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LX1745CPW Datasheet(PDF) 9 Page - Microsemi Corporation |
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LX1745CPW Datasheet(HTML) 9 Page - Microsemi Corporation |
9 / 16 page LX1745 PRODUCTION DATASHEET Microsemi Integrated Products Division 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 9 Triple Output Boost – LED Driver / LCD Bias Copyright © 2000 Rev. 1.2, 2006-02-27 TM ® APPLICATION NOTE negative voltage transition that is greater than the output voltage. POWER MOSFET SELECTION The LX1745 can source up to 100mA of gate current. An logic level N-channel MOSFET with a low turn on threshold voltage, low gate charge and low RDS(ON) is required to optimize overall circuit performance. OVER VOLTAGE PROTECTION PROGRAMMING Since the output of the LED Driver is a current mode configuration, it may be desirable to protect the output from an over-voltage condition in the event the load is removed or not present. The LX1745 includes an over voltage monitor that is easily programmed with two external resistors (Figure 6). This feature eliminates the need for a Zener Diode clamp on the output. Programming is accomplished by first selecting ROVP_2 and then calculating ROVP_1 using the following equation. REF REF OVP 2 _ OVP 1 _ OVP V V - V R R = eq. 9 where VOVP is the desired maximum voltage on the output. This voltage should be selected to accommodate the maximum forward voltage of all the LEDs, over temperature, plus the maximum feedback voltage. Conversely, it may also be selected according to the maximum VDS voltage of the output MOSFET. INDUCTOR CURRENT LIMIT PROGRAMMING Setting of the peak inductor current limit is an important aspect of the PFM constant off-time architecture; it determines the maximum output power capability and has a marked effect on efficiency. It is recommended that the peak inductor current be set to approximately two times the expected maximum DC input current. This setting will minimize the inductor size, the input ripple current, and the output ripple voltage. Care should be taken to use inductors that will not saturate at the peak inductor current level. The desired peak inductor current can be estimated by the following equation: IN OUT PK V P 2 I ⋅ η ⋅ = eq. 10 where POUT is the total output power, η is the expected conversion efficiency, and VIN is the input voltage. From the calculated desired IPK an RCS resistance value can be chosen from the following equation: 6 PK CS 10 30 185 . 0 I R − ⋅ − ≅ eq. 11 which is taken from the following graph (Figure 7). 0 200 400 600 800 1000 0 5 10 15 20 RCS (kΩ ) Figure 7 – Peak Current Programming Resistor This graph characterizes the relationship between peak inductor current, the inductance value, and the RCS programming resistor. INDUCTOR SELECTION An inductor value of 47 μH has been show to yield very good results. Choosing a lower value emphasizes peak current overshoot, effectively raises the switching frequency, and increases the dissipative losses due to increased currents. OUTPUT CAPACITOR SELECTION Output voltage ripple is a function of the several parameters: inductor value, output capacitance value, peak switch current, load current, input voltage, and the output voltage. All of these factors can be summarized by the following equation: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − + ⋅ + + − ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ⋅ ⋅ ≅ IN F OUT OUT PK L SW IN OUT OUT PK RIPPLE V V V I I ) V V ( V 1 C I I L V eq. 12 where VL is the voltage drop across the inductor, VF is the forward voltage of the output catch diode, and VSW is the voltage drop across the power switch. VL+VSW can be approximated at 0.4V and VF can be approximated at 0.4V. |
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