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1-A SIMPLE STEP-DOWN SWITCHING VOLTAGE REGULATOR
SLVS569E – JANUARY 2005 – REVISED JANUARY 2006
APPLICATION INFORMATION
Layout Guidelines
With any switching regulator, circuit layout plays an important role in circuit performance. Wiring and parasitic
inductances, as well as stray capacitances, are subjected to rapidly switching currents, which can result in
unwanted voltage transients. To minimize inductance and ground loops, the length of the leads indicated by
heavy lines (see
should be minimized. Optimal results can be achieved by single-point grounding or by
ground-plane construction. For the same reasons, the two programming resistors used in the adjustable version
should be located as close as possible to the regulator to keep the sensitive feedback wiring short.
Adjustable Output Voltage Versions
FEEDBACK
7
LM2575
(ADJ)
OUTPUT
3
7-V to 60-V
Unregulated
DC Input
+
5, 12, 13 GND
C
IN
100
µF
9 ON/OFF
L1
330
µH
R2
D1
11DQ06
+
C
OUT
330
µF
R1
L
O
A
D
V
OUT
+V
IN
16
V
OUT
= V
REF
(1 + R2/R1) = 5 V
Where
V
REF
= 1.23 V
R1 = 2 kW
R2 = 6.12 kW
Figure 1. Test Circuit and Layout Guidelines
Input Capacitor (C
IN
)
For stability concerns, an input bypass capacitor (electrolytic, C
IN
≥
47
µF)
needs to be located as close as
possible to the regulator. For operating temperatures below –25°C, C
IN
may need to be larger in value. In
addition, since most electrolytic capacitors have decreasing capacitances and increasing ESR as temperature
drops, adding a ceramic or solid tantalum capacitor in parallel increases the stability in cold temperatures.
To extend the capacitor operating lifetime, the capacitor RMS ripple current rating should be:
I
C,RMS
u
1.2(
t
on
)I
, where:
T
LOAD
V
t
on
+
OUT
{buck regulator}, and
T
V
IN
|V
OUT
|
t
on
+
{buck−boost regulator}
(|V
OUT
|
)
V
IN
)
T
Output Capacitor (C
OUT
)
For both loop stability and filtering of ripple voltage, an output capacitor also is required, again in close proximity
to the regulator. For best performance, low-ESR aluminum electrolytics are recommended, although standard
aluminum electrolytics may be adequate for some applications. Based on the following equation:
Output Ripple Voltage = (ESR of C
OUT
)
×
(inductor ripple current)
5
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