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LM3500TL-16 Arkusz danych(PDF) 12 Page - National Semiconductor (TI) |
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LM3500TL-16 Arkusz danych(HTML) 12 Page - National Semiconductor (TI) |
12 / 18 page Operation The LM3500 utilizes a synchronous Current Mode PWM control scheme to regulate the feedback voltage over almost all load conditions. The DC/DC controller acts as a controlled current source ideal for white LED applications. The LM3500 is internally compensated preventing the use of any external compensation components providing a compact overall so- lution. The operation can best be understood referring to the block diagram in Figure 1. At the start of each cycle, the oscillator sets the driver logic and turns on the NMOS power device conducting current through the inductor and turns off the PMOS power device isolating the output from the V SW pin. The LED current is supplied by the output capacitor when the NMOS power device is active. During this cycle, the output voltage of the EAMP controls the current through the inductor. This voltage will increase for larger loads and decrease for smaller loads limiting the peak current in the inductor minimizing EMI radiation. The EAMP voltage is compared with a voltage ramp and the sensed switch volt- age. Once this voltage reaches the EAMP output voltage, the PWM COMP will then reset the logic turning off the NMOS power device and turning on the PMOS power de- vice. The inductor current then flows through the PMOS power device to the white LED load and output capacitor. The inductor current recharges the output capacitor and supplies the current for the white LED branches. The oscil- lator then sets the driver logic again repeating the process. The Duty Limit Comp is always operational preventing the NMOS power switch from being on more than one cycle and conducting large amounts of current. The LM3500 has dedicated protection circuitry active during normal operation to protect the IC and the external compo- nents. The Thermal Shutdown circuitry turns off both the NMOS and PMOS power devices when the die temperature reaches excessive levels. The LM3500 has a UVP Comp that disables both the NMOS and PMOS power devices when battery voltages are too low preventing an on state of the power devices which could conduct large amounts of current. The OVP Comp prevents the output voltage from increasing beyond 15.5V(LM3500-16) and 20.5V(LM3500- 21) when the primary white LED network is removed or if there is an LED failure, allowing the use of small (16V for LM3500-16 and 25V for LM3500-21) ceramic capacitors at the output. This comparator has hysteresis that will regulate the output voltage between 15.5V and 14.6V typically for the LM3500-16, and between 20.5V and 19.5V for the LM3500- 21. The LM3500 features a shutdown mode that reduces the supply current to 0.1uA and isolates the input and output of the converter. 20065704 FIGURE 1. LM3500 Block Diagram www.national.com 12 |
Podobny numer części - LM3500TL-16 |
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Podobny opis - LM3500TL-16 |
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