SLVS638C January   2006  – November 2014

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Functional Block Diagram
  5. Revision History
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 Handling Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics — TL2575
    6. 7.6 Electrical Characteristics — TL2575HV
    7. 7.7 Typical Characteristics
  8. Parameter Measurement Information
    1. 8.1 Test Circuits
  9. Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Feedback Connection
      2. 9.3.2 ON/OFF Input
    4. 9.4 Device Functional Modes
      1. 9.4.1 Standby Mode
  10. 10Application and Implementation
    1. 10.1 Typical Application
      1. 10.1.1 Design Requirements
      2. 10.1.2 Detailed Design Procedure
        1. 10.1.2.1 Input Capacitor (CIN)
        2. 10.1.2.2 Output Capacitor (COUT)
        3. 10.1.2.3 Catch Diode
        4. 10.1.2.4 Inductor
        5. 10.1.2.5 Output Voltage Ripple and Transients
        6. 10.1.2.6 Grounding
        7. 10.1.2.7 Reverse Current Considerations
        8. 10.1.2.8 Buck Regulator Design Procedure
        9. 10.1.2.9 Inductor Selection Guide
      3. 10.1.3 Application Curves
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
  13. 13Device and Documentation Support
    1. 13.1 Related Links
    2. 13.2 Trademarks
    3. 13.3 Electrostatic Discharge Caution
    4. 13.4 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

10 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

10.1 Typical Application

typ_app_fix_lvs638.gif
Pin numbers are for the KTT (TO-263) package.
Figure 12. Typical Application Circuit (Fixed Version)

10.1.1 Design Requirements

  • Input capacitor for stability
  • Output capacitor for loop stability and ripple filtering
  • Catch diode to filter noise
  • Output inductor depending on the mode of operation

10.1.2 Detailed Design Procedure

10.1.2.1 Input Capacitor (CIN)

For stability concerns, an input bypass capacitor (electrolytic, CIN ≥ 47 μF) needs to be located as close as possible to the regulator. For operating temperatures below –25°C, CIN 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 calculated as shown in Equation 1.

Equation 1. IC,RMS >  1.2 (ton / T) ILOAD

where

  • ton/T = VOUT/VIN \{buck regulator\}
  • ton/T = |VOUT|/(|VOUT| + VIN) \{buck-boost regulator\}

10.1.2.2 Output Capacitor (COUT)

For both loop stability and filtering of ripple voltage, an output capacitor 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 as shown in Equation 2.

Equation 2. Output ripple voltage = (ESR of COUT) × (inductor ripple current)

Output ripple of 50 mV to 150 mV typically can be achieved with capacitor values of 220 μF to 680 μF. Larger COUT can reduce the ripple 20 mV to 50 mV peak to peak. To improve further on output ripple, paralleling of standard electrolytic capacitors may be used. Alternatively, higher-grade capacitors such as high frequency, low inductance, or low ESR can be used.

The following should be taken into account when selecting COUT:

  • At cold temperatures, the ESR of the electrolytic capacitors can rise dramatically (typically 3× nominal value at –25°C). Because solid-tantalum capacitors have significantly better ESR specifications at cold temperatures, they should be used at operating temperature lower than –25°C. As an alternative, tantalums can also be paralleled to aluminum electrolytics and should contribute 10% to 20% to the total capacitance.
  • Low ESR for COUT is desirable for low output ripple. However, the ESR should be greater than 0.05 Ω to avoid the possibility of regulator instability. Hence, a sole tantalum capacitor used for COUT is most susceptible to this occurrence.
  • The ripple current rating of the capacitor, 52 kHz, should be at least 50% higher than the peak-to-peak inductor ripple current.

10.1.2.3 Catch Diode

As with other external components, the catch diode should be placed close to the output to minimize unwanted noise. Schottky diodes have fast switching speeds and low forward voltage drops and, thus, offer the best performance, especially for switching regulators with low output voltages (VOUT < 5 V). If a high-efficiency, fast-recovery, or ultra-fast-recovery diode is used in place of a Schottky, it should have a soft recovery (versus abrupt turn-off characteristics) to avoid the chance of causing instability and EMI. Standard 50- to 60-Hz diodes, such as the 1N4001 or 1N5400 series, are not suitable.

10.1.2.4 Inductor

Proper inductor selection is key to the performance-switching power-supply designs. One important factor to consider is whether the regulator is used in continuous mode (inductor current flows continuously and never drops to zero) or in discontinuous mode (inductor current goes to zero during the normal switching cycle). Each mode has distinctively different operating characteristics and, therefore, can affect the regulator performance and requirements. In many applications, the continuous mode is the preferred mode of operation, since it offers greater output power with lower peak currents, and also can result in lower output ripple voltage. The advantages of continuous mode of operation come at the expense of a larger inductor required to keep inductor current continuous, especially at low output currents and/or high input voltages.

The TL2575 and TL2575HV devices can operate in either continuous or discontinuous mode. With heavy load currents, the inductor current flows continuously and the regulator operates in continuous mode. Under light load, the inductor fully discharges and the regulator is forced into the discontinuous mode of operation. For light loads (approximately 200 mA or less), this discontinuous mode of operation is perfectly acceptable and may be desirable solely to keep the inductor value and size small. Any buck regulator eventually operates in discontinuous mode when the load current is light enough.

The type of inductor chosen can have advantages and disadvantages. If high performance or high quality is a concern, then more-expensive toroid core inductors are the best choice, as the magnetic flux is contained completely within the core, resulting in less EMI and noise in nearby sensitive circuits. Inexpensive bobbin core inductors, however, generate more EMI as the open core does not confine the flux within the core. Multiple switching regulators located in proximity to each other are particularly susceptible to mutual coupling of magnetic fluxes from each other’s open cores. In these situations, closed magnetic structures (such as a toroid, pot core, or E-core) are more appropriate.

Regardless of the type and value of inductor used, the inductor never should carry more than its rated current. Doing so may cause the inductor to saturate, in which case the inductance quickly drops, and the inductor looks like a low-value resistor (from the dc resistance of the windings). As a result, switching current rises dramatically (until limited by the current-by-current limiting feature of the TL2575 and TL2575HV devices) and can result in overheating of the inductor and the IC itself.

NOTE

Different types of inductors have different saturation characteristics.

10.1.2.5 Output Voltage Ripple and Transients

As with any switching power supply, the output of the TL2575 and TL2575HV devices have a sawtooth ripple voltage at the switching frequency. Typically about 1% of the output voltage, this ripple is due mainly to the inductor sawtooth ripple current and the ESR of the output capacitor (see Output Capacitor (COUT)). Furthermore, the output also may contain small voltage spikes at the peaks of the sawtooth waveform. This is due to the fast switching of the output switch and the parasitic inductance of COUT. These voltage spikes can be minimized through the use of low-inductance capacitors.

There are several ways to reduce the output ripple voltage: a larger inductor, a larger COUT, or both. Another method is to use a small LC filter (20 μH and 100 μF) at the output. This filter can reduce the output ripple voltage by a factor of 10 (see Figure 11).

10.1.2.6 Grounding

The power and ground connections of the TL2575 and TL2575HV devices must be low impedance to help maintain output stability. For the 5-pin packages, both pin 3 and tab are ground, and either connection can be used as they are both part of the same lead frame. With the 16-pin package, all the ground pins (including signal and power grounds) should be soldered directly to wide PCB copper traces to ensure low-inductance connections and good thermal dissipation.

10.1.2.7 Reverse Current Considerations

There is an internal diode from the output to VIN. Therefore, the device does not protect against reverse current and care must be taken to limit current in this scenario.

10.1.2.8 Buck Regulator Design Procedure

PROCEDURE (Fixed Output) EXAMPLE (Fixed Output)

Known:

VOUT = 3.3 V, 5 V, 12 V, or 15 V

VIN(Max) = Maximum input voltage

ILOAD(Max) = Maximum load current

Known:

VOUT = 5 V

VIN(Max) = 20 V

ILOAD(Max) = 1 A

1. Inductor Selection (L1)

1. Inductor Selection (L1)

A. From Figure 13 through Figure 16, select the appropriate inductor code based on the intersection of VIN(Max) and ILOAD(Max).

A. From Figure 14 (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 × ILOAD(Max) to allow for the ripple current. The actual peak current in L1 (in normal operation) can be calculated as follows:

IL1(pk) = ILOAD(Max) + (VIN – VOUT) × ton / 2L1

Where ton = VOUT / VIN × (1 / fosc)

B. L330 → L1 = 330 μH

Choose from:

34042 (Schott)

PE-52627 (Pulse Engineering)

RL1952 (Renco)

2. Output Capacitor Selection (COUT)

2. Output Capacitor Selection (COUT)

A. The TL2575 control loop has a two-pole two-zero frequency response. The dominant pole-zero pair is established by COUT and L1. To meet stability requirements while maintaining an acceptable output ripple voltage (Vripple ≉ 0.01 × VOUT), the recommended range for a standard aluminum electrolytic COUT is between 100 μF and 470 μF.

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

B. COUT should have a voltage rating of at least 1.5 × VOUT. 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 COUT rated at 8 V is sufficient for VOUT = 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 1)

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

A. In normal operation, the catch diode requires a current rating of at least 1.2 × ILOAD(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 VOUT.

A. Pick a diode with 3-A rating.

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

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

4. Input Capacitor (CIN)

CIN = 100 μF, 25 V, aluminum electrolytic

PROCEDURE (Adjustable Output) EXAMPLE (Adjustable Output)

Known:

VOUT(Nom)

VIN(Max) = Maximum input voltage

ILOAD(Max) = Maximum load current

Known:

VOUT = 10 V

VIN(Max) = 25 V

ILOAD(Max) = 1 A

1. Programming Output Voltage (Selecting R1 and R2)

Referring to Figure 2, VOUT is defined by:

q_vout_pr.gif

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

q_r2_pr.gif

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 = (VIN – VOUT) × ton

E × T = (VIN – VOUT) × (VOUT / VIN) × \{1000 / fosc(in kHz)\} [V × μs]

NOTE: Along with ILOAD, 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 17, select the appropriate inductor code based on the intersection of E × T value and ILOAD(Max).

B. Using Figure 17, 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 ILOAD(Max) to allow for the ripple current. The actual peak current in L1 (in normal operation) can be calculated as follows:

IL1(pk) = ILOAD(Max) + (VIN – VOUT) × ton / 2L1

Where ton = VOUT / VIN × (1 / fosc)

C. H470 → L1 = 470 μH

Choose from:

34048 (Schott)

PE-53118 (Pulse Engineering)

RL1961 (Renco)

3. Output Capacitor Selection (COUT)

3. Output Capacitor Selection (COUT)

A. The TL2575 control loop has a two-pole two-zero frequency response. The dominant pole-zero pair is established by COUT and L1. To meet stability requirements, COUT must meet the following requirement:

q_cout_pr.gif

However, COUT may need to be several times larger than the calculated value above in order to achieve an acceptable output ripple voltage of ~0.01 × VOUT.

A. COUT ≥ 7785 × 25 / (10 × 470) [μF]

COUT ≥ 41.4 μF

To obtain an acceptable output voltage ripple → COUT = 220 μF electrolytic

B. COUT should have a voltage rating of at least 1.5 × VOUT. 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 at least 1.2 × ILOAD(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 VOUT.

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

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

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

5. Input Capacitor (CIN)

CIN = 100 μF, 35 V, aluminum electrolytic

Table 1. Diode Selection Guide

VR SCHOTTKY FAST RECOVERY
1A 3A 1A 3A
20 V 1N5817
MBR120P
SR102
1N5820
MBR320
SR302
The following diodes are all rated to 100 V:
11DF1
MUR110
HER102
The following diodes are all rated to 100 V:
31DF1
MURD310
HER302
30 V 1N5818
MBR130P
11DQ03
SR103
1N5821
MBR330
31DQ03
SR303
40 V 1N5819
MBR140P
11DQ04
SR104
IN5822
MBR340
31DQ04
SR304
50 V MBR150
11DQ05
SR105
MBR350
31DQ05
SR305
60 V MBR160
11DQ06
SR106
MBR360
31DQ06
SR306

10.1.2.9 Inductor Selection Guide

Inductor Value Selection Guide for Continuous-Mode Operation
ai_induct_33v.gifFigure 13. TL2575-33
ai_induct_12v.gifFigure 15. TL2575-12
ai_induct_adj.gifFigure 17. TL2575-ADJ
ai_induct_50v.gifFigure 14. TL2575-50
ai_induct_15v.gifFigure 16. TL2575-15

10.1.3 Application Curves

g_sw_waves_a.gif
A. Output pin voltage, 10 V/Div
B. Output pin current, 1 A/Div
C. Inductor current, 0.5 A/Div
D. Output ripple voltage, 20 mV/Div
Figure 18. Switching Waveforms
fig_14_graph_slvs638.gifFigure 19. Load Transient Response