LM3477/LM3477A
High Efficiency High-Side N-Channel Controller for
Switching Regulator
General Description
The LM3477/A is a high-side N-channel MOSFET switching
regulator controller. It can be used in topologies requiring a
high side MOSFET such as buck, inverting (buck-boost) and
zeta regulators. The LM3477/A's internal push pull driver
allows compatibility with a wide range of MOSFETs. This, the
wide input voltage range, use of discrete power components
and adjustable current limit allows the LM3477/A to be opti-
mized for a wide variety of applications.
The LM3477/A uses a high switching frequency of 500kHz to
reduce the overall solution size. Current-mode control re-
quires only a single resistor and capacitor for frequency
compensation. The current mode architecture also yields
superior line and load regulation and cycle-by-cycle current
limiting. A 5A shutdown state can be used for power sav-
ings and for power supply sequencing. Other features in-
clude internal soft-start and output over voltage protection.
The internal soft-start reduces inrush current. Over voltage
protection is a safety feature to ensure that the output volt-
age stays within regulation.
The LM3477A is similar to the LM3477. The primary differ-
ence between the two is the point at which the device
transitions into hysteretic mode. The hysteretic threshold of
the LM3477A is one-third of the LM3477.
Features
n
500kHz switching frequency
n
Adjustable current limit
n
1.5% reference
n
Thermal shutdown
n
Frequency compensation optimized with a single
capacitor and resistor
n
Internal softstart
n
Current mode operation
n
Undervoltage lockout with hysteresis
n
8-lead Mini-SO8 (MSOP-8) package
Applications
n
Local Voltage Regulation
n
Distributed Power
n
Notebook and Palmtop Computers
n
Internet Appliances
n
Printers and Office Automation
n
Battery operated Devices
n
Cable Modems
n
Battery Chargers
Hysteretic
Threshold
LM3477
) 36% of
programmed current
limit
LM3477A
) 12% of
programmed current
limit
*See Hysteretic Threshold and section for more information.
Typical Application Circuit
20003333
Typical High Efficiency Step-Down (Buck) Converter
June 2002
LM3477/LM3477A
High
Efficiency
High-Side
N-Channel
Controller
for
Switching
Regulator
2002 National Semiconductor Corporation
DS200033
www.national.com
Connection Diagram
20003302
8 Lead Mini SO8 Package (MSOP-8 Package)
Package Marking and Ordering Information
Order Number
Package Type
Package Marking
Supplied As:
LM3477MM
MSOP-8
S13B
1000 units on Tape and Reel
LM3477MMX
MSOP-8
S13B
3500 units on Tape and Reel
LM3477AMM
MSOP-8
S13A
1000 units on Tape and Reel
LM3477AMMX
MSOP-8
S13A
3500 units on Tape and Reel
Pin Description
Pin Name
Pin Number
Description
I
SEN
1
Current sense input pin. Voltage generated across an external sense resistor is fed
into this pin.
COMP/SD
2
Compensation pin. A resistor-capacitor combination connected to this pin provides
compensation for the control loop. Pull this pin below 0.65V to shutdown.
FB
3
Feedback pin. The output voltage should be adjusted using a resistor divider to
provide 1.270V at this pin.
GND
4
Ground pin.
SW
5
Switch Node. Source of the external MOSFET is connected to this node.
DR
6
Drive pin. The gate of the external MOSFET should be connected to this pin.
CB
7
Boot-strap pin. A capacitor must be connected between this pin and SW pin (pin 5)
for proper operation. The voltage developed across this capacitor provides the gate
drive for the external MOSFET.
V
IN
8
Power Supply Input pin.
LM3477/LM3477A
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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.
Input Voltage
36V
Peak Driver Output Current (
<
10s)
1.0A
CB Pin Voltage (Note 2)
43V
I
SEN
Pin Voltage
500mV
Power Dissipation
Internally Limited
Storage Temperature Range
-65C to +150C
Junction Temperature
+150C
ESD Susceptibilty (Note 3)
Human Body Model
Machine Model
2kV
200V
Lead Temperature for MSOP Package
Vapor Phase (60 sec.)
Infared (15 sec.)
215C
220C
Operating Ratings
(Note 1)
Supply Voltage
2.97V
V
IN
35V
Junction
Temperature Range
-40C
T
J
+125C
Electrical Characteristics
(Note 4)
Specifications in Standard type face are for T
J
= 25C, and in bold type face apply over the full Operating Temperature
Range. Unless otherwise specified, V
IN
= 12V.
Symbol
Parameter
Conditions
Typical
Limit
Units
V
FB
Feedback Voltage
V
COMP
= 1.4V,
2.97V
V
IN
36V
1.270
1.260/1.252
1.288/1.290
V
V(min)
V(max)
V
LINE
Feedback Voltage Line
Regulation
2.97V
V
IN
36V
0.001
%/V
V
LOAD
Output Voltage Load Regulation
0.5
%/V (max)
V
UVLO
Input Undervoltage Lock-out
2.87
2.97
V
V(max)
V
UV(HYS)
Input Undervoltage Lock-out
Hysteresis
180
130
225
mV
mV (min)
mV (max)
F
SW
Switching Frequency
500
435
575
kHz
kHz(min)
kHz(max)
R
DS1 (ON)
Driver Switch On Resistance
(top)
I
DR
= 0.2A, V
IN
= 5V
7
R
DS2 (ON)
Driver Switch On Resistance
(bottom)
I
DR
= 0.2A
4
(V
CB
-V
SW
)
max
Maximum Boot Voltage
V
IN
<
7.2V
V
IN
V
V
IN
7.2V
7.2
D
max
Maximum Duty Cycle
93
88
%
%(min)
T
min
(on)
Minimum On Time
330
230
495
nsec
nsec(min)
nsec(max)
I
SUPPLY
Supply Current (switching)
(Note 5)
2.0
3.0
mA
mA (max)
I
Q
Quiescent Current in Shutdown
Mode
(Note 6), V
IN
= 5V
5
8
A
A (max)
LM3477/LM3477A
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3
Electrical Characteristics
(Note 4)
(Continued)
Specifications in Standard type face are for T
J
= 25C, and in bold type face apply over the full Operating Temperature
Range. Unless otherwise specified, V
IN
= 12V.
Symbol
Parameter
Conditions
Typical
Limit
Units
V
CL(O)
Current Limit Voltage at 0% Duty
Cycle
LM3477
155
130/125
185/190
mV
mV (min)
mV (max)
LM3477A
165
140/135
195/200
mV
mV (min)
mV (max)
V
CL(100)
Current Limit Voltage at 100%
Duty Cycle
LM3477
74
50/43
98/98
mV
mV (min)
mV (max)
LM3477A
65
41/25
89/98
mV
mV (min)
mV (max)
V
SC
Short-Circuit Current Limit Sense
Voltage
V
IN
= 5V, LM3477
350
270
420
mV
mV (min)
mV (max)
V
IN
= 5V, LM3477A
310
260
380
mV
mV (min)
mV (max)
V
SL
Internal Compensation Ramp
Voltage Height
V
IN
= 5V, LM3477
83
mV
V
IN
= 5V, LM3477A
103
V
OVP
Output Over-voltage Protection
(with respect to feedback
voltage) (Note 7)
V
COMP
= 1.4V
50
32/25
78/85
mV
mV(min)
mV(max)
V
OVP(HYS)
Output Over-Voltage Protection
Hysteresis(Note 7)
V
COMP
= 1.4V
60
20
110
mV
mV(min)
mV(max)
Gm
Error Amplifier
Transconductance
V
COMP
= 1.4V
I
EAO
= 100A
(Source/Sink)
750
600/365
1000/1265
mho
mho (min)
mho (max)
A
VOL
Error Amplifier Voltage Gain
V
COMP
= 1.4V
I
EAO
= 100A
(Source/Sink)
38
30
42
V/V
V/V (min)
V/V (max)
I
EAO
Error Amplifier Output Current
(Source/ Sink)
Source, V
COMP
= 1.4V,
V
FB
= 0V
100
75/50
130/160
A
A (min)
A (max)
Sink, V
COMP
= 1.4V, V
FB
= 1.4V
-140
-110/-95
-170/-180
A
A (min)
A (max)
V
EAO
Error Amplifier Output Voltage
Swing
Upper Limit
V
FB
= 0V
COMP Pin = Floating
2.2
2.0
2.35
V
V(min)
V(max)
Lower Limit
V
FB
= 1.4V
0.75
0.5
0.95
V
V(min)
V(max)
T
SS
Internal Soft-Start Delay
V
FB
= 1.2V, V
COMP
=
Floating
5
msec
T
r
Drive Pin Rise Time
C
GS
= 3000pF, V
DR
= 0
to 3V
25
ns
T
f
Drive Pin Fall Time
C
GS
= 3000pF, V
DR
= 0
to 3V
25
ns
LM3477/LM3477A
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4
Electrical Characteristics
(Note 4)
(Continued)
Specifications in Standard type face are for T
J
= 25C, and in bold type face apply over the full Operating Temperature
Range. Unless otherwise specified, V
IN
= 12V.
Symbol
Parameter
Conditions
Typical
Limit
Units
V
SD
Shutdown Threshold (Note 8)
Output = High
1.15
1.35
V
V (max)
Output = Low
0.65
0.3
V
V (min)
I
SD
Shutdown Pin Current
V
SD
= 5V
-1
A
V
SD
= 0V
+1
TSD
Thermal Shutdown
165
C
T
SH
Thermal Shutdown Hysteresis
10
C
JA
Thermal Resistance
MM Package
200
C/W
LM3477/LM3477A
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