SLVSAN6B February   2011  – September 2016 TPS61181A

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Device Comparison Table
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Supply Voltage
      2. 8.3.2 Boost Regulator
      3. 8.3.3 Enable and Start-Up
      4. 8.3.4 Overcurrent, Overvoltage, and Short-Circuit Protection
      5. 8.3.5 IFB Pin Unused
    4. 8.4 Device Functional Modes
      1. 8.4.1 Current Program and PWM Dimming
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Inductor Selection
        2. 9.2.2.2 Output Capacitor Selection
        3. 9.2.2.3 Audible Noise Reduction
        4. 9.2.2.4 Isolation MOSFET Selection
      3. 9.2.3 Application Curves
    3. 9.3 Additional Application Circuits
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device And Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
    2. 12.2 Receiving Notification of Documentation Updates
    3. 12.3 Community Resources
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

9 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.

9.1 Application Information

The TPS61176 provides a high-performance LED lighting solution for tablets, notebooks and other low power LCD backlit displays. The device can drive 6 strings of 10 series LEDs in a compact and high efficient solution. The LED current is controlled via a logic level PWM input and the LED current level is set using an ISET resistor.

9.2 Typical Application

TPS61181A app_sch_slvsan6.gif Figure 9. TPS61181A Typical Application

9.2.1 Design Requirements

For typical LED driver applications, use the parameters listed in Table 2.

Table 2. Design Parameters

DESIGN PARAMETER EXAMPLE VALUE
Minimum input voltage 4.5 V
Output voltage Vin to 38 V
Current accuracy 20 mA (typical)
Oscillator frequency 1 MHz (typical)

9.2.2 Detailed Design Procedure

9.2.2.1 Inductor Selection

Because the selection of the inductor affects the steady-state operation of a power supply, transient behavior and loop stability, the inductor is the most important component in switching power regulator design. There are three specifications most important to the performance of the inductor: inductor value, DC resistance, and saturation current. The TPS61181A device is designed to work with inductor values between 4.7 μH and 10 μH. A 4.7-μH inductor may be available in a smaller or lower profile package, while 10 μH may produce higher efficiency due to lower inductor ripple. If the boost output current is limited by the overcurrent protection of the device, using a 10-μH inductor can offer higher output current.

The internal loop compensation for the PWM control is optimized for the recommended component values, including typical tolerances. 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 zero current value depending on how the inductor vendor defines saturation

In a boost regulator, the inductor DC current can be calculated as:

Equation 2. TPS61181A q2_idc_lvs801.gif

where

  • VO = boost output voltage
  • Io = boost output current
  • Vin = boost input voltage
  • η = power conversion efficiency, use 90% for TPS61181A applications

The inductor current peak-to-peak ripple can be calculated as:

Equation 3. TPS61181A q3_ipp_lvs801.gif

where

  • Ipp = inductor peak-to-peak ripple
  • L = inductor value
  • Fs= switching frequency
  • Vbat= boost input voltage

Therefore, the peak current seen by the inductor is

Equation 4. TPS61181A q4_ip_lvs801.gif

Select the inductor with saturation current at least 25% higher than the calculated peak current. To calculate the worse case inductor peak current, use minimum input voltage, maximum output voltage and maximum load current.

Regulator efficiency is dependent on the resistance of its high current path, switching losses associated with the PWM switch and power diode. Although the TPS61181A device have optimized the internal switch resistance, the overall efficiency still relies on the DC resistance (DCR) of the inductor; lower DCR improves efficiency. However, there is a trade off between DCR and inductor footprint. Furthermore, shielded inductors typically have a higher DCR than unshielded ones. Table 3 lists recommended inductor models.

Table 3. Recommended Inductors for TPS61181A

L
(μH)
DCR TYPICAL
(mΩ)
Isat
(A)
SIZE
(L×W×H mm)
TOKO
A915AY-4R7M 4.7 38 1.87 5.2 × 5.2 × 3
A915AY-100M 10 75 1.24 5.2 × 5.2 × 3
TDK
SLF6028T-4R7M1R6 4.7 28.4 1.6 6 × 6 × 2.8
SLF6028T-100M1R3 10 53.2 1.3 6 × 6 × 2.8

9.2.2.2 Output Capacitor Selection

During PWM brightness dimming, the load transient causes voltage ripple on the output capacitor. Since the PWM dimming frequency is in the audible frequency range, the ripple can produce audible noises on the output ceramic capacitor. There are two ways to reduce or eliminate this audible noise. The first option is to select PWM dimming frequency outside the audible range. This means the dimming frequency needs be to lower than 200 Hz or higher than 30 KHz. The potential issue with a very low dimming frequency is that WLED on/off can become visible and thus cause a flickering effect on the display. On the other hand, high dimming frequency can compromise the dimming range since the LED current accuracy and current match are difficult to maintain at low dimming duty cycle. The second option is to reduce the amount of the output ripple, and therefore minimize the audible noise.

The TPS61181A adopts a patented technology to limit output ripple even with small output capacitance. In a typical application, the output ripple is less than 200 mV during PWM dimming with a 4.7-μF output capacitor, and the audible noise is not noticeable. The devices are designed to be stable with output capacitor down to 1 μF. However, the output ripple will increase with lower output capacitor.

Care must be taken when evaluating the derating of a ceramic capacitor due to applied dc voltage, aging and over frequency. For example, larger form factor capacitors (in 1206 size) have their self resonant frequencies in the switching frequency range of the TPS61181A. So the effective capacitance is significantly lower. Therefore, it may be necessary to use small capacitors in parallel instead of one large capacitor.

9.2.2.3 Audible Noise Reduction

Ceramic capacitors can produce audible noise if the frequency of its AC voltage ripple is in the audible frequency range. In TPS61181A applications, both input and output capacitors are subject to AC voltage ripple during PWM brightness dimming. The device integrates a patented technology to minimize the ripple voltage, and thus audible noises.

To further reduce the audible noise, one effective way is to use two or three small size capacitors in parallel instead of one large capacitor. The application circuit in Figure 16 uses two 2.2-μF/25-V ceramic capacitors at the input and two 1-μF/50-V ceramic capacitors at the output. All of the capacitors are in 0805 package. Although the output ripple during PWM dimming is higher than with one 4.7 μF in a 1206 package, the overall audible noise is lower.

In addition, connecting a 10-nF/50V ceramic capacitor between the VO pin and IFB1 pin can further reduce the output AC ripple during the PWM dimming. Since this capacitor is subject to large AC ripple, choose a small package such as 0402 to prevent it from producing noise.

9.2.2.4 Isolation MOSFET Selection

The TPS61181A provides a gate driver to an external P-channel MOSFET which can be turned off during device shutdown or fault condition. This MOSFET can provide a true shutdown function, and also protect the battery from output short circuit conditions. The source of the PMOS must be connected to the input, and a pullup resistor is required between the source and gate of the FET to keep the FET off during device shutdown. To turn on the isolation FET, the Fault pin is pulled low, and clamped at 8 V below the VBAT pin voltage.

During device shutdown or fault condition, the isolation FET is turned off, and the input voltage is applied on the isolation MOSFET. During a short circuit condition, the catch diode (D2 in typical application circuit) is forward biased when the isolation FET is turned off. The drain of the isolation FET swings below ground. The voltage across the isolation FET can be momentarily greater than the input voltage. Therefore, select a 30-V MOSFET for a 24-V maximum input. The on resistance of the FET has a large impact on power conversion efficiency since the FET carries the input voltage. Select a MOSFET with Rds(on) less than 100 mΩ to limit the power losses.

9.2.3 Application Curves

TPS61181A eff_io1a_lvsan6.gif
Figure 10. Efficiency vs Output Current
TPS61181A eff_dc1_lvsan6.gif
Figure 12. Efficiency vs Dimming Duty Cycle
TPS61181A eff_dc3_lvsan6.gif
Figure 14. Efficiency vs Dimming Duty Cycle
TPS61181A eff_io2a_lvsan6.gif
Figure 11. Efficiency vs Output Current
TPS61181A eff_dc2_lvsan6.gif
Figure 13. Efficiency vs Dimming Duty Cycle
TPS61181A eff_dc4_lvsan6.gif
Figure 15. Efficiency vs Dimming Duty Cycle

9.3 Additional Application Circuits

TPS61181A app_sch3_lvsan6.gif Figure 16. Audible Noise-Reduction Circuit
TPS61181A app_led3_lvsan6.gif Figure 17. TPS61181A for Three Strings of LEDs
TPS61181A app_led3dc_lvsan6.gif Figure 18. TPS61181A for Three Strings of LEDs With Double Current
TPS61181A app_led2hd_lvsan6.gif Figure 19. TPS61181A for Two Strings, High-Brightness LEDs Application
TPS61181A app_led1hb_lvsan6.gif Figure 20. TPS61181A for One-String, High-Brightness LED Application
TPS61181A app_led10ser_lvsan6.gif Figure 21. TPS61181A Driving External PFET for True Shutdown Application
TPS61181A app_led6ser_lvsan6.gif Figure 22. TPS61181A With Separate VBAT Power for Low Voltage-Input Application
TPS61181A app_led620_lvsan6.gif Figure 23. TPS61181A + TPS60151 for One Cell Li-Ion Battery Power Application