V

CC

Soldering Information

Lead Temperature (soldering, 10sec) 260˚C

Infrared or Convection (20sec) 235˚C

ESD Rating (Note 3) 2kV

Operating Ratings

Supply Voltage Range, VCC(Note 2) 3V to 6V

BOOT Voltage Range 1V to 17V

Junction Temperature Range (T

Thermal Resistance (θ

) 155˚C/W

JA

) −40˚C to +125˚C

J

Electrical Characteristics

VCC= 3.3V 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

V

FB

V

ON

I

Q_VCC

t

PWGD1

t

PWGD2

I

SS-ON

I

SS-OC

I

SEN-TH

I

FB

ERROR AMPLIFIER

GBW Error Amplifier Unity Gain

G Error Amplifier DC Gain 118 dB

SR Error Amplifier Slew Rate 2 V/µs

I

EAO

V

EAO

FB Pin Voltage VCC=3Vto6V 0.594 0.6 0.606 V

UVLO Thresholds VCCRising

Falling

V

CC

V

= 3.3V, VSD= 3.3V

CC

= 600 kHz

f

Operating VCCCurrent

Shutdown V

Current VCC= 3.3V, VSD=0V 1 3 µA

CC

SW

V

= 5V, VSD= 3.3V

CC

= 600 kHz

f

SW

1.1 1.7 2.3

1.3 2 2.6

2.79

2.42

PWGD Pin Response Time VFBRising 10 µs

PWGD Pin Response Time VFBFalling 10 µs

SS Pin Source Current VSS=0V 7 10 14 µA

SS Pin Sink Current During Over
Current

I

Pin Source Current Trip

SEN

Point

VSS= 2.0V

90 µA

25 40 55 µA

FB Pin Current Sourcing 20 nA

Bandwidth

EAO Pin Current Sourcing and
Sinking Capability

9 MHz

14
16

Error Amplifier Output Voltage Minimum 1 V

Maximum 2.2 V

V

mA

mA

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Electrical Characteristics (Continued)

VCC= 3.3V unless otherwise indicated. Typicals and limits appearing in plain type apply for TA=TJ= 25˚C. Limits appearing in

LM2747

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

GATE DRIVE

I

Q-BOOT

R

HG_UP

BOOT Pin Quiescent Current V

High-Side MOSFET Driver
Pull-Up ON resistance

R

HG_DN

High-Side MOSFET Driver
Pull-Down ON resistance

R

LG_UP

Low-Side MOSFET Driver Pull-Up
ON resistance

R

LG_DN

Low-Side MOSFET Driver
Pull-Down ON resistance

OSCILLATOR

PWM Frequency

f

SW

External Synchronizing Signal
Frequency

SYNC

SYNC

D

MAX

Synchronization Signal Low

L

Threshold

Synchronization Signal High

H

Threshold

Max High-Side Duty Cycle fSW= 300 kHz

LOGIC INPUTS AND OUTPUTS

V

STBY-IH

V

STBY-IL

V

SD-IH

V

SD-IL

V

PWGD-TH-LO

V

PWGD-TH-HI

V

PWGD-HYS

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 power MOSFETs can run on a separate 1V to 14V rail (Input voltage, V
See the MOSFET GATE DRIVERS section under Application Information for further details.

Note 3: ESD using the human body model which is a 100pF capacitor discharged through a 1.5 kΩ resistor into each pin.

Standby High Trip Point VFB= 0.575V, V

Standby Low Trip Point VFB= 0.575V, V

SD Pin Logic High Trip Point VSDRising 1.3 V

SD Pin Logic Low Trip Point VSDFalling 0.8 V

PWGD Pin Trip Points VFBFalling 0.408 0.434 0.457 V

PWGD Pin Trip Points VFBRising 0.677 0.710 0.742 V

PWGD Hysteresis VFBFalling

= 12V, VSD=0 18 90 µA

BOOT

=5V@350 mA Sourcing 2.7 Ω

V

BOOT

350 mA Sinking 0.8 Ω

=5V@350 mA Sourcing 2.7 Ω

V

BOOT

350 mA Sinking 0.8 Ω

= 750 kΩ 50

R

FADJ

R

= 100 kΩ 300

FADJ

R

= 42.2 kΩ 475 600 725

FADJ

R

= 18.7 kΩ 1000

FADJ

Voltage Swing = 0V to V

= 250 kHz to 1 MHz 1 V

f

SW

= 250 kHz to 1 MHz 2 V

f

SW

CC

250 1000

86

= 600 kHz

f

SW

f

SW

V

SD

V

SD

= 1 MHz

Rising

Falling

BOOT

BOOT

= 3.3V

= 3.3V

0.232

78
67

60

Rising

V

FB

). Practical lower limit of VINdepends on selection of the external MOSFET.

IN

90

1.1

kHz

%

V

V

mV

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Typical Performance Characteristics

LM2747

Efficiency (V

= 3.3V, fSW= 1 MHz Internal Reference Voltage vs Temperature

V

CC

OUT

= 1.2V)

20150940 20150958

Frequency vs Temperature Output Voltage vs Output Current

Switch Waveforms

= 3.3V, VIN= 5V, V

V

CC

= 3A, CSS= 12 nF, fSW= 1 MHz

I

OUT

OUT

20150960 20150956

Start-Up (Full-Load)

= 1.2V

20150946

V

= 3.3V, VIN= 5V, V

CC

= 3A, CSS= 12 nF, fSW= 1 MHz

I

OUT

OUT

= 1.2V

20150948

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Typical Performance Characteristics (Continued)

LM2747

Start-Up (No-Load)

V

= 3.3V, VIN= 5V, V

CC

= 12 nF, fSW= 1 MHz

C

SS

Load Transient Response

= 3.3V, VIN= 14V, V

V

CC

f

SW

= 1 MHz

OUT

OUT

Shutdown (Full-Load)

= 1.2V

20150949 20150950

= 1.2V

V

= 3.3V, VIN= 5V, V

CC

= 3A, CSS= 12 nF, fSW= 1 MHz

I

OUT

Line Transient Response (V

= 3.3V, V

V

CC

= 2A, fSW= 1 MHz

I

OUT

OUT

OUT

IN

= 1.2V

= 1.2V

=3Vto9V)

20150953 20150954

Frequency vs. Frequency Adjust Resistor

20150955

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Maximum Duty Cycle vs Frequency

VCC= 3.3V

20150992

Typical Performance Characteristics (Continued)

LM2747

Maximum Duty Cycle vs V

fSW= 600 kHz

CC

Maximum Duty Cycle vs V

fSW= 1 MHz

20150993 20150994

CC

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