FAN5616 Datasheet, PDF (11/14 Pages) Fairchild Semiconductor – High-Efficiency, Constant-Current LED Driver with Adaptive Charge Pump

2. Digital Control
A built-in 2-bit DAC is used to digitally control the LED’s
brightness through the EN1 and EN2 inputs. Once the
desired maximum LED current is set by the external
R_SET resistor, the percentage of the maximum LED cur-
rent is selected, as described in Table 2, to perform the
dimming operation.
Table 2. LED Dimming with Internal 2-bit DAC
EN2
0
0
1
1
EN1
0
1
0
1
LED Current % of Maximum
0%
33.3% (1/3)
66.7% (2/3)
100% (3/3)
100
50
0
0
50
100
PWM Signal Duty Cycle (%)
3. PWM Control
In addition to the digital LED brightness control, the
FAN5616 features a PWM control. The LED current var-
ies according to the width of the PWM signal applied to
the EN1/EN2 input pins. Once the desired maximum
LED current (I_LED-MAX) is set by the external R_SET resis-
tor, the percentage of I_LED-MAX to perform the dimming
operation depends on the configuration of the PWM sig-
nal with respect to the 2-bit DAC inputs (EN1 and EN2)
as described in Table 2.
For example, if R_SET = 7.8kΩ then I_LED-MAX = 20mA. If
EN1 and EN2 are tied together and a PWM signal is
applied, the LED current will vary between 0% and 100%
(0mA and 20mA) of the maximum LED current according
to the duty cycle of the PWM signal.
The PWM signal can be applied to either enable input
pins (EN1/EN2) or to both tied together. Depending upon
the configuration, the average LED current can be
adjusted within any range limited by 0, 1/3, 2/3, 3/3 of the
maximum LED current as described in Table 3. The
PWM duty cycle is assumed to be between 10% and
90%.
Table 3. Average LED Dimming Range
Enable Input Pins
EN2
EN1
LOW
TOGGLE
HIGH
TOGGLE
TOGGLE LOW
TOGGLE HIGH
TOGGLE TOGGLE
Average I_LED
Adjustment Range
3% to 30% of I_LED-MAX
70% to 97% of I_LED-MAX
6% to 60% of I_LED-MAX
39% to 93% of I_LED-MAX
10% to 90% of I_LED-MAX
Figure 7. Ideal PWM Dimming Response
4. Dimming with DC Voltage
The brightness control using a variable DC voltage is
shown in Figure 8. If R1 = 78kΩ, R2 = 7.8kΩ, adjusting
V_EXT in the 0V to 0.6V range results in dimming the LED
current from 22mA to 2mA.
R2
R1
V_EXT
I_SET
FAN5616
Figure 8. DC Voltage Control
The FAN5616’s internal circuit maintains a constant
V_SET = 0.6V. Adjusting V_EXT changes the I_SET and I_LED
accordingly.
By selecting different values for R1, R2 and V_EXT, the
I_LED variation range can be changed according to the fol-
lowing equation:
I_LED =
156 ₊ (156 – 260 x ) V_{EXT mA}
R1
R2
Where, 0V < V_EXT < 0.6V (1+R2/R1) and R1 and R2 are
in kΩ.
The recommended PWM frequency range is 200Hz to
50kHz for an acceptable linear response. At higher
frequencies, the current waveform can no longer follow
the PWM signal waveform, resulting in a significant
difference between the value of the average I_LED and the
theoretical calculation.
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FAN5616 Rev. 1.0.0
www.fairchildsemi.com