TL2575_10 Datasheet, PDF (18/29 Pages) Texas Instruments – 1-A SIMPLE STEP-DOWN SWITCHING VOLTAGE REGULATORS

TL2575, TL2575HV
1-A SIMPLE STEP-DOWN SWITCHING VOLTAGE REGULATORS
SLVS638B – MAY 2006 – REVISED JANUARY 2007
www.ti.com
PROCEDURE (Adjustable Output)
EXAMPLE (Adjustable Output)
Known:
V_OUT(Nom)
V_IN(Max) = Maximum input voltage
I_LOAD(Max) = Maximum load current
1. Programming Output Voltage (Selecting R1 and R2)
Referring to Fig. 2, V_OUT is defined by:
( ^{V = V OUT}
REF
1
+
R2
R1
where V_REF = 1.23 V
Choose a value for R1 between 1 kΩ and 5 kΩ (use 1% metal-film
resistors for best temperature coefficient and stability over time).
( ^{R2 = R1}
V_OUT
V_REF
–1
Known:
V_OUT = 10 V
V_IN(Max) = 25 V
I_LOAD(Max) = 1 A
1. Programming Output Voltage (Selecting R1 and R2)
Select R1 = 1 kΩ
R2 = 1 (10/1.23 – 1) = 7.13 kΩ
Select R2 = 7.15 kΩ (closest 1% value)
2. Inductor Selection (L1)
2. Inductor Selection (L1)
A. Calculate the "set" volts-second (E•T) across L1:
E•T = (V_IN – V_OUT) × t_on
E•T = (V_IN – V_OUT) × (V_OUT/V_IN) × {1000/f_osc(in kHz)} [V•µs]
NOTE: Along with I_LOAD, the "set" volts-second (E•T) constant
establishes the minimum energy storage requirement for the
inductor.
A. Calculate the "set" volts-second (E•T) across L1:
E•T = (25 – 10) × (10/25) × (1000/52) [V•µs]
E•T = 115 V•µs
B. Using Figure 19, select the appropriate inductor code based on
the intersection of E•T value and I_LOAD(Max).
C. From Table 2, choose the appropriate inductor based on the
inductor code. Parts from three well-known inductor manufacturers
are given. The inductor chosen should be rated for operation at
52-kHz and have a current rating of at least 1.15 x I_LOAD(Max) to
allow for the ripple current. The actual peak current in L1 (in normal
operation) can be calculated as follows:
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)
A. The TL2575 control loop has a two-pole two-zero frequency
response. The dominant pole-zero pair is established by C_OUT and
L1. To meet stability requirements, C_OUT must meet the following
requirement:
C_OUT
³
7758
V_IN(Max)
(µF)
V_OUT · L1(µH)
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.
B. Using Figure 19, the intersection of 115 V•µs and 1 A
corresponds to an inductor code of H470.
C. H470 → L1 = 470 µF
Choose from:
34048 (Schott)
PE-53118 (Pulse Engineering)
RL1961 (Renco)
3. Output Capacitor Selection (C_OUT)
A.C_OUT ≥ 7785 × 25/(10 × 470) [µF]
C_OUT ≥ 41.4 µF
To obtain an acceptable output voltage ripple →
C_OUT = 220 µF electrolytic
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
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
1.25 × V_IN(Max).
A. Pick a diode with a 3-A rating.
B. Pick a 40-V rated Schottky diode (1N5822, MBR340, 31QD04, or
SR304) or 100-V rated Fast Recovery diode (31DF1, MURD310, or
HER302)
5. Input Capacitor (C_IN)
An aluminum electrolytic or tantalum capacitor is needed for input
bypassing. Locate C_IN as close to V_IN and GND pins as possible.
5. Input Capacitor (C_IN)
C_IN = 100 µF, 35 V, aluminum electrolytic
18
Submit Documentation Feedback