10
2007 Semtech Corp.
www.semtech.com
SC4505
POWER MANAGEMENT
Applications Information
An additional advantage of PWM dimming comes to
customers who prefer to avoid inrush currents when filling
the boost output capacitor – simply PWM the device at
12% duty for a millisecond or two, reducing inrush current
to less than 50mA. This dim time will vary based on
number of LED and size of output capacitor, but can be
easily determined on the bench, and programmed into
the uC firmware.
For an example, suppose the current flowing through
the I
O1
channel is programmed at 20mA by R
O1SET
. A 1kHz
PWM signal with the duty ratio of 20% is applied to the
EN_BL pin of the SC4505. Then the average current
flowing through the I
O1
channel is
4mA20mA 20% I
O1AVG
=×=
A startup delay time between the enable signal goes high
and the internal current regulator actually turns on is
about 1.6µs, which causes a small offset dependent on
PWM frequency. As the PWM signal frequency goes
higher, the effect of the delay will get more obvious to
customers.
However, since PWM is always linear, offset can be easily
corrected in software. The offset correction factor can
be described as:
F
PWM
is in KHz.
For an example, at 20kHz, D
CORRECTION
= 3%. So for 50%
of the nominal LED current in I
O1
channel, the PWM signal
should have a duty ratio of 53%.
Flash Timeout Programming
When Channel IO2 is in flash mode, a timer is available
to prevent LED overstress. The timer is only active in
Flash mode – not active in torch mode. The capacitor
tied between the FTO pin and the AGND sets the time
duration of the flash mode. In flash mode, an external
capacitor is charged with 1.2µA. When the voltage on
this capacitor reaches the 1.25V threshold Channel 2 is
turned off. The timer can be reset by entering torch mode
or turning off channel 2. The FTO pin can simply be
grounded to disable this feature, as would be necessary
when using I
O2
for sub display.
PWMCORRECTION
FS µ 1.6 100 D ××=
The relationship between the EN_FL, FL_TRB, FTO pin
voltage and the current flowing through the IO2 pin is
illustrated in Figure 4 below.
Figure 4. Relationship between RN_FL, FL_TRB, FTO
and I
O2
Current
To calculate the FTO capacitor needed for a desired
timeout, a simple formula can be used as shown below:
t100.862C
-6
⋅⋅=
Where C is in Farads, and t is in Seconds.
EN_FL
FL_TRB
t
t
t
t
FTO
I
O2
I
O2MAX
20% x I
O2MAX
0
0
0
0
1.25V
Over Voltage Protection (OVP)
SC4505 includes a built-in overvoltage protection circuit
to prevent damage to the IC and output capacitor in the
event of open-circuit condition. The output voltage of
the boost converter is detected at the VO pin, and divided
down by an internal resistor strand of 500kΩ. If the
voltage at the VO pin exceeds 28V, the boost converter
will be shut down, and a strong pulldown will be applied
to the VO pin to quickly discharge the output capacitor.
This additional level of protection prevents a condition
where the output capacitor and Schottky diode must
endure high voltage for an extended period of time, which
can pose a reliability risk for the user’s system.
The boost OVP triggering point can be adjusted by adding
an external resistor divider at VO pin as shown in Figure
5.
