SNVSA76B March   2015  – March 2017 TPS61177A

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 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 I2C Timing Requirements
    7. 6.7 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Supply Voltage
      2. 7.3.2  Boost Regulator
      3. 7.3.3  Programmable Switch Frequency and Slew Rate
      4. 7.3.4  LED Current Sinks
      5. 7.3.5  Enable and Start-Up Timing
      6. 7.3.6  Input Undervoltage Protection (UVLO)
      7. 7.3.7  Overvoltage Protection (OVP)
      8. 7.3.8  Current-Sink Open Protection
      9. 7.3.9  Overcurrent Protection
      10. 7.3.10 Thermal Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Mode Selection
      2. 7.4.2 Analog and PWM Mixed Dimming Mode
      3. 7.4.3 Analog Dimming Mode
      4. 7.4.4 Direct PWM Dimming
    5. 7.5 Programming
      1. 7.5.1 Configuration Parameters
    6. 7.6 Register Maps
      1. 7.6.1  MODE (A0h)
      2. 7.6.2  CS (A1h)
      3. 7.6.3  UVLO (A2h)
      4. 7.6.4  FREQ (A3h)
      5. 7.6.5  SR (A4h)
      6. 7.6.6  ILIM (A5h)
      7. 7.6.7  Control (FFh)
      8. 7.6.8  Example - Writing to a Single RAM Register
      9. 7.6.9  Example - Writing to Multiple RAM Registers
      10. 7.6.10 Example - Saving Contents of all RAM Registers to E2PROM
      11. 7.6.11 Example - Reading from a Single RAM Register
      12. 7.6.12 Example - Reading from a Single E2PROM Register
      13. 7.6.13 Example - Reading from Multiple RAM Registers
      14. 7.6.14 Example - Reading from Multiple E2PROM Registers
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 CS Pin Unused
      2. 8.1.2 Brightness Dimming Control
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Inductor Selection
        2. 8.2.2.2 Output Capacitor Selection
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
    2. 11.2 Community Resources
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

8 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.

8.1 Application Information

8.1.1 CS Pin Unused

The TPS61177A has open/short string detection. For an unused CS string, simply short it to ground or leave it open. If the CS pin is open, the boost output voltage ramps up to overvoltage threshold during start-up. The device then detects the zero current string and removes it from the feedback loop. If the CS pin is shorted to ground, the device detects the short and immediately removes it (or them) out of feedback loop within 4 ms after device enable, and the boost output voltage does not go up to overvoltage threshold. Instead, it ramps to the regulation voltage after soft start.

Shorting unused CS pins to ground for faster start-up is recommended.

8.1.2 Brightness Dimming Control

The TPS61177A has three dimming methods. See Mode Selection section for dimming mode selection. With analog and PWM mixed dimming or pure analog dimming through the PWM control interface, the internal decoder block detects duty information from the input PWM signal, saves it in an up to 10-bits register and delivers to either a mixed mode dimming control circuit or pure analog dimming control circuit. In mixed dimming mode, the output dimming control circuit sets the DC current of six current sinks linearly between 25% and 100% at same scale to the value in up to a 10-bits register. When the brightness level is below 25% to full-scale value, the dimming control circuit turns on/off six output current sinks at same frequency with PWMB and duty cycle out of shift register. See Analog and PWM Mixed Dimming Mode section for more explanation. While in pure analog dimming mode, the output dimming control circuit sets the DC current of six current sinks linearly between 1% and 100% at same scale to the value in up to a 10-bits register. See Analog Dimming Mode section for more detail explanation.

The TPS61177A also has direct PWM dimming control through the PWM control interface. In direct PWM mode, each current sink turns on/off at the same frequency and duty cycle as the input PWM signal. See Direct PWM Dimming section for more explanation.

When in analog and PWM mixed mode, insertion of a series 10-kΩ to 20-kΩ resistor close to PWMB pin is recommended. This resistor, together with an internal capacitor, forms a low pass R-C filter with a 30-ns to 60-ns time constant. This prevents possible high frequency noise being coupled into the input PWM signal and causing interference to the internal duty cycle decoding circuit. However, it is not necessary for direct PWM mode since the duty cycle decoding circuit is disabled during direct PWM mode.

8.2 Typical Application

TPS61177A typical_application_slvsbo0.gif Figure 36. TPS61177A Typical Application

8.2.1 Design Requirements

DESIGN PARAMETER EXAMPLE VALUE
Inductor 10 µH
Minimum input voltage 2.5 V
Number of series LED 12
LED maximum forward voltage (Vf) 3.3 V
Schottky diode forward voltage (Vf) 0.2 V
Efficiency (η) 85%
Switching frequency 600 kHz
PWM input frequency 1 kHz
Maximum LED string current 30 mA

The TPS61177A is designed to support up to 2.2 A (typical) SW current. Thus, SW current must be carefully calculated with factors such as inductor, target efficiency, output voltage, load current, and so forth. In most cases, the voltage ratio between input and boost output must be < 10.

8.2.2 Detailed Design Procedure

8.2.2.1 Inductor Selection

Because selection of the inductor affects power supply steady-state operation, 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 TPS61177A is designed to work with inductor values between 4.7 µH and 22 µH. A 10-µH inductor is typically available in a smaller or lower profile package, while a 22-µH inductor may produce higher efficiency due to a slower switching frequency and/or lower inductor ripple. If the boost output current is limited by the overcurrent protection of the device, using a 10-µH inductor and the highest switching frequency maximizes controller output current capability.

Internal loop compensation for PWM control is optimized for the external component values, including typical tolerances, recommended in Table 10. 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. In a boost regulator, the inductor DC current can be calculated with Equation 1.

Equation 1. TPS61177A eq1_ILdc_lusbo0.gif

where

  • VOUT = boost output voltage
  • IOUT = boost output current
  • VIN = boost input voltage
  • η = power conversion efficiency, use 90% for TPS61177A applications

The inductor current peak-to-peak ripple can be calculated with Equation 2.

Equation 2. TPS61177A eq2_ILPP_lusbo0.gif

where

  • ΔIL(P-P) = inductor peak-to-peak ripple
  • L = inductor value
  • FS = Switching frequency
  • VOUT = boost output voltage
  • VIN = boost input voltage

Therefore, the peak current seen by the inductor is calculated with Equation 3.

Equation 3. TPS61177A eq3_ILP_lusbo0.gif

Select an inductor with a saturation current over the calculated peak current. To calculate the worst-case inductor peak current, use the minimum input voltage, maximum output voltage, and maximum load current.

Regulator efficiency is dependent on the resistance of its high current path and switching losses associated with the power FET switch and power diode. Although the TPS61177A device has optimized the internal switch resistances, the overall efficiency is affected by the inductor DC resistance (DCR). Lower DCR improves efficiency. However, there is a trade off between DCR and inductor footprint; furthermore, shielded inductors typically have higher DCR than unshielded ones. Table 10 lists the recommended inductors.

Table 10. Recommended Inductor

L (µH) DCR (mΩ) ISAT (A) Size (L × W × H mm)
Cyntec
PCMB051H-100MS 10 140 3 5.4 × 5.2 × 1.6
Taiyo
NRA6012T 100ME 10 335 1.45 6 × 6 × 1.2

8.2.2.2 Output Capacitor Selection

The output capacitor is mainly selected to meet the requirement for output ripple and loop stability. This ripple voltage is related to the capacitance of the capacitor and its equivalent series resistance (ESR). Assuming a capacitor with zero ESR, the minimum capacitance needed for a given ripple can be calculated with Equation 4:

Equation 4. TPS61177A eq4_COUT_lusbo0.gif

where

  • Vripple = peak-to-peak output ripple.

The additional part of the ripple caused by ESR is calculated using: Vripple_ESR = IOUT × RESR

Due to its low ESR, Vripple_ESR can be neglected for ceramic capacitors, but must be considered if tantalum or electrolytic capacitors are used. The controller output voltage also ripples due to the load transient that occurs during PWM dimming. The TPS61177A adopts a patented technology to limit this type of output ripple even with the minimum recommended output capacitance. In a typical application, the output ripple is less than 250 mV during PWM dimming with a 4.7-µF output capacitor. However, the output ripple decreases with higher output capacitances.

8.2.3 Application Curves

TPS61177A figure17_slvsbo0.gif
Figure 37. Start-Up Waveform
TPS61177A figure18_slvsbo0.gif
Figure 38. Start-Up Waveform