IS31BL3508A
Integrated Silicon Solution, Inc. – www.issi.com
Rev. A, 05/10/2013
9
APPLICATION INFORMATION
INPUT AND OUTPUT CAPACITOR
A 2.2μF input capacitor is used to reduce input ripple
and noise, where the input ripple amplitude is inversely
proportional to the value of the input capacitance. The
input and output capacitors should be placed as close
to the device as possible, so as to reduce the effect of
voltage ripple. The value of the output capacitor is
dependent upon the output ripple. A 2.2μF or 10μF
ceramic capacitor with a 50V rating is sufficient for the
majority of applications.
INDUCTOR VALUE
The choice of the inductor value will affect the
performance of the device. Large value inductors will
reduce the current ripple in the inductor, thus reducing
output voltage ripple. However large value inductors of
the same current rating, typically also have larger DC
resistance, thus reducing the system efficiency. For
example in Figure 1, the loads are 10 WLEDs and
output current is 20mA. The maximum input current
can reach 600mA. For this application a 10μH 700mA
inductor is sufficient.
DIODE
The diode, D
1
, should use schottky diode to optimize
the efficiency of system. The rated current and peak
current of D
1
should be above output current and peak
current of L
1
. And the recurrent peak reverse voltage of
D
1
should be above OVP voltage. A 1A 60V schottky
diode is recommended.
LED CURRENT SETPOINT
When the IS31BL3508A is used for LED driving
applications (Figure 1), the LED current flows through
the external feedback resistor R
SET
to regulate the LED
current. The internal feedback regulation point is 0.3V.
The maximum LED current may be calculated by the
following Equation (1).
SETFBMAX
RVI / (1)
For accurate LED current, resistors should be 1% or
better tolerance.
LED INTENSITY CONTROL
EN PIN DIMMING
The EN pin of the IS31BL3508A can accept a PWM
signal to implement LED dimming (Figure 1). LED
current may be computed using the following Equation
(2).
PWMMAXLED
DII (2)
I
MAX
is computed using equation (1). Recommended
PWM frequency is in the range of 500Hz ~ 500kHz. So
as to guarantee a reasonably good dimming effect, the
minimum duty cycle should be about 10%.
Driving the EN pin with a PWM signal can effectively
adjust the LED intensity. The PWM signal voltage
levels must meet the EN pin input voltage levels,
V
EN_ON
and V
EN_OFF
.
IS31BL3508A can also use a DC voltage or PWM
signal to directly control the LED current, and thus
provide fine adjustment for the LED intensity.
FB PIN DC VOLTAGE DIMMING
The schematic of Figure 5 show an example of using a
DC input voltage at the FB pin to implement LED
dimming. When the DC input voltage increases, the
current through R
2
and R
3
increases producing an
offset voltage across R
2
, thereby causing the LED
current to be reduced. The LED current may be
computed using the following Equation (3). V
FB
is 0.3V
(typ).
SET
FBDC
FB
LED
R
R
VVR
V
I
3
2
)(
(3)
When the DC voltage range is 0V~1.8V, a good value
for R
2
is 75kΩ，and for R
3
is 360kΩ.
FB PIN PWM DIMMING
A PWM signal connected to the FB pin can also be
used to directly control the LED current (Figure 6). A
low pass filter converts the PWM duty cycle to a DC
level, which is in turn summed together with the
feedback voltage of RSET to effectively reduce the
LED current proportionally. Due to the filtering of the
PWM signal, the frequency of the PWM signal can
have an affect on the output voltage ripple. For a PWM
signal input signal of 2.8V, the recommended
frequency is greater than 2kHz. Using a fixed
frequency PWM signal and adjusting the PWM duty
cycle effectively adjusts the LED intensity. LED current
may be computed using the following Equation (4).
V
FB
is 0.3V (typ).
SET
FBPWM
FB
LED
R
RR
VDutyVR
V
I
34
2
)(
(4)
For a PWM signal range of 0V~1.8V, a suitable value
for R
2
is 75kΩ, R
3
is 150kΩ, R
4
is 200kΩ.
SETTING THE OUTPUT VOLTAGE
The IS31BL3508A can be configured to provide a
constant output voltage (Figure 4). The output voltage
level can be computed using the following Equation (5).
The device internal V
FB
is 0.3V (typ).
SETSETFBOUT
RRRVV /
1
(5)
