LM2746
Low Voltage N-Channel MOSFET Synchronous Buck
Regulator Controller
General Description
The LM2746 is a high-speed synchronous buck regulator
controller with a feedback voltage accuracy of
1%. It can
provide simple down conversion to output voltages as low as
0.6V. Though the control sections of the IC are rated for 3 to
5.5V, the driver sections are designed to accept input supply
rails as high as 16V. The use of 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 MOS-
FETs 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 fixed-frequency voltage-mode PWM control architecture
is adjustable from 50 kHz to 1 MHz with one external resis-
tor. This wide range of switching frequency gives the power
supply designer the flexibility to make better tradeoffs be-
tween component size, cost and efficiency.
Features include 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.
Features
n
Power stage input voltage from 1V to 16V
n
Control stage input voltage from 3V to 5.5V
n
Output voltage adjustable down to 0.6V
n
Power good flag and shutdown
n
Output overvoltage and undervoltage detection
n
1% feedback voltage accuracy over temperature
n
Low-side adjustable current sensing
n
Adjustable soft-start
n
Tracking and sequencing with shutdown and soft start
pins
n
Switching frequency from 50 kHz to 1 MHz
n
Exposed pad TSSOP-14 package
Applications
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
20147701
May 2005
LM2746
Low
V
oltage
N-Channel
MOSFET
Synchronous
Buck
Regulator
Controller
2005 National Semiconductor Corporation
DS201477
www.national.com
Connection Diagram
20147702
14-Lead Plastic eTSSOP
(MXA14A)
Ordering Information
Order Number
Package Type
NSC Package Drawing
Supplied As
LM2746MXA
eTSSOP-14
MXA14A
94 Units, Rail
LM2746MXAX
eTSSOP-14
MXA14A
2500 Units on Tape and Reel
Pin Description
BOOT (Pin 1) - Bootstrap pin. This is the supply rail for the
gate drivers. 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
IN
to properly turn on the
MOSFET. See MOSFET Gate Drivers in the Application
Information section for more details on how to select MOS-
FETs.
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 pins must be con-
nected together on the PCB to form a ground plane, which is
usually 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.
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 recom-
mended value for the pull-up resistor is 100 k
. The voltage
on this pin is thus pulled low under output undervoltage or
overvoltage fault conditions and also under input UVLO.
I
SEN
(Pin 7) - Current limit threshold setting pin. This sources
a fixed 40 A current. A resistor of appropriate value should
be connected between this pin and the drain of the low-side
MOSFET (switch node). The minimum value for this resistor
is 1 k
.
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 refer-
ence voltage. For the basic soft-start function, a capacitor of
minimum value 1nF is connected from this pin to ground. To
track the rising ramp of another power supply's output, con-
nect 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 (Pin 11) - Frequency adjust pin. The switching fre-
quency is set by connecting a resistor of suitable value
between 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 k
for 100 kHz, 97.6 k for 300 kHz, 56.2
k
for 500 kHz, 24.9 k 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
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.
EXPOSED PAD - Connect to ground.
LM2746
www.national.com
2
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
-0.3 to 6.25V
BOOT Voltage
-0.3 to 21V
All other pins
-0.3 to V
CC
+ 0.3V
Junction Temperature
150C
Storage Temperature
-65C to 150C
Soldering Information
Lead Temperature
(soldering, 10sec)
260C
Infrared or Convection (20sec)
235C
ESD Rating (Note 2)
2 kV
Operating Ratings
Supply Voltage Range (V
CC
)
3V to 5.5V
Power Stage Input Voltage (V
IN
)
1V to 16V
Junction Temperature Range
(T
J
)
-40C to +125C
Thermal Resistance (
JA
)
38C/W
Electrical Characteristics
V
CC
= 3.3V unless otherwise indicated. Limits in standard type are for T
J
= 25C only; limits in boldface type apply over the
junction temperature (T
J
) range of -40C to +125C. Minimum and Maximum limits are guaranteed through test, design, or sta-
tistical correlation. Typical values represent the most likely parametric norm at T
J
= 25C, and are provided for reference pur-
poses only.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
V
FB
FB Pin Voltage
V
CC
= 3V to 5.5V
V
T
J
= -25C to +85C
0.594
0.6
0.606
T
J
= -40C to +125C
0.592
0.6
0.608
V
ON
UVLO Thresholds
Rising
Falling
2.76
2.42
V
I
Q_VCC
Operating V
CC
Current
V
CC
= 3.3V, V
SD
= 3.3V
Fsw = 600kHz
1.0
1.5
2.1
mA
V
CC
= 5V, V
SD
= 3.3V
Fsw = 600kHz
1.0
1.7
2.1
Shutdown V
CC
Current
V
CC
= 3.3V, V
SD
= 0V
1
5
A
t
PWGD1
PWGD Pin Response Time
V
FB
Rising
10
s
t
PWGD2
PWGD Pin Response Time
V
FB
Falling
10
s
I
SS-ON
SS Pin Source Current
V
SS
= 0V
7
10
14
A
I
SS-OC
SS Pin Sink Current During
Over Current
V
SS
= 2.5V
100
A
I
SEN-TH
I
SEN
Pin Source Current Trip
Point
25
40
55
A
ERROR AMPLIFIER
GBW
Error Amplifier Unity Gain
Bandwidth
9
MHz
G
Error Amplifier DC Gain
106
dB
SR
Error Amplifier Slew Rate
3.2
V/s
I
EAO
EAO Pin Current Sourcing and
Sinking Capability
V
EAO
= 1.5, FB = 0.55V
V
EAO
= 1.5, FB = 0.65V
2.6
9.2
mA
V
EA
Error Amplifier Output Voltage
Minimum
0
V
Maximum
3.3
V
LM2746
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3
Electrical Characteristics
(Continued)
V
CC
= 3.3V unless otherwise indicated. Limits in standard type are for T
J
= 25C only; limits in boldface type apply over the
junction temperature (T
J
) range of -40C to +125C. Minimum and Maximum limits are guaranteed through test, design, or sta-
tistical correlation. Typical values represent the most likely parametric norm at T
J
= 25C, and are provided for reference pur-
poses only.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
GATE DRIVE
I
Q-BOOT
BOOT Pin Quiescent Current
V
BOOT
= 12V, V
SD
= 0
18
90
A
R
HG_UP
High-Side MOSFET Driver
Pull-Up ON resistance
V
BOOT
= 5V
@
350mA Sourcing
3
R
HG_DN
High-Side MOSFET Driver
Pull-Down ON resistance
V
BOOT
= 5V
@
350mA Sinking
2
R
LG_UP
Low-Side MOSFET Driver
Pull-Up ON resistance
V
BOOT
= 5V
@
350mA Sourcing
3
R
LG_DN
Low-Side MOSFET Driver
Pull-Down ON resistance
V
BOOT
= 5V
@
350mA Sinking
2
OSCILLATOR
f
SW
PWM Frequency
R
FADJ
= 702.1 k
50
kHz
R
FADJ
= 98.74 k
300
R
FADJ
= 45.74 k
475
600
725
R
FADJ
= 24.91 k
1000
D
Max High-Side Duty Cycle
f
SW
= 300kHz
f
SW
= 600kHz
f
SW
= 1MHz
80
76
73
%
LOGIC INPUTS AND OUTPUTS
V
SD-IH
SD Pin Logic High Trip Point
V
SD
Rising
1.3
V
V
SD-IL
SD Pin Logic Low Trip Point
V
SD
Falling
0.8
V
V
PWGD-TH-LO
PWGD Pin Trip Points
FB Falling
0.408
0.434
0.457
V
V
PWGD-TH-HI
PWGD Pin Trip Points
FB Rising
0.677
0.710
0.742
V
V
PWGD-HYS
PWGD Hysteresis
FB Falling
FB Rising
60
90
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. Opearting 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.
LM2746
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4
Typical Performance Characteristics
Efficiency (V
OUT
= 1.2V)
V
CC
= 3.3V, f
SW
= 300kHz
Efficiency (V
OUT
= 2.5V)
V
CC
= 3.3V, f
SW
= 300kHz
20147740
20147757
Efficiency (V
OUT
= 3.3V)
V
CC
= 5V, f
SW
= 300kHz
V
CC
Operating Current plus BOOT Current vs Frequency
FDS6898A FET (T
A
= 25C)
20147741
20147745
BOOT Pin Current vs Temperature for
BOOT Voltage = 3.3V
f
SW
= 300kHz, FDS6898A FET, No-Load
BOOT Pin Current vs Temperature for
BOOT Voltage = 5V
f
SW
= 300kHz, FDS6898A FET, No-Load
20147742
20147743
LM2746
www.national.com
5
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