LM1771
Low-Voltage Synchronous Buck Controller with
Precision Enable and No External Compensation
General Description
The LM1771 is an efficient synchronous buck switching con-
troller with a precision enable requiring no external compen-
sation. The constant on-time control scheme provides a
simple design free of compensation components, allowing
minimal component count and board space. The precision
enable pin allows flexibility in sequencing multiple rails and
setting UVLO. The LM1771 also incorporates a unique input
feed-forward to maintain a constant frequency independent
of the input voltage. The LM1771 is optimized for a low
voltage input range of 2.8V to 5.5V and can provide an
adjustable output as low as 0.8V. Driving an external high
side PFET and low side NFET it can provide efficiencies as
high as 95%.
Three versions of the LM1771 are available depending on
the switching frequency desired for the application. Nominal
switching frequencies are in the range of 100kHz to
1000kHz.
Features
n
Input voltage range of 2.8V to 5.5V
n
0.8V reference voltage
n
Precision enable
n
No compensation required
n
Constant frequency across input range
n
Low quiescent current of 400 A
n
Internal soft-start
n
Short circuit protection
n
Tiny LLP-6 package and MSOP-8 package
Applications
n
Simple To Design, High Efficiency Step Down Switching
Regulators
n
FPGAs, DSPs, and ASIC Power Supplies
n
Set-Top Boxes
n
Cable Modems
n
Printers
n
Digital Video Recorders
n
Servers
n
Graphic Cards
Typical Application Circuit
20189001
June 2006
LM1771
Low-V
oltage
Synchronous
Buck
Controller
with
Precision
Enable
and
No
External
Compensation
2006 National Semiconductor Corporation
DS201890
www.national.com
Connection Diagrams
Top View
Top View
20189002
6-Lead LLP (3mm x 3mm)
NS Package Number SDE06A
20189040
MSOP-8
NS Package Number MUA08A
Ordering Information
Order Number
Timing Option
Package Type
NSC Package
Drawing
Top Mark
Supplied As
LM1771SSD
500ns
6-Lead LLP
SDE06A
1771S
1000 units Tape and Reel
LM1771SSDX
1771S
4500 units Tape and Reel
LM1771TSD
1000ns
1771T
1000 units Tape and Reel
LM1771TSDX
1771T
4500 units Tape and Reel
LM1771USD
2000ns
1771U
1000 units Tape and Reel
LM1771USDX
1771U
4500 units Tape and Reel
Order Number
Timing Option
Package Type
NSC Package Drawing
Supplied As
LM1771SMM
500ns
MSOP-8
MUA08A
Available Soon
LM1771SMMX
LM1771TMM
1000ns
LM1771TMMX
LM1771UMM
2000ns
LM1771UMMX
Pin Descriptions
Pin #
Name
Function
1
FB
Feedback Pin
2
GND
Ground
3
HG
PFET Gate Drive
4
LG
NFET Gate Drive
5
VIN
Input supply
6
EN
Enable Pin
DAP
-
Die Attach Pad is internally connected to GND, but it cannot be
used as the primary GND connection
LM1771
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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
IN
-0.3V to 6V
EN, FB, HG, LG
-0.3V to V
IN
Storage Temperature Range
-65C to 150C
Junction Temperature
150C
Lead Temperature
(soldering, 10sec)
260C
ESD Rating
2kV
Operating Ratings
V
IN
to GND
2.8V to 5.5V
Junction Temperature Range (T
J
)
-40C to +125C
Electrical Characteristics
Specifications with standard typeface are for T
J
= 25C, and those in bold face
type apply over the full Junction Temperature Range (-40C to +125C). Minimum and Maximum limits are guaranteed
through test, design or statistical correlation. Typical values represent the most likely parametric norm at T
J
= 25C and are
provided for reference purposes only. Unless otherwise specified V
IN
= 3.3V.
Symbol
Parameter
Conditions
Min
Typ
Max
Unit
V
FB
Feedback pin voltage
0.782
0.8
0.818
V
I
Q
Quiescent current
V
FB
= 0.9V
400
700
A
T
ON
Switch On-Time
LM1771S - (500ns)
0.4
0.5
0.6
s
LM1771T - (1000ns)
0.8
1.0
1.2
LM1771U - (2000ns)
1.6
2.0
2.4
T
OFF_MIN
Minimum Off-Time
LM1771S - (500ns)
150
250
ns
LM1771T - (1000ns)
135
225
LM1771U - (2000ns)
120
220
T
D
Gate Drive Dead-Time
70
ns
V
IH_EN
EN Pin Rising Threshold
1.15
1.2
1.25
V
V
EN_HYS
EN Pin Hysteresis
50
200
mV
I
FB
Feedback pin bias current
V
FB
= 0.9V
50
nA
V
UVLO
Under-voltage lock out
V
IN
Rising Edge
2.65
2.8
V
V
UVLO_HYS
Under-voltage lock out hysteresis
50
mV
V
SC_TH
Feedback pin Short Circuit Latch
Threshold
0.42
0.55
0.65
V
R
DS(ON) 1
HG FET driver pull-up On resistance
I
HG
= 20 mA
4
R
DS(ON) 2
HG FET driver pull-down On resistance I
HG
= 20 mA
6
R
DS(ON) 3
LG FET driver pull-up On resistance
I
LG
= 20 mA
4
R
DS(ON) 4
LG FET driver pull-down On resistance I
LG
= 20 mA
6
Note 1: Absolute Maximum Ratings indicate limits beyond which damage may occur to the device. Operating Ratings indicate conditions for which the device is
intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications, see Electrical Characteristics.
LM1771
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3
Typical Performance Characteristics
All curves taken at VIN = 3.3V with configuration in typical ap-
plication circuit shown in Application Information section of this datasheet. T
J
= 25C, unless otherwise specified.
T
ON
vs V
IN
(LM1771S)
T
ON
vs V
IN
(LM1771T)
20189007
20189009
T
ON
vs V
IN
(LM1771U)
T
ON
vs Temperature (LM1771S)
20189011
20189008
T
ON
vs Temperature (LM1771T)
T
ON
vs Temperature (LM1771U)
20189010
20189012
LM1771
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4
Typical Performance Characteristics
All curves taken at VIN = 3.3V with configuration in typical
application circuit shown in Application Information section of this datasheet. T
J
= 25C, unless otherwise
specified. (Continued)
T
OFF
vs Temperature (LM1771S)
T
OFF
vs Temperature (LM1771T)
20189013
20189041
T
OFF
vs Temperature (LM1771U)
Feedback Voltage vs Temperature
20189042
20189017
V
EN
Threshold vs Temperature
Short Circuit Threshold vs Temperature
20189014
20189018
LM1771
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