Datasheet

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BUCK REGULATOR DESIGN PROCEDURE
TL2575 , TL2575HV
1-A SIMPLE STEP-DOWN SWITCHING VOLTAGE REGULATORS
SLVS638B MAY 2006 REVISED JANUARY 2007
PROCEDURE (Fixed Output) EXAMPLE (Fixed Output)
Known: Known:
V
OUT
= 3.3 V, 5 V, 12 V, or 15 V V
OUT
= 5 V
V
IN(Max)
= Maximum input voltage V
IN(Max)
= 20 V
I
LOAD(Max)
= Maximum load current I
LOAD(Max)
= 1 A
1. Inductor Selection (L1) 1. Inductor Selection (L1)
A. From Figure 15 through Figure 18 , select the appropriate inductor A. From Figure 16 (TL2575-05), the intersection of 20-V line and
code based on the intersection of V
IN(Max)
and I
LOAD(Max)
. 1-A line gives an inductor code of L330.
B. From Table 2 , choose the appropriate inductor based on the B. L330 L1 = 330 µ H
inductor code. Parts from three well-known inductor manufacturers
Choose from:
are given. The inductor chosen should be rated for operation at
34042 (Schott)
52-kHz and have a current rating of at least 1.15 × I
LOAD(Max)
to
PE-52627 (Pulse Engineering)
allow for the ripple current. The actual peak current in L1 (in normal
operation) can be calculated as follows:
RL1952 (Renco)
I
L1(pk)
= I
LOAD(Max)
+ (V
IN
V
OUT
) × t
on
/2L1
Where t
on
= V
OUT
/V
IN
× (1/f
osc
)
2. Output Capacitor Selection (C
OUT
) 2. Output Capacitor Selection (C
OUT
)
A. The TL2575 control loop has a two-pole two-zero frequency A. C
OUT
= 100- µ F to 470- µ F, standard aluminum electrolytic
response. The dominant pole-zero pair is established by C
OUT
and
L1. To meet stability requirements while maintaining an acceptable
output ripple voltage (V
ripple
0.01 × V
OUT
), the recommended range
for a standard aluminum electrolytic C
OUT
is between 100 µ F and
470 µ F.
B. C
OUT
should have a voltage rating of at least 1.5 × V
OUT
. But if a B. Although a C
OUT
rated at 8 V is sufficient for V
OUT
= 5 V, a
low output ripple voltage is desired, choose capacitors with a higher-voltage capacitor is chosen for its typically lower ESR (and
higher-voltage ratings than the minimum required, due to their hence lower output ripple voltage) Capacitor voltage
typically lower ESRs. rating = 20 V.
3. Catch Diode Selection (D1) (see Table 1 ) 3. Catch Diode Selection (D1) (see Table 1 )
A. In normal operation, the catch diode requires a current rating of A. Pick a diode with 3-A rating.
at least 1.2 × I
LOAD(Max)
. For the most robust design, D1 should be
rated to handle a current equal to the TL2575 maximum switch peak
current; this represents the worst-case scenario of a continuous
short at V
OUT
.
B. The diode requires a reverse voltage rating of at least B. Pick 30-V rated Schottky diode (1N5821, MBR330, 31QD03, or
1.25 × V
IN(Max)
. SR303) or 100-V rated Fast Recovery diode (31DF1, MURD310, or
HER302).
4. Input Capacitor (C
IN
) 4. Input Capacitor (C
IN
)
An aluminum electrolytic or tantalum capacitor is needed for input C
IN
= 100 µ F, 25 V, aluminum electrolytic
bypassing. Locate C
IN
as close to the V
IN
and GND pins as
possible.
17
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