Datasheet

www.ti.com
V = V
OUT REF
1 +
where V = 1.23 V
REF
(
(
R2
R1
(
(
R2 = R1
– 1
V
OUT
V
REF
C 7758
OUT
³
(µF)
V
IN(Max)
V L1(µH)
OUT
·
TL2575 , TL2575HV
1-A SIMPLE STEP-DOWN SWITCHING VOLTAGE REGULATORS
SLVS638B MAY 2006 REVISED JANUARY 2007
PROCEDURE (Adjustable Output) EXAMPLE (Adjustable Output)
Known: Known:
V
OUT(Nom)
V
OUT
= 10 V
V
IN(Max)
= Maximum input voltage V
IN(Max)
= 25 V
I
LOAD(Max)
= Maximum load current I
LOAD(Max)
= 1 A
1. Programming Output Voltage (Selecting R1 and R2) 1. Programming Output Voltage (Selecting R1 and R2)
Referring to Fig. 2, V
OUT
is defined by: Select R1 = 1 k
R2 = 1 (10/1.23 1) = 7.13 k
Select R2 = 7.15 k (closest 1% value)
Choose a value for R1 between 1 k and 5 k (use 1% metal-film
resistors for best temperature coefficient and stability over time).
2. Inductor Selection (L1) 2. Inductor Selection (L1)
A. Calculate the "set" volts-second (E T) across L1: A. Calculate the "set" volts-second (E T) across L1:
E T = (V
IN
V
OUT
) × t
on
E T = (25 10) × (10/25) × (1000/52) [V µ s]
E T = (V
IN
V
OUT
) × (V
OUT
/V
IN
) × {1000/f
osc
(in kHz)} [V µ s] E T = 115 V µ s
NOTE: Along with I
LOAD
, the "set" volts-second (E T) constant
establishes the minimum energy storage requirement for the
inductor.
B. Using Figure 19 , select the appropriate inductor code based on B. Using Figure 19 , the intersection of 115 V µ s and 1 A
the intersection of E T value and I
LOAD(Max)
. corresponds to an inductor code of H470.
C. From Table 2 , choose the appropriate inductor based on the C. H470 L1 = 470 µ F
inductor code. Parts from three well-known inductor manufacturers
Choose from:
are given. The inductor chosen should be rated for operation at
34048 (Schott)
52-kHz and have a current rating of at least 1.15 x I
LOAD(Max)
to
PE-53118 (Pulse Engineering)
allow for the ripple current. The actual peak current in L1 (in normal
operation) can be calculated as follows:
RL1961 (Renco)
I
L1(pk)
= I
LOAD(Max)
+ (V
IN
V
OUT
) × t
on
/2L1
Where t
on
= V
OUT
/V
IN
× (1/f
osc
)
3. Output Capacitor Selection (C
OUT
) 3. Output Capacitor Selection (C
OUT
)
A. The TL2575 control loop has a two-pole two-zero frequency A.C
OUT
7785 × 25/(10 × 470) [ µ F]
response. The dominant pole-zero pair is established by C
OUT
and
C
OUT
41.4 µ F
L1. To meet stability requirements, C
OUT
must meet the following
To obtain an acceptable output voltage ripple
requirement:
C
OUT
= 220 µ F electrolytic
However, C
OUT
may need to be several times larger than the
calculated value above in order to achieve an acceptable output
ripple voltage of ~0.01 × V
OUT
.
B. C
OUT
should have a voltage rating of at least 1.5 × V
OUT
. But if a
low output ripple voltage is desired, choose capacitors with a higher
voltage ratings than the minimum required due to their typically
lower ESRs.
4. Catch Diode Selection (D1) (see Table 1 ) 4. Catch Diode Selection (D1) (see Table 1 )
A. In normal operation, the catch diode requires a current rating of A. Pick a diode with a 3-A rating.
at least 1.2 × I
LOAD(Max)
. For the most robust design, D1 should be
rated for 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 a 40-V rated Schottky diode (1N5822, MBR340, 31QD04, or
1.25 × V
IN(Max)
. SR304) or 100-V rated Fast Recovery diode (31DF1, MURD310, or
HER302)
5. Input Capacitor (C
IN
) 5. Input Capacitor (C
IN
)
An aluminum electrolytic or tantalum capacitor is needed for input C
IN
= 100 µ F, 35 V, aluminum electrolytic
bypassing. Locate C
IN
as close to V
IN
and GND pins as possible.
18
Submit Documentation Feedback