FAN5616 Datasheet, PDF (9/14 Pages) Fairchild Semiconductor – High-Efficiency, Constant-Current LED Driver with Adaptive Charge Pump

VIN
GND
VOUT = 2 x VIN
VIN
+
CAP1
–
+
CAP2
–
VOUT
COUT
GND
Figure 4. Switch Configuration
Smart Soft-Start
When the FAN5616 is enabled, the Smart Soft-Start cir-
cuit limits the switcher’s in-rush current. Figure 5 shows
the timing diagram of the Smart Soft-Start circuit and
associated signals.
After a power up, the FAN5616 is placed in low power
mode until a logic “High” is applied to the enable (EN1/
EN2) pin enabling the device. Within the first 500µS of
enabling the device the output voltage (V_OUT) is increased
linearly until it reaches its nominal level. When a logic
“Low” is applied to EN1/EN2 for more than 5mS, the
device is placed in a low power mode and the output volt-
age is turned off. The LED current is controlled by apply-
ing a PWM signal to the EN1/EN2 pin. To avoid
interference between the PWM signal and the soft-start
circuit, the PWM signal applied must be faster than
200Hz but not greater than 50kHz. The soft-start circuit
will be reactivated with each low to high transition on the
EN1/EN2 pin. As shown in Figure 5, the PWM signal,
ideally, should be controlled so that the initial logic “High”
is at least 250µS before returning to its standard fre-
quency. This allows V_OUT to ramp to its nominal level.
However, as shown in Figure 6, in most cases a fre-
quency greater than a 1kHz PWM signal is applied to the
EN1/EN2 pin such that the V_OUT ramp rate slows accord-
ingly. The V_OUT ramp will begin during the initial high
level (ON state) of the PWM signal while its voltage will
be maintained during its low level (OFF state).
The following formula explains the relationship between
duty cycle (D) and soft-start V_OUT ramp time (T_STR),
T_STR
~~
250µS
D
Where, D = ^{T ON}
T_ON +T_OFF
For example, a PWM signal with a 50% duty ratio
(D=0.5) generates a 500µS soft-start V_OUT ramp.
9
FAN5616 Rev. 1.0.0
www.fairchildsemi.com