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SE3362 Datasheet(PDF) 5 Page - Seaward Electronics Inc. |
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SE3362 Datasheet(HTML) 5 Page - Seaward Electronics Inc. |
5 / 8 page 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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