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LM3224 Arkusz danych(PDF) 9 Page - National Semiconductor (TI) |
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LM3224 Arkusz danych(HTML) 9 Page - National Semiconductor (TI) |
9 / 18 page Operation CONTINUOUS CONDUCTION MODE The LM3224 is a current-mode, PWM boost regulator. A boost regulator steps the input voltage up to a higher output voltage. In continuous conduction mode (when the inductor current never reaches zero at steady state), the boost regu- lator operates in two cycles. In the first cycle of operation, shown in Figure 1 (a), the transistor is closed and the diode is reverse biased. Energy is collected in the inductor and the load current is supplied by C OUT. The second cycle is shown in Figure 1 (b). During this cycle, the transistor is open and the diode is forward biased. The energy stored in the inductor is transferred to the load and output capacitor. The ratio of these two cycles determines the output voltage. The output voltage is defined approximately as: where D is the duty cycle of the switch, D and D' will be required for design calculations. SETTING THE OUTPUT VOLTAGE The output voltage is set using the feedback pin and a resistor divider connected to the output as shown in the typical operating circuit. The feedback pin voltage is 1.26V, so the ratio of the feedback resistors sets the output voltage according to the following equation: SOFT-START CAPACITOR The LM3224 has a soft-start pin that can be used to limit the inductor inrush current on start-up. The external SS pin is used to tailor the soft-start for a specific application but is not required for all applications and can be left open when not needed. When used, a current source charges the external soft-start capacitor, Css. The soft-start time can be estimated as: Tss = Css*1.24V/Iss THERMAL SHUTDOWN The LM3224 includes thermal shutdown protection. If the die temperature exceeds 140˚C the regulator will shut off the power switch, significantly reducing power dissipation in the device. The switch will remain off until the die temperature is reduced to approximately 120˚C. If the cause of the excess heating is not removed (excessive ambient temperature, excessive power dissipation, or both) the device will con- tinue to cycle on and off in this manner to protect from damage. 20097602 FIGURE 1. Simplified Boost Converter Diagram (a) First Cycle of Operation (b) Second Cycle Of Operation www.national.com 9 |
Podobny numer części - LM3224 |
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Podobny opis - LM3224 |
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