LM1770
SOT-23 Synchronous Buck Controller
General Description
The LM1770 is an efficient synchronous buck switching con-
troller in a tiny SOT23 package. The constant on-time control
scheme provides a simple design free of compensation com-
ponents, allowing minimal component count and board
space. It also incorporates a unique input feed-forward to
maintain a constant frequency independent of the input volt-
age. The LM1770 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 LM1770 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
No compensation required
n
Constant frequency across input range
n
Low quiescent current of 400A
n
Internal soft-start
n
Short circuit protection
n
Tiny SOT-23 package
Applications
n
Simple To Design, High Efficiency Step Down Switching
Regulators
n
Set-Top Boxes
n
Cable Modems
n
Printers
n
Digital Video Recorders
n
Servers
Typical Application Circuit
20166201
September 2005
LM1770
SOT
-23
Synchronous
Buck
Controller
2005 National Semiconductor Corporation
DS201662
www.national.com
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
Storage Temperature Range
-65C to 150C
Junction Temperature
150C
Lead Temperature
(soldering, 10sec)
260C
ESD Rating
2.5kV
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
VFB
Feedback pin voltage
V
IN
= 3.3V
0.782
0.80
0.818
V
V
IN
= 5.0V
0.772
0.79
0.808
V
FB
/
V
IN
Line Regulation
V
IN
= 2.8V to 5.5V
-5
mV/V
I
Q
Operating Quiescent current
V
FB
= 0.9V
400
600
A
T
ON
Switch On-Time
LM1770S - (500ns)
0.4
0.5
0.6
s
LM1770T - (1000ns)
0.8
1.0
1.2
LM1770U - (2000ns)
1.6
2.0
2.4
T
OFF_MIN
Minimum Off-Time
LM1770S - (500ns)
150
250
ns
LM1770T - (1000ns)
135
225
LM1770U - (2000ns)
120
220
T
D
Gate Drive Dead-Time
70
ns
I
FB
Feedback pin bias current
V
FB
= 0.9V
50
nA
V
UVLO
Under-voltage lock out
V
IN
Rising Edge
2.6
2.8
V
V
UVLO_HYS
Under-voltage lock out hysteresis
30
mV
V
SC_TH
Feedback pin Short Circuit Latch
Threshold
0.5
0.55
0.65
V
R
DS(ON) 1
HG FET driver pull-up On resistance
I
HG
= 20 mA
5
R
DS(ON) 2
HG FET driver pull-down On resistance I
HG
= 20 mA
9
R
DS(ON) 3
LG FET driver pull-up On resistance
I
LG
= 20 mA
9
R
DS(ON) 4
LG FET driver pull-down On resistance I
LG
= 20 mA
5
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.
LM1770
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3
Typical Performance Characteristics
T
ON
vs Temperature (LM1770S)
Quiescent Current vs Temperature
20166203
20166204
T
OFF
vs Temperature
Feedback Voltage vs Temperature
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20166206
Deadtime vs Temperature
Short Circuit Threshold vs Temperature
20166225
20166226
LM1770
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