SLVS641B April   2008  – March  2015 TL4242

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 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 PWM Input
      2. 7.3.2 ST Output
        1. 7.3.2.1 Function and Timing Diagram
    4. 7.4 Device Functional Modes
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Input Supply Voltage
      2. 8.1.2 Power Dissipation in TL4242
      3. 8.1.3 Setting the Output Current
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Input Voltage
        2. 8.2.2.2 Shunt Resistor
      3. 8.2.3 Application Curve
  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 Trademarks
    2. 11.2 Electrostatic Discharge Caution
    3. 11.3 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

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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 Input Supply Voltage

The input supply voltage calculates as the sum of the LED forward voltages, the TL4242 drop voltage, and the voltage drop across the shunt resistor, RREF. The total LED forward voltage depends on the type of LED and the number of LEDs in the string. The TL4242 drop voltage must be greater than Vdr (typically 350 mV), but must not be too high as this could cause excessive power dissipation inside the device. The voltage drop across the shunt resistor is typically 177 mV.

8.1.2 Power Dissipation in TL4242

Power dissipation in the TL4242 will come from two sources:

  • Quiescent power: (Input voltage × Supply current)
  • Power dissipation in the pass element:
    • Equation 3. ((VI – VQ) × IQ)

The power dissipation in the pass element can be significant if the input voltage, VI, is much higher than VQ. The power dissipation is also dependent on the LED current. Equation 4 is an example calculation using the design parameters listed in Table 2.

Table 2. Design Parameters

DESIGN PARAMETER EXAMPLE VALUE
VI 13.5 V
VQ 7 V
IQ 0.2 A
Equation 4. ((VI – VQ) × IQ) = (13.5 – 7) × 0.2 = 1.3 W

In Equation 4, there is 1.3 W of power dissipation in the pass element of the TL4242. This power dissipation will cause the junction temperature of the device to increase. The increase in temperature is equal to RθJA × 1.3 W. Please note that RθJA is dependent on the PCB layout.

8.1.3 Setting the Output Current

An external shunt resistor in the ground path of the connected LEDs is used to sense the LED current. A regulation loop holds the voltage drop at the shunt resistor at a constant level of 177 mV (typical). The constant-current level can be adjusted by selecting the shunt resistance, RREF. Calculate the typical output current using the equation:

Equation 5. IQ,typ = VREF/RREF

where

The equation applies for RREF = 0.39 Ω to 10 Ω.

The output current is shown as a function of the reference resistance in Figure 1.

8.2 Typical Application

Figure 6 shows a typical application with the TL4242 driving three LEDs in series.

TL4242 ai_app_cx_lvs732.gifFigure 6. Application Circuit

8.2.1 Design Requirements

For this design example, use the following as the input parameters in Table 3.

Table 3. Design Parameters

DESIGN PARAMETERS EXAMPLE VALUE
# of LEDs 3
Forward Voltage of each LED 3.5 V
LED Current 377 mA

8.2.2 Detailed Design Procedure

8.2.2.1 Input Voltage

The input voltage must be greater than the sum of the LED forward voltages, the TL4242 drop voltage, and the voltage drop across the shunt resistor, RREF. In this design example, the total LED forward voltage is 3 × 3.5 V = 10.5 V. The typical TL4242 drop voltage is 350 mV. The typical voltage drop across the shunt resistor is 177 mV. In sum, the input voltage must be greater than 10.5 + 0.350 + 0.177 = 11.027 V. An appropriate input voltage for this application would be 12 V.

8.2.2.2 Shunt Resistor

The shunt resistor value, RREF, can be calculated based on the desired LED current.

Equation 6. IQ,typ = VREF/RREF

where

As shown in Design Requirements, the desired LED current is 377 mA. The appropriate RREF value for this application calculates to be 0.47 Ω.

8.2.3 Application Curve

TL4242 g_iout_rref_lvs641.gifFigure 7. Output Current vs External Resistor