LM2576
PIN FUNCTION DESCRIPTION
Pin
1
Symbol
V
in
Description (Refer to Figure 1)
This pin is the positive input supply for the LM2576 step−down switching regulator. In order to minimize voltage
transients and to supply the switching currents needed by the regulator, a suitable input bypass capacitor must be
present (C
in
in Figure 1).
This is the emitter of the internal switch. The saturation voltage V
sat
of this output switch is typically 1.5 V. It should
be kept in mind that the PCB area connected to this pin should be kept to a minimum in order to minimize coupling
to sensitive circuitry.
Circuit ground pin. See the information about the printed circuit board layout.
This pin senses regulated output voltage to complete the feedback loop. The signal is divided by the internal resistor
divider network R2, R1 and applied to the non−inverting input of the internal error amplifier. In the Adjustable version
of the LM2576 switching regulator this pin is the direct input of the error amplifier and the resistor network R2, R1 is
connected externally to allow programming of the output voltage.
It allows the switching regulator circuit to be shut down using logic level signals, thus dropping the total input supply
current to approximately 80
mA.
The threshold voltage is typically 1.4 V. Applying a voltage above this value (up to
+V
in
) shuts the regulator off. If the voltage applied to this pin is lower than 1.4 V or if this pin is left open, the
regulator will be in the “on” condition.
2
Output
3
4
GND
Feedback
5
ON/OFF
DESIGN PROCEDURE
Buck Converter Basics
The LM2576 is a “Buck” or Step−Down Converter which
is the most elementary forward−mode converter. Its basic
schematic can be seen in Figure 16.
The operation of this regulator topology has two distinct
time periods. The first one occurs when the series switch is
on, the input voltage is connected to the input of the inductor.
The output of the inductor is the output voltage, and the
rectifier (or catch diode) is reverse biased. During this
period, since there is a constant voltage source connected
across the inductor, the inductor current begins to linearly
ramp upwards, as described by the following equation:
I
L(on)
+
V
in
– V out t on
L
This period ends when the power switch is once again
turned on. Regulation of the converter is accomplished by
varying the duty cycle of the power switch. It is possible to
describe the duty cycle as follows:
t
d
+
on , where T is the period of switching.
T
For the buck converter with ideal components, the duty
cycle can also be described as:
V
d
+
out
V
in
Figure 17 shows the buck converter, idealized waveforms
of the catch diode voltage and the inductor current.
V
on(SW)
Power
Switch
Diode Voltage
During this “on” period, energy is stored within the core
material in the form of magnetic flux. If the inductor is
properly designed, there is sufficient energy stored to carry
the requirements of the load during the “off” period.
L
Power
Switch
Off
V
D
(FWD)
Power
Switch
On
Power
Switch
Off
Power
Switch
On
V
in
D
C
out
R
Load
Time
Figure 16. Basic Buck Converter
I
pk
Inductor Current
I
Load
(AV)
I
min
Diode
Power
Switch
Power
Switch
Time
The next period is the “off” period of the power switch.
When the power switch turns off, the voltage across the
inductor reverses its polarity and is clamped at one diode
voltage drop below ground by the catch diode. The current
now flows through the catch diode thus maintaining the load
current loop. This removes the stored energy from the
inductor. The inductor current during this time is:
I
L(off)
+
V out – V
L
D
t
off
Diode
Figure 17. Buck Converter Idealized Waveforms
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