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
200µA 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 < 200µA
• 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
7V–40V
Unregulated
DC Input
LM2576
+
C
IN
100µF
+V
IN
1
3
Gnd
5
On/Off
2
Output
4
Feedback
L1
100µH
+
D1
1N5822
C
OUT
1000µF
R2 3.09k
R1 1k
V
OUT
= 1.23 1 +
( )
+5V, 3A
Regulated
Output
Note: Pin numbers are for TO-220 Package
7V–40V
Unregulated
DC Input
LM2576-5.0
+
C
IN
100µF
+V
IN
1
3
Gnd
5
On/Off
2
Output
4
Feedback
L1
100µH
+
D1
1N5822
C
OUT
1000µF
+5V, 3A
Regulated
Output
Note: Pin numbers are for TO-220 Package
Fixed Regulator in Typical Application Adjustable Regulator in Fixed Output Application
