FAN5611_06 Datasheet, PDF (9/14 Pages) Fairchild Semiconductor – Low-Dropout LED Drivers for White, Blue, or any Color LED

Example 3: Drive white, blue, red, amber LEDs string
Assuming a boost circuit or existing voltage bus, the
FAN561X series of LED drivers can be used to drive a
whole string of LEDs with flexible brightness control via
analog and/or PWM. .
V_IN
'N' number of
LEDs Per Branch
– V_DROP < 0.3V
– V_{IN_MIN} = N ¥ V_F + V_DROP
– V_{IN _MAX} = N ¥ V_F + 5.5V
where V_IN = existing bus, boost voltage.
Key advantage:
• No need for current matching resistors and discrete
transistor for brightness control.
I_SET
I4
I3
I2
I1
V_CONTROL
CTRL
R_SET
ENABLE
FAN5613
ON/OFF
GND
LED Brightness Control
All of the FAN561X LED drivers feature analog and 2. FAN5612
PWM controls to give designers flexible brightness con-
V_IN
trol. These control methods can be applied to the circuit
in two different ways to provide more flexibility than other
solutions. To determine the value of R_SET, use the I_SET
vs. V_CTRL graph in Figure 4.
1. FAN5611
V_IN
I_SET
I3
I2
I1
V_CONTROL
CTRL
FAN5612
R_SET
ENABLE ^ON/OFF
GND
I_SET
I4
I3
I2
I1
V_CONTROL (3V)
CTRL _FAN5611
R_SET
GND
- Analog
Set V_CONTROL and R_SET for LED current using:
I_LED = ²----⁰---⁰-----^×-----⁽--^V----^C----^O---_R-^N----_ST---^RE----_TO----^L----–----^V-----^C---^T----^R---^L---⁾-
- PWM
V_CONTROL = PWM
– Amplitude sets maximum LED current.
– Pulse width controls between 0 and maximum.
- Analog
Set V_CONTROL and R_SET for LED current using:
- PWM - 1
I_LED = ²----⁰---⁰-----^×-----⁽--^V----^C----^O---_R-^N----_ST---^RE----_TO----^L----–----^V-----^C---^T----^R---^L---⁾-
V_CONTROL = PWM
– Amplitude sets maximum LED current.
– Pulse width controls between 0 and maximum.
- PWM - 2
Set V_CONTROL and R_SET for maximum LED current using:
I_LED ∼ 200 × I_SET
ON/OFF = PWM
– Amplitude has no effect on current.
– Pulse width controls between 0 and maximum.
© 2003 Fairchild Semiconductor Corporation
FAN5611/FAN5612/FAN5613/FAN5614 Rev. 1.0.9
9
www.fairchildsemi.com