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SE3362 Datasheet(HTML) 5 Page - Seaward Electronics Inc.

Part No. SE3362
Description  4-Channel Regulated Charge Pump White LED Driver
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Maker  SEAWARD [Seaward Electronics Inc.]
Homepage  http://www.seawardinc.com
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SE3362 Datasheet(HTML) 5 Page - Seaward Electronics Inc.

   
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Revision 12/4/2008
Preliminary and all contents are subject to change without prior notice
© Seaward Electronics, Inc., 2007. • www.seawardinc.com.cn • Page 5
Applications Information
Selecting Capacitors
To get the better performance of SE3362, the selecting of
peripherally appropriate capacitor and value is very
important. These capacitors determine some parameters
such as input and output ripple, power efficiency, maximum
supply current by charge pump, and start-up time. To
reduce the input and output ripple effectively, the low ESR
ceramic capacitors are recommended. Generally, to
reduce the output ripple, increasing the output capacitance
COUT is necessary. However, this will increase the start-up
time of output voltage.
For LED driving applications, the input voltage ripple is
more important than output ripple. Input ripple is controlled
by input capacitor CIN, increasing the value of input
capacitance can further reduce the ripple. Practically, the
input voltage ripple depends on the power supply’s
impedance. If a single input capacitor CIN cannot satisfy the
requirement of application, it is necessary to add a
low-pass filter as shows in Figure 1.
Figure1. C-R-C filter used to reduce input ripple
The flying capacitor C1 determines the supply current
capability of the charge pump and to influence the overall
efficiency of system. The lower value will improve
efficiency, but it will limit the LED
’ s current at low input
voltage.
Setting the LED Current
The SE3362 can be set a fixed LED’s current by a resister
RSET connected from RSET to GND. RSET establishes the
reference current and mirrors the current into LED1, LED2,
LED3, and LED4. The current into LED is about 400 times
of the current flows through the RSET, the approximate
setting formula is given as follows:
ILED(A) =
)
(
)
(
500
Ω
SET
R
V
LED current and Table 2 shows the values of RSET for a
fixed LED current.
ILED(mA)
RSET (KΩ)
5
100
10
50
15
33.3
20
25.0
25
20
Table 2
30
16.7
LED current setting with NMOS
LED current setting control can also be achieved by using
the external NMOS to change equivalent resister of RSET
pin. Figure 2 shows this application circuit of method. For
this example, a 3 bit signals can set 8 kinds of different
equivalent resister of RSET pin, i.e. produce 8 kinds of LED
current level. Table 3 shows the relation between
equivalent resister of RSET pin and control signal.
Figure 2. The application circuit of setting LED current
which using a NMOS to set RSET.


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