4-120
October 1998
LM2576
Micrel
LM2576
52kHz Simple 3A Buck Voltage Regulator
General Description
The LM2576 series of monolithic integrated circuits provide
all the active functions for a step-down (buck) switching
regulator. Fixed versions are available with a 3.3V, 5V, or 12V
fixed output. Adjustable versions have an output voltage
range from 1.23V to 37V. Both versions are capable of driving
a 3A load with excellent line and load regulation.
These regulators are simple to use because they require a
minimum number of external components and include internal
frequency compensation and a fixed-frequency oscillator.
The LM2576 series offers a high efficiency replacement for
popular three-terminal adjustable linear regulators. It
substantially reduces the size of the heat sink, and in many
cases no heat sink is required.
A standard series of inductors available from several different
manufacturers are ideal for use with the LM2576 series. This
feature greatly simplifies the design of switch-mode power
supplies.
The feedback voltage is guaranteed to
2% tolerance for
adjustable versions, and the output voltage is guaranteed to
3% for fixed versions, within specified input voltages and
output load conditions. The oscillator frequency is guaranteed
to
10%. External shutdown is included, featuring less than
200
A standby current. The output switch includes cycle-by-
cycle current limiting and thermal shutdown for full protection
under fault conditions.
870 20
Typical Applications
Features
3.3V, 5V, 12V, and adjustable output versions
Voltage over specified line and load conditions:
Fixed version:
3% max. output voltage
Adjustable version:
2% max. feedback voltage
Guaranteed 3A output current
Wide input voltage range:
4V to 40V
Wide output voltage range
1.23V to 37V
Requires only 4 external components
52kHz fixed frequency internal oscillator
Low power standby mode I
Q
typically < 200
A
80% efficiency (adjustable version typically > 80%)
Uses readily available standard inductors
Thermal shutdown and current limit protection
100% electrical thermal limit burn-in
Applications
Simple high-efficiency step-down (buck) regulator
Efficient pre-regulator for linear regulators
On-card switching regulators
Positive to negative converter (inverting Buck-Boost)
Isolated Flyback Converter using minimum number of
external components
Negative Boost Converter
R2
R1
7V40V
Unregulated
DC Input
LM2576
+
C
IN
100F
+V
IN
1
3
Gnd
5
On/Off
2
Output
4
Feedback
L1
100H
+
D1
1N5822
C
OUT
1000F
R2 3.09k
R1 1k
V
OUT
= 1.23 1 +
( )
+5V, 3A
Regulated
Output
Note: Pin numbers are for TO-220 Package
7V40V
Unregulated
DC Input
LM2576-5.0
+
C
IN
100F
+V
IN
1
3
Gnd
5
On/Off
2
Output
4
Feedback
L1
100H
+
D1
1N5822
C
OUT
1000F
+5V, 3A
Regulated
Output
Note: Pin numbers are for TO-220 Package
Fixed Regulator in Typical Application
Adjustable Regulator in Fixed Output Application
October 1998
4-121
LM2576
Micrel
4
Part Number
Temperature Range
Package
LM2576BT*
40
C to +85
C
5-lead TO-220
LM2576-3.3BT
40
C to +85
C
5-lead TO-220
LM2576-5.0BT
40
C to +85
C
5-lead TO-220
LM2576-12BT
40
C to +85
C
5-lead TO-220
LM2576BU*
40
C to +85
C
5-lead TO-263
LM2576-3.3BU
40
C to +85
C
5-lead TO-263
LM2576-5.0BU
40
C to +85
C
5-lead TO-263
LM2576-12BU
40
C to +85
C
5-lead TO-263
* Adjustable output regulators.
Contact factory for bent or staggered leads option.
Pin Configurations
GND
5- ON/OFF
4- FEEDBACK
3- GROUND
2- OUTPUT
1- VIN
5-LEAD TO-263 (U)
GND
5- ON/OFF
4- FEEDBACK
3- GROUND
2- OUTPUT
1- VIN
5-LEAD TO-220 (T)
Ordering Information
4-122
October 1998
LM2576
Micrel
Electrical Characteristics
Specifications with standard typeface are for T
J
= 25
C, and those with boldface type apply
over full Operating Temperature Range. Unless otherwise specified, V
IN
= 12V, and I
LOAD
= 500mA.
LM2576
Symbol
Parameter
Conditions
Typ
Limit
Units
(Note 2)
(Limits)
SYSTEM PARAMETERS, ADJUSTABLE REGULATORS (Note 3) Test Circuit
Figure 1
V
OUT
Feedback Voltage
V
IN
= 12V , I
LOAD
= 0.5A
1.230
V
V
OUT
= 5V
1.217
V(min)
1.243
V(max)
V
OUT
Feedback Voltage
0.5A
I
LOAD
3A, 6V
V
IN
40V
1.230
V
LM2576
V
OUT
= 5V
1.193/1.180
V(min)
1.267/1.280
V(max)
Efficiency
V
IN
= 12V, I
LOAD
= 3A, V
OUT
= 5V
82
%
SYSTEM PARAMETERS, 3.3V REGULATORS (Note 3) Test Circuit
Figure 1
V
OUT
Output Voltage
V
IN
= 12V , I
LOAD
= 0.5A
3.3
V
V
OUT
= 3.3V
3.234
V(min)
3.366
V(max)
V
OUT
Output Voltage
0.5A
I
LOAD
3A, 6V
V
IN
40V
3.3
V
LM2576-3.3
V
OUT
= 3.3V
3.168/3.135
V(min)
3.432/3.465
V(max)
Efficiency
V
IN
= 12V, I
LOAD
= 3A
75
%
SYSTEM PARAMETERS, 5V REGULATORS (Note 3) Test Circuit
Figure 1
V
OUT
Output Voltage
V
IN
= 12V , I
LOAD
= 0.5A
5.0
V
V
OUT
= 5V
4.900
V(min)
5.100
V(max)
V
OUT
Output Voltage
0.5A
I
LOAD
3A, 8V
V
IN
40V
5.0
V
LM2576-5.0
V
OUT
= 5V
4.800/4.750
V(min)
5.200/5.250
V(max)
Efficiency
V
IN
= 12V, I
LOAD
= 3A
82
%
SYSTEM PARAMETERS, 12V REGULATORS (Note 3) Test Circuit
Figure 1
V
OUT
Output Voltage
V
IN
= 25V , I
LOAD
= 0.5A
12
V
V
OUT
= 12V
11.760
V(min)
12.240
V(max)
V
OUT
Output Voltage
0.5A
I
LOAD
3A, 15V
V
IN
40V
12
V
LMLM2576-12
V
OUT
= 12V
11.520/11.400
V(min)
12.480/12.600
V(max)
Efficiency
V
IN
= 25V, I
LOAD
= 3A
88
%
Absolute Maximum Ratings
(Note 1)
Maximum Supply Voltage
45V
ON/OFF Pin Input Voltage
0.3V
V
+40V
Output Voltage to Ground (Steady State)
1V
Power Dissipation
Internally Limited
Storage Temperature Range
65
C to +150
C
Minimum ESD Rating
C = 100pF, R = 1.5k
2 kV
FB Pin
1 kV
Lead Temperature (soldering, 10 sec.)
260
C
Maximum Junction Temperature
150
C
Operating Ratings
Temperature Range
40
C
T
J
+125
C
Supply Voltage
40V
October 1998
4-123
LM2576
Micrel
4
Electrical Characteristics (continued)
LM2576
Units
Symbol
Parameter
Conditions
Typ
Limit
(Limits)
(Note 2)
DEVICE PARAMETERS, ADJUSTABLE REGULATOR
I
B
Feedback Bias Current
V
OUT
= 5V
50
100/500
nA
DEVICE PARAMETERS, FIXED and ADJUSTABLE REGULATORS
f
O
Oscillator Frequency
52
kHz
47/42
kHz (min)
58/63
kHz (max)
V
SAT
Saturation Voltage
I
OUT
= 3A (Note 4)
1.4
V
1.8/2.0
V(max)
DC
Max Duty Cycle (ON)
(Note 5)
98
%
93
%(min)
I
CL
Current Limit
Peak Current, t
ON
3
s (Note 4)
5.8
A
4.2/3.5
A(min)
6.9/7.5
A(max)
I
L
Output Leakage Current
V
IN
= 40V, (Note 6),
Output = 0V
2
mA(max)
Output = 1V
7.5
mA
(Note 6)
Output = 1V
30
mA(max)
I
Q
Quiescent Current
(Note 6)
5
mA
10
mA(max)
I
STBY
Standby Quiescent
ON/OFF Pin = 5V (OFF)
50
A
Current
200
A(max)
JA
Thermal Resistance
T,U Package, Junction to Ambient (Note 7)
65
C/W
JA
T,U Package, Junction to Ambient (Note 8)
45
JC
T,U Package, Junction to Case
2
4-124
October 1998
LM2576
Micrel
Electrical Characteristics (continued)
LM2576
Units
Symbol
Parameter
Conditions
Typ
Limit
(Limits)
(Note 2)
ON/OFF CONTROL, FIXED and ADJUSTABLE REGULATORS Test Circuit
Figure 1
V
IH
ON/OFF Pin Logic
V
OUT
= 0V
1.4
2.2/2.4
V(min)
V
IL
Input Level
V
OUT
= 5V
1.2
1.0/0.8
V(max)
I
IH
ON /OFF Pin Logic
ON /OFF Pin = 5V (OFF)
4
A
Current
30
A(max)
I
IL
ON/OFF Pin = 0V (ON)
0.01
A
10
A(max)
Note 1:
Absolute Maximum Rating indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device
is intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical
Characteristics.
Note 2:
All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face) . All room temperature limits are
100% production tested. All limits at temperature extreme are guaranteed via testing.
Note 3:
External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the
LM2576/LM1576 is used as shown in
Figure 1 test circuit, system performance will be shown in system parameters section of Electrical
Characteristics.
Note 4:
Output (pin 2) sourcing current. No diode, inductor or capacitor connected to output.
Note 5:
Feedback (pin 4) removed from output and connected to 0V.
Note 6:
Feedback (pin 4) removed from output and connected to 12V to force the output transistor OFF.
Note 7:
Junction to ambient thermal resistance (no external heat sink) for the 5-lead TO-220 package mounted vertically, with 1/2" leads in a socket, or
on PC board with minimum copper area.
Note 8:
Junction to ambient thermal resistance (no external heat sink) for the 5-lead TO-220 package mounted vertically, with 1/4" leads soldered to PC
board containing approximately 4 square inches of copper area surrounding the leads.
Note 9:
Junction to ambient thermal resistance with approximately 1 square inch of pc board copper surrounding the leads. Additional copper will lower
thermal resistance further.
Typical Performance Characteristics
Normalized
Feedback Voltage*
JUNCTION TEMPERATURE (C)
NORMALIZED FEEDBACK VOLTAGE (mV)
25
20
15
10
5
0
-5
-10
-15
-20
-25
-75 -50 -25 0
25 50 75 100 125 150
VIN = 12V
ILOAD = 200mA
Normalized to TJ = 25C
Feedback Voltage
vs Duty Cycle*
DUTY CYCLE (%)
FEEDBACK VOLTAGE CHANGE (mV)
20
15
10
5
0
-5
-10
-15
-20
ILOAD = 200 mA
VIN = 40V
VIN = 7V
0
20
40
60
80
100
* Adjustable version only
October 1998
4-125
LM2576
Micrel
4
Typical Performance Characteristics (continued)
(Circuit of
Figure 1)
Supply Current
INPUT VOLTAGE (V)
SUPPLY CURRENT (mA)
20
Measured at
Ground Pin
TJ = 25C
VOUT = 5V
18
16
14
12
10
8
6
4
0
10
20
30
40
50
60
ILOAD to 1A
Supply Current
vs. Duty Cycle
DUTY CYCLE (%)
20.0
17.5
15.0
12.5
10.0
7.5
5.0
2.5
0
0
20
40
60
80
100
VIN from 7V to 40V
ILOAD = 200 mA
SUPPLY CURRENT (mA)
JUNCTION TEMPERATURE (C)
200
150
100
50
0
-75
VIN = 12V
STANDBY QUIESCENT CURRENT (A)
Standby
Quiescent Current
VIN = 40V
-50 -25
0 25 50 75 100 125 150
Minimum Operating
Voltage
JUNCTION TEMPERATURE (C)
5.0
ILOAD = 200 mA
INPUT VOLTAGE (V)
Line Regulation
INPUT VOLTAGE (V)
14
0
NORMALIZED FEEDBACK VOLTAGE (mV)
JUNCTION TEMPERATURE (C)
100
-100
-75
STANDBY QUIESCENT CURRENT (A)
Feedback Pin Current
-50 -25 0
25 50 75 100 125 150
VOUT = 1.23V
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
-75 -50 -25 0
25 50 75 100 125 150
12
10
8
6
4
2
0
-2
-4
-6
5
10
15
20
25
30
75
50
25
0
-25
-50
-75
JUNCTION TEMPERATURE (C)
+8
-8
-75
NORMALIZED FREQUENCY (%)
Oscillator Frequency
-50 -25
0 25 50 75 100 125 150
+6
+4
+2
0
-2
-4
--6
VIN = 12V
VIN = 40V
Normalized at 25C
Normalized to VIN = 10V
VOUT = 5V
ILOAD = 500mA
TJ = 25
VON/OFF = 5V
Normalized
Output Voltage
JUNCTION TEMPERATURE (C)
OUTPUT VOLTAGE CHANGE (mV)
100
75
50
25
0
-25
-50
-75
-100
-75 -50 -25
0
25 50 75 100 125 150
VIN = 12V
ILOAD = 200mA
Normalized to TJ = 25C
Current Limit
JUNCTION TEMPERATURE (C)
6.5
6.0
5.5
5.0
4.5
4.0
-75
VIN = 12V
OUTPUT CURRENT (A)
Switch
Saturation Voltage
SWITCH CURRENT (A)
1.5
-55C
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
SATURATION VOLTAGE (V)
INPUT VOLTAGE (V)
100
EFFICIENCY (%)
Efficiency
VOUT = 12V
-50 -25
0
25 50 75 100 125 150
0.5
1.0
1.5
2.0
2.5
3.0
25C
150C
95
90
85
80
75
70
65
60
200 mA
200 mA
VOUT = 3V
1A
3A
1A
3A
0
5
10 15 20
25 30 35 40
Dropout Voltage
JUNCTION TEMPERATURE (C)
INPUT-OUTPUT DIFFERENTIAL (V)
2.0
-75 -50 -25
0
25 50 75 100 125 150
ILOAD = 3A
L1 = 330 H
ILOAD = 1A
ILOAD = 200 mA
1.5
1.0
0.5
0
4-126
October 1998
LM2576
Micrel
100 S/div.
Load Transient Response
+100mV
0
-100mV
Output
Voltage
Change
3A
2A
1A
0
Output
Current
50V
0
4A
2A
0
4A
2A
0
A
B
C
D
5 S/div.
Switching Waveforms
A: Output pin voltage 50V/div
B: Output pin current 2A/div
C: Inductor current 2A/div
D: Output ripple voltage 50 mV/div., AC coupled
Horizontal Time Base: 5S/div
V
OUT = 5V
V
IN = 45V
Typical Performance Characteristics
(Circuit of
Figure 1)
October 1998
4-127
LM2576
Micrel
4
LM2575
+
100 F
+V
IN
1
3
GND
5
ON/OFF
2
OUTPUT
4
FEEDBACK
L1
330 H
+
D1
MBR360
C
OUT
470F
R2
R1
V
OUT
5.00V
7V - 40V
UNREGULATED
DC INPUT
C
IN
L
O
A
D
C
IN
-- 100F, 75V Aluminum Electrolytic
C
OUT
-- 470F, 15V Aluminum Electrolytic
D1 -- Schottky, MBR360
L1 -- 330H, 415-0926 (AIE)
R1 -- 1k, 0.01%
R2 -- 3.065k, 0.01%
5-pin TO-220 socket--2936 (Loranger Mfg. Co.)
4-pin TO-3 socket--8112-AG7 (Augat Inc.)
LM2575-5.0
+
100 F
+V
IN
1
3
GND
5
ON/OFF
2
OUTPUT
4
FEEDBACK
L1
330 H
+
D1
11DQ06
C
OUT
220F
V
OUT
5.00V
Note: Pin numbers are for TO-220 Package
7V - 40V
UNREGULATED
DC INPUT
C
IN
C
IN
-- 100F, 75V Aluminum Electrolytic
C
OUT
-- 330F, 15V Aluminum Electrolytic
D1 -- Schottky, 11DQ06
L1 -- 100H, 415-0926 (AIE)
5-pin TO-220 socket--2936 (Loranger Mfg. Co.)
4-pin TO-3 socket--8112-AG7 (Augat Inc.)
L
O
A
D
ON/OFF
COMPARATOR
FIXED GAIN
ERROR AMP
3 AMP
SWITCH
OUTPUT
DRIVER
+
-
+
-
VIN
FEED-
BACK
1.23V
BAND-GAP
REFERENCE
52 kHz
OSCILLATOR
RESET
THERMAL
SHUTDOWN
CURRENT
LIMIT
5
2
3
1
Note: Pin numbers are for the TO-220 package
INTERNAL
REGULATOR
ON/OFF
GND
ON/OFF
COMPARATOR
FIXED GAIN
ERROR AMP
3 AMP
SWITCH
OUTPUT
DRIVER
+
-
+
-
VIN
FEED-
BACK
1.23V
BAND-GAP
REFERENCE
52 kHz
OSCILLATOR
RESET
THERMAL
SHUTDOWN
CURRENT
LIMIT
5
2
3
1
4
INTERNAL
REGULATOR
ON/OFF
GND
3.1k
1.0k
4
Block Diagrams
As in any switching regulator, layout is very important. Rapidly switching currents associated with wiring inductance generate voltage transients
which can cause problems. For minimal stray inductance and ground loops, the length of the leads indicated by heavy lines should be kept as
short as possible. Single-point grounding (as indicated) or ground plane construction should be used for best results.
Fixed Regulator
Adjustable Regulator
Test Circuits and Layout Guidelines
Figure 1.
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