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CD43-470 Folha de dados(PDF) 5 Page - Microsemi Corporation |
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CD43-470 Folha de dados(HTML) 5 Page - Microsemi Corporation |
5 / 8 page Microsemi Microsemi Integrated Products 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 5 Copyright © 2002 Rev. 3.0a, 2005-03-14 LX1734 1.0MHz Inverting DC/DC Converter TM ® TH EORY OF O PERAT ION The LX1734 is a fixed frequency current mode controller designed to develop a negative output voltage from a positive input voltage. The switching transistor and current sense resistor are integrated into the part. The PWM functions in a peak current regulation mode using the amplified error signal to determine the peak switch current each cycle. Slope compensation is added to provide stable operation at high duty cycles. A current limit detector overrides the regulation loop and prevents the switch current from exceeding the over current threshold level. The bandgap control circuit keeps Q1 biased on and produces a reference current (IREF) that produces a voltage drop across the internal resistance that has a positive temperature coefficient. When this resistor voltage drop is added to the negative temperature coefficient of the base-emitter voltage drop of Q1, the result is a temperature compensated reference voltage (VREF) at the NFB pin. The summing node from the external feedback network is connected directly to NFB pin, which is relatively high impedance (typically 150k). The feedback loop minimizes the error current, (IERROR) which effectively regulates the voltage at the NFB pin. As with a conventional error amplifier, the error signal is proportional to the difference between the temperature compensated reference voltage (VREF) and the summing node voltage. A slight correction factor is necessary to account for the added summing node voltage due to the reference current (IREF, typically 4µADC) flowing through the Thevenin equivalent summing node external resistance. APPLI CATI ON NOTE The LX1734 can be used in several topologies that generate a negative output voltage from a positive input voltage. The LX1734 can be used in a dual inductor converter with coupled or uncoupled inductors (see Figure 1); this topology is required if the absolute value of the output voltage is less than or equal to the input voltage but can also be used for higher voltage outputs. The following components or their equivalents can be used to implement the converter in Figure 1, which produces a –5V output at 150mA from a +5V input. The reference design has an efficiency of greater than 72% and an input ripple voltage of less than 6mVP-P and an output ripple voltage of less than 300µVP-P. Ref Description Part Number Manufacturer C1 Ceramic, 4.7uF, 6.3V (0805) JMK212BJ475MG Taiyo Yuden C2 Ceramic, 1uF, 16V (0805) GRM40X7R105M16 Murata C3 Ceramic, 22uF, 6.3V (1210) JMK325BJ226MM Taiyo Yuden C4 Ceramic, 470pF, 50V (0402) GRM36X7R471K050 Murata D1 Diode, 0.5A, 30V UPS530 Microsemi L1 Inductor, Coupled, 22uH CLS62-220NC Sumida Table 1 - Part List for Figure 1 (All Parts Are Surface Mount). Separate inductors (not on a common core) can be used in place of the coupled inductor (L1) of Figure 1. In this case the only component that changes in the parts list is L1, which now would be two separate inductors (L1, formerly L1A, and L2, formerly L1B). With the separate inductors the peak-to-peak voltage ripple on the input the output were less the 2mVP-P and less than 500µVP-P, respectively. Table 2 - Part List For Alternative Inductors Ref. Designator Description Part Number Manufacturer L1, L2 Inductor, 47uH, (1812) LQH4C470K04M00 Murata Inductor Selection When the LX1734 is used in a dual inductor converter with coupled inductors, a parallel winding inductor value of 22µH works well for a 5V input and a -5V output at 150mA. The inductor value can be scaled to the particular set of operating conditions based on the input voltage, output voltage, and output current. The new value of coupled inductor parallel inductance can be calculated using the following equation: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − × ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ × = OUT OUT IN NEW V V 5 I mA 150 V 5 V µH 22 L The inductor value should be rounded to the nearest available value. The parallel saturation current rating of a coupled inductor should be sized to carry the summation of the peak input and peak output inductor currents. When the LX1734 is used in a dual inductor converter with two separate (uncoupled) inductors or when using the boost converter with an inverting charge pump output configuration, the inductance value for each inductor should be about twice the value recommended for a coupled inductor. The peak current in the inductor is the DC current plus ½ of the peak-to-peak ripple current. The saturation current rating of the inductors should be sized to carry the peak inductor current. The peak-to-peak ripple current can be calculated based on the inductor value, the terminal voltage (input or output), and the duty cycle. The DC inductor current is the same as the DC output current on the output inductor. The DC input current includes the power for the LX1734, but is still a good approximation for the DC inductor current for higher power applications. For simplicity, the calculations below ignore the voltage drops of the switch and diode. The duty cycle, D, for the dual inductor topology (assuming continuous inductor current mode operation) is approximately: |
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