SLVSA18A September   2009  – July 2015 TPS61161-Q1

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
  6. Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Soft Start-Up
      2. 7.3.2 Open LED Protection
      3. 7.3.3 Shutdown
      4. 7.3.4 Undervoltage Lockout
      5. 7.3.5 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 LED Brightness Dimming Mode Selection
      2. 7.4.2 PWM Brightness Dimming
      3. 7.4.3 Digital 1 Wire Brightness Dimming
      4. 7.4.4 EasyScale: 1-Wire Digital Dimming
  8. Applications and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Current Program
        2. 8.2.2.2 Maximum Output Current
        3. 8.2.2.3 Inductor Selection
        4. 8.2.2.4 Schottky Diode Selection
        5. 8.2.2.5 Compensation Capacitor Selection
        6. 8.2.2.6 Input and Output Capacitor Selection
      3. 8.2.3 Application Curves
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
    3. 10.3 Thermal Considerations
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Community Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

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8 Applications 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.

8.1 Application Information

In the application, TPS61161-Q1 drives 10 LEDs, the output current is set at 20mA, the circuit can support wide range input voltage from 3 V to 18 V. By applying PWM signal on CTRL pin, the circuit can realize PWM dimming control.

8.2 Typical Application

TPS61161-Q1 typ_app_lvs791.gifFigure 15. LED Drivers With 10 White LEDs Schematic

8.2.1 Design Requirements

Table 4 lists the input parameters for this design example.

Table 4. Design Parameters

PARAMETER EXAMPLE VALUE
Brightness control PWM Dimming
Input voltage 3 V to 18 V
Output current 20 mA
LED loads 10 LEDs

8.2.2 Detailed Design Procedure

8.2.2.1 Current Program

The FB voltage is regulated by a low 0.2-V reference voltage. The LED current is programmed externally using a current-sense resistor in series with the LED string. The value of the RSET is calculated using Equation 2:

Equation 2. TPS61161-Q1 q1_iled_lvs791.gif

where

  • ILED = output current of LEDs
  • VFB = regulated voltage of FB
  • RSET = current sense resistor

The output current tolerance depends on the FB accuracy and the current sensor resistor accuracy.

8.2.2.2 Maximum Output Current

The overcurrent limit in a boost converter limits the maximum input current and thus maximum input power for a given input voltage. Maximum output power is less than maximum input power due to power conversion losses. Therefore, the current limit setting, input voltage, output voltage and efficiency can all change maximum current output. The current limit clamps the peak inductor current; therefore, the ripple must be subtracted to derive maximum dc current. The ripple current is a function of switching frequency, inductor value and duty cycle. The following equations take into account of all of the previous factors for maximum output current calculation.

Equation 3. TPS61161-Q1 q3r_lvs791.gif

where

  • IP = inductor peak to peak ripple
  • L = inductor value
  • VF = Schottky diode forward voltage
  • FS = switching frequency
  • VOUT = output voltage of the boost converter. It is equal to the sum of VFB and the voltage drop across LEDs.
Equation 4. TPS61161-Q1 q4_ioutmax_lvs791.gif

where

  • IOUT_MAX = maximum output current of the boost converter
  • ILIM = overcurrent limit
  • η = efficiency

For instance, when VIN is 3 V, 8 LEDs output equivalent to VOUT of 26 V, the inductor is 22 µH, the Schottky forward voltage is 0.2 V; and then the maximum output current is 65 mA in typical condition. When VIN is 5 V, 10 LEDs output equivalent to VOUT of 32 V, the inductor is 22 µH, the Schottky forward voltage is 0.2 V; and then the maximum output current is 85 mA in typical condition.

8.2.2.3 Inductor Selection

The selection of the inductor affects steady state operation as well as transient behavior and loop stability. These factors make it the most important component in power regulator design. There are three important inductor specifications, inductor value, dc resistance and saturation current. Considering inductor value alone is not enough.

The inductor value determines the inductor ripple current. Choose an inductor that can handle the necessary peak current without saturating, according to half of the peak-to-peak ripple current given by Equation 3, pause the inductor dc current given by:

Equation 5. TPS61161-Q1 q5_iindc_lvs791.gif

Inductor values can have ±20% tolerance with no current bias. When the inductor current approaches saturation level, its inductance can decrease 20% to 35% from the 0-A value depending on how the inductor vendor defines saturation current. Using an inductor with a smaller inductance value forces discontinuous PWM when the inductor current ramps down to zero before the end of each switching cycle. This reduces the boost converter’s maximum output current, causes large input voltage ripple and reduces efficiency. Large inductance value provides much more output current and higher conversion efficiency. For these reasons, TI recommends a 10-µH to 22-µH inductor value range. A 22-µH inductor optimized the efficiency for most application while maintaining low inductor peak to peak ripple. Table 5 lists the recommended inductor for the TPS61161-Q1. When recommending inductor value, the factory has considered –40% and 20% tolerance from its nominal value.

TPS61161-Q1 has built-in slope compensation to avoid sub-harmonic oscillation associated with current mode control. If the inductor value is lower than 10 µH, the slope compensation may not be adequate, and the loop can be unstable. Therefore, customers must verify the inductor in their application if it is different from the recommended values.

Table 5. Recommended Inductors for TPS61161-Q1

PART NUMBER L
(µH)
DCR MAX
(Ω)
SATURATION CURRENT
(mA)
SIZE
(L × W × H mm)
VENDOR
LQH3NPN100NM0 10 0.3 750 3×3×1.5 Murata
VLCF5020T-220MR75-1 22 0.4 750 5×5×2 TDK
CDH3809/SLD 10 0.3 570 4×4×1 Sumida
A997AS-220M 22 0.4 510 4×4×1.8 TOKO

8.2.2.4 Schottky Diode Selection

The high switching frequency of the TPS61161-Q1 demands a high-speed rectification for optimum efficiency. Ensure that the diode average and peak current rating exceeds the average output current and peak inductor current. In addition, the diode’s reverse breakdown voltage must exceed the open LED protection voltage. The ONSemi MBR0540 and the ZETEX ZHCS400 are recommended for TPS61161-Q1.

8.2.2.5 Compensation Capacitor Selection

The compensation capacitor C3 (see the block diagram), connected from COMP pin to GND, is used to stabilize the feedback loop of the TPS61161-Q1. Use a 220-nF ceramic capacitor for C3.

8.2.2.6 Input and Output Capacitor Selection

The output capacitor is mainly selected to meet the requirements for the output ripple and loop stability. This ripple voltage is related to the capacitor’s capacitance and its equivalent series resistance (ESR). Assuming a capacitor with zero ESR, the minimum capacitance needed for a given ripple can be calculated by

Equation 6. TPS61161-Q1 q6_cout_lvs791.gif

where

  • VRIPPLE = peak-to-peak output ripple.

The additional output ripple component caused by ESR is calculated using:

Equation 7. TPS61161-Q1 q7_vripp_lvs791.gif

Due to its low ESR, Vripple_ESR can be neglected for ceramic capacitors, but must be considered if tantalum or electrolytic capacitors are used.

Take care when evaluating a ceramic capacitor’s derating under dc bias, aging, and ac signal. For example, larger form factor capacitors (in 1206 size) have a resonant frequencies in the range of the switching frequency. So the effective capacitance is significantly lower. The dc bias can also significantly reduce capacitance. Ceramic capacitors can loss as much as 50% of its capacitance at its rated voltage. Therefore, leave the margin on the voltage rating to ensure adequate capacitance at the required output voltage.

TI recommends the capacitor in the range of 1 µF to 4.7 µF for input side. The output requires a capacitor in the range of 0.47 µF to 10 µF. The output capacitor affects the loop stability of the boost regulator. If the output capacitor is below the range, the boost regulator can potentially become unstable. For example, if use the output capacitor of 0.1 µF, a 470 nF compensation capacitor must be used for the loop stable.

The popular vendors for high value ceramic capacitors are:

TDK (http://www.component.tdk.com/components.php)

Murata (http://www.murata.com/cap/index.html)

8.2.3 Application Curves

TPS61161-Q1 wvfrm_01_12v_slvsa18.png
Figure 16. Input Voltage 12 V
TPS61161-Q1 wvfrm_03_4v_slvsa18.png
Figure 18. Input Voltage 4 V
TPS61161-Q1 wvfrm_02_18v_slvsa18.png
Figure 17. Input Voltage 18 V