LM2574, NCV2574
http://onsemi.com
9
PIN FUNCTION DESCRIPTION
Pin
Symbol Description (Refer to Figure 1)
SO−16W PDIP−8
12 5 V
in
This pin is the positive input supply for the LM2574 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).
14 7 Output This is the emitter of the internal switch. The saturation voltage V
sat
of this output switch is
typically 1.0 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.
4 2 Sig Gnd Circuit signal ground pin. See the information about the printed circuit board layout.
6 4 Pwr Gnd Circuit power ground pin. See the information about the printed circuit board layout.
3 1 Feedback 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 LM2574 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.
5 3 ON/OFF 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 input threshold voltage is typically 1.5 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 1.5 V or if this pin is left open, the regulator will be in the “on” condition.
DESIGN PROCEDURE
Buck Converter Basics
The LM2574 is a “Buck” or Step−Down Converter which
is the most elementary forward−mode converter. Its basic
schematic can be seen in Figure 17.
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)
+
ǒ
V
in
–V
out
Ǔ
t
on
L
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.
Figure 17. Basic Buck Converter
DV
in
R
Load
L
C
out
Power
Switch
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. 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)
+
ǒ
V
out
–V
D
Ǔ
t
off
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:
d +
t
on
T
, where T is the period of switching.
For the buck converter with ideal components, the duty
cycle can also be described as:
d +
V
out
V
in
Figure 18 shows the buck converter idealized waveforms
of the catch diode voltage and the inductor current.
Figure 18. Buck Converter Idealized Waveforms
Power
Switch
Power
Switch
Off
Power
Switch
Off
Power
Switch
On
Power
Switch
On
V
on(SW)
V
D
(FWD)
Time
Time
I
Load
(AV)
I
min
I
pk
Diode Diode
Power
Switch
Diode VoltageInductor Current
