TLED2043 Datasheet, PDF (11/15 Pages) Texas Instruments – 4-CHANNEL WHITE LED DRIVER

TLED2043
4-CHANNEL WHITE LED DRIVER
SLDS147C – JUNE 2002 – REVISED NOVEMBER 2002
TYPICAL CHARACTERISTICS
efficiency
Laboratory measurements and calculations have shown that the application circuit in Figure 8 typically has the
following efficiency:
• Battery Voltage = 3.6 V: h = 78.8% (I_LED = 12.2 mA)
• Battery Voltage = 3.3 V: h = 89% (I_LED = 7.8 mA)
• Battery Voltage = 3.1 V: h = 80.4% (I_LED = 4.5 mA)
NOTE: Efficiency, as mentioned previously, refers to the efficiency of the overall solution and is defined simply
as:
h = P_LED/P_BATTERY
where P_BATTERY is the total power delivered by the battery to the entire circuit.
Thus, the efficiency indicates how effectively the TLED2043 delivers power to the LEDs. As such, LED selection
can have a significant impact on calculated efficiency. For instance, two different brands of LEDs may have
different values of V_F for the same value of I_F. According to the equation above, the LED with the larger V_F will
dissipate more power (P_LED = V_F × I_F) and thus will achieve higher efficiency. This result simply states how much
power is dissipated in the LED, not how well that particular LED converts the electrical power into light–this, of
course, solely depends on the LED construction and characteristics.
It should be pointed out that although a lower calculated efficiency is achieved using the LED with the lower V_F
(for the same I_F), the benefit of the lower V_F is that there is now more headroom for LED operation as battery
voltage drops.
LED brightness control
The brightness of an LED is proportional to its bias current. Since the current control amplifier determines the
LED current by sensing the difference between its two inputs, changing the voltage at either IN– or IN+ (or both)
affects LED current and brightness. There are several methods to achieve this.
1. Variable V_REF: use an adjustable shunt regulator (e.g., TL431/TLV431) and a variable resistor to change
voltage applied to IN– of the current-control amplifier.
• V_REF ↑, brightness ↓
2. External Analog Voltage, V_BRIGHT: applied to either IN– or IN+ of the current-control amplifier.
• IN–: V_BRIGHT ↑ , brightness ↓
• IN+: V_BRIGHT ↑ , brightness ↑
NOTE: If using this method, V_BRIGHT should be coupled resistively to the Current Control Amplifier via
Switch 3, allowing it to be disconnected from the amplifier when the switches are opened. Doing so prevents
wasted current flow from V_BRIGHT to ground when no power is applied to the TLED2043.
3. Variable Resistors: use a variable resistor anywhere along the signal path of IN– or IN+ (R1–R8); this
allows for adjustment of the voltage at IN– or IN+ (or both).
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