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LM2743 Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller
January 6, 2011
LM2743
Low Voltage N-Channel MOSFET Synchronous Buck
Regulator Controller
General Description
The LM2743 is a high-speed synchronous buck regulator
controller with an accurate feedback voltage accuracy of
±2%. It can provide simple down conversion to output volt-
ages as low as 0.6V. Though the control sections of the IC
are rated for 3 to 6V, the driver sections are designed to ac-
cept input supply rails as high as 16V. 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 pro-
viding a simple, adjustable current limit.
The fixed-frequency voltage-mode PWM control architecture
is adjustable from 50 kHz to 1 MHz with one external resistor.
This wide range of switching frequency gives the power sup-
ply designer the flexibility to make better tradeoffs between
component size, cost and efficiency.
Features include soft-start, input under-voltage lockout (UV-
LO) and Power Good (based on both under-voltage and over-
voltage 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.
Features
■
■
■
■
■
■
■
■
■
Power stage input voltage from 1V to 16V
Control stage input voltage from 3V to 6V
Output voltage adjustable down to 0.6V
Power good flag and shutdown
Output over-voltage and under-voltage detection
±2% feedback voltage accuracy over temperature
Low-side adjustable current sensing
Adjustable soft-start
Tracking and sequencing with shutdown and soft-start
pins
■
Switching frequency from 50 kHz to 1 MHz
■
TSSOP-14 package
Applications
■
■
■
■
■
3.3V Buck Regulation
Cable Modem, DSL and ADSL
Laser Jet and Ink Jet Printers
Low Voltage Power Modules
DSP, ASIC, Core and I/O
Typical Application
20095201
© 2011 National Semiconductor Corporation
200952
www.national.com
LM2743
Connection Diagram
20095202
14-Lead Plastic TSSOP
θ
JA
= 155°C/W
Ordering Information
Order Number
LM2743MTC
LM2743MTCX
Package Type
TSSOP-14
TSSOP-14
NSC Package Drawing
MTC14
MTC14
Supplied As
94 Units, Raill
2500 Units on Tape and Reel
Pin Descriptions
BOOT (Pin 1)
- Bootstrap pin. This is the supply rail for the
gate drivers. When the high-side MOSFET turns on, the volt-
age on this pin should be at least one gate threshold above
the regulator input voltage V
IN
to properly turn on the MOS-
FET. See MOSFET Gate Drivers in the Application Informa-
tion 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 shoot-
through.
PGND (Pins 3, 13)
- Power ground. This is also the ground
for the low-side MOSFET driver. Both the pins must be con-
nected 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 ap-
propriately to the ground plane with due regard to good layout
practices in switching power regulator circuits.
V
CC
(Pin 5)
Supply rail for the control sections of the IC.
PWGD (Pin 6)
- Power Good pin. This is an open drain output,
which is typically meant to be connected to V
CC
or 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. For most applications a recommended value for the
pull-up resistor is 100 kΩ. The voltage on this pin is thus pulled
low under output under-voltage or over-voltage fault condi-
tions and also under input UVLO.
I
SEN
(Pin 7)
- Current limit threshold setting pin. This sources
a fixed 40 µA current. A resistor of appropriate value should
be connected between this pin and the drain of the low-side
MOSFET (switch node).
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 volt-
age. 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 re-
sistor 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 (Pin 11)
- Frequency adjust pin. The switching fre-
quency is set by connecting a resistor of suitable value be-
tween this pin and ground. The equation for calculating the
exact value is provided in Application Information, but some
typical values (rounded up to the nearest standard values) are
324 kΩ for 100 kHz, 97.6 kΩ for 300 kHz, 56.2 kΩ for 500 kHz,
24.9 kΩ for 1 MHz.
SD (Pin 12)
- IC shutdown pin. Pull this pin to V
CC
to ensure
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 sequenc-
ing 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.
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2
LM2743
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
I
SEN
All other pins
Junction Temperature
Storage Temperature
-0.3 to 7V
-0.3 to 21V
-0.3 to 9.5V
-0.3 to V
CC
+ 0.3V
150°C
−65°C to 150°C
Soldering Information
Lead Temperature
(soldering, 10sec)
Infrared or Convection (20sec)
ESD Rating (Note
3)
260°C
235°C
2 kV
Operating Ratings
Supply Voltage Range (V
CC
)
Junction Temperature Range (T
J
)
Thermal Resistance (θ
JA
)
3V to 6V
−40°C to +125°C
155°C/W
Electrical Characteristics
V
CC
= 3.3V unless otherwise indicated. Typicals and limits appearing in plain type apply for T
A
= T
J
= 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
V
FB
V
ON
Parameter
FB Pin Voltage
UVLO Thresholds
Rising
Falling
V
CC
= 3.3V, V
SD
= 3.3V
Fsw = 600kHz
V
CC
= 5V, V
SD
= 3.3V
Fsw = 600kHz
V
CC
= 3.3V, V
SD
= 0V
V
FB
Rising
V
FB
Falling
V
SS
= 0V
V
SS
= 2.5V
25
7
1.0
1.0
Conditions
V
CC
= 3V to 6V
Min
0.588
Typ
0.6
2.76
2.42
1.5
1.7
110
6
6
10
90
40
55
14
2.1
mA
2.1
185
µA
µs
µs
µA
µA
µA
Max
0.612
Units
V
V
I
Q_VCC
Operating V
CC
Current
Shutdown V
CC
Current
t
PWGD1
t
PWGD2
I
SS-ON
I
SS-OC
I
SEN-TH
GBW
G
SR
I
EAO
V
EA
PWGD Pin Response Time
PWGD Pin Response Time
SS Pin Source Current
SS Pin Sink Current During Over
Current
I
SEN
Pin Source Current Trip Point
Error Amplifier Unity Gain
Bandwidth
Error Amplifier DC Gain
Error Amplifier Slew Rate
EAO Pin Current Sourcing and
Sinking Capability
Error Amplifier Output Voltage
ERROR AMPLIFIER
9
106
3.2
V
EAO
= 1.5, FB = 0.55V
V
EAO
= 1.5, FB = 0.65V
Minimum
Maximum
2.6
9.2
1
2
MHz
dB
V/µs
mA
V
V
3
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LM2743
Symbol
GATE DRIVE
I
Q-BOOT
R
HG_UP
R
HG_DN
R
LG_UP
R
LG_DN
OSCILLATOR
Parameter
BOOT Pin Quiescent Current
Conditions
V
BOOT
= 12V, V
SD
= 0
Min
Typ
18
3
2
3
2
Max
90
Units
µA
Ω
Ω
Ω
Ω
High-Side MOSFET Driver Pull-Up
V
BOOT
= 5V @ 350mA Sourcing
ON resistance
High-Side MOSFET Driver Pull-
Down ON resistance
HG = 5V @ 350mA Sourcing
Low-Side MOSFET Driver Pull-Up
V
BOOT
= 5V @ 350mA Sourcing
ON resistance
Low-Side MOSFET Driver Pull-
Down ON resistance
LG = 5V @ 350mA Sourcing
R
FADJ
= 702.1 kΩ
f
SW
PWM Frequency
R
FADJ
= 98.74 kΩ
R
FADJ
= 45.74 kΩ
R
FADJ
= 24.91 kΩ
D
Max High-Side Duty Cycle
f
SW
= 300kHz
f
SW
= 600kHz
f
SW
= 1MHz
V
FB
= 0.575V, V
BOOT
= 3.3V, V
SD
Rising
V
FB
= 0.575V, V
BOOT
= 3.3V, V
SD
Falling
V
SD
Rising
V
SD
Falling
FB Falling
FB Rising
FB Falling
FB Rising
0.8
0.408
0.677
0.232
475
50
300
600
1000
80
76
73
1.1
%
725
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
Standby High Trip Point
Standby Low Trip Point
SD Pin Logic High Trip Point
SD Pin Logic Low Trip Point
PWGD Pin Trip Points
PWGD Pin Trip Points
PWGD Hysteresis
V
V
1.3
0.434
0.710
60
90
0.457
0.742
V
V
V
V
mV
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 16V rail (Input voltage, V
IN
). Practical lower limit of V
IN
depends on selection of the external MOSFET.
Note 3:
The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin.
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