Known:

V_OUT = 3.3 V, 5 V, 12 V, or 15 V

V_IN(Max) = Maximum input voltage

I_LOAD(Max) = Maximum load current

Known:

V_OUT = 5 V

V_IN(Max) = 20 V

I_LOAD(Max) = 1 A

1. Inductor Selection (L1)

1. Inductor Selection (L1)

A. From Figure 9-2 through Figure 9-5, select the appropriate inductor code based on the intersection of V_IN(Max) and I_LOAD(Max).

A. From Figure 9-3 (TL2575-05), the intersection of 20-V line and 1-A line gives an inductor code of L330.

B. The inductor chosen should be rated for operation at 52-kHz and have a current rating of at least 1.15 × 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)

B. L330 → L1 = 330 μH

Choose from:

34042 (Schott)

PE-52627 (Pulse Engineering)

RL1952 (Renco)

2. Output Capacitor Selection (C_OUT)

2. 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 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 μF and 470 μF.

A. C_OUT = 100-μF to 470-μF, standard aluminum electrolytic

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. Although a C_OUT rated at 8 V is sufficient for V_OUT = 5 V, a higher-voltage capacitor is chosen for its typically lower ESR (and hence lower output ripple voltage) → Capacitor voltage rating = 20 V.

3. Catch Diode Selection (D1) (see Table 9-1)

3. Catch Diode Selection (D1) (see Table 9-1)

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 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.

A. Pick a diode with 3-A rating.

B. The diode requires a reverse voltage rating of at least 1.25 × V_IN(Max).

B. Pick 30-V rated Schottky diode (1N5821, MBR330, 31QD03, or SR303) or 100-V rated Fast Recovery diode (31DF1, MURD310, or HER302).

4. Input Capacitor (C_IN)

An aluminum electrolytic or tantalum capacitor is needed for input bypassing. Locate C_IN as close to the V_IN and GND pins as possible.

4. Input Capacitor (C_IN)

C_IN = 100 μF, 25 V, aluminum electrolytic

Known:

V_OUT(Nom)

V_IN(Max) = Maximum input voltage

I_LOAD(Max) = Maximum load current

Known:

V_OUT = 10 V

V_IN(Max) = 25 V

I_LOAD(Max) = 1 A

1. Programming Output Voltage (Selecting R1 and R2)

Referring to Figure 6-2, V_OUT is defined by:

Choose a value for R1 between 1 kΩ and 5 kΩ (use 1% metal-film resistors for best temperature coefficient and stability over time).

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]

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 9-6, select the appropriate inductor code based on the intersection of E × T value and I_LOAD(Max).

B. Using Figure 9-6, the intersection of 115 V • μs and 1 A corresponds to an inductor code of H470.

C. 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)

C. H470 → L1 = 470 μH

Choose from:

34048 (Schott)

PE-53118 (Pulse Engineering)

RL1961 (Renco)

3. Output Capacitor Selection (C_OUT)

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:

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.

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

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 9-1)

4. Catch Diode Selection (D1) (see Table 9-1)

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.

A. Pick a diode with a 3-A rating.

B. The diode requires a reverse voltage rating of at least 1.25 × V_IN(Max).

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