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LM3503SQ-44 数据表(PDF) 6 Page - Texas Instruments |
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LM3503SQ-44 数据表(HTML) 6 Page - Texas Instruments |
6 / 28 page Driver Logic + - Oscillator Current Sense + - Thermal Shutdown Duty Limit Reference Duty Limit Comparator Fb UVP Comparator OVP Comparator PWM Comparator Light Load Comparator Light Load Reference VIN Sw VOUT1 VOUT2 Fb En2 En1 PGND Cntrl AGND N1 N2 P1 Error Amplifier UVP Reference OVP Reference Current Limit Soft Start 9 15,16 12 10 14 4 7 6 2,3 13 FET Logic + - + - + - + - LM3503 SNVS329D – JULY 2005 – REVISED AUGUST 2006 www.ti.com Block Diagram Figure 3. Block Diagram Detailed Description of Operation The LM3503 utilizes an asynchronous current mode pulse-width-modulation (PWM) control scheme to regulate the feedback voltage over specified load conditions. The DC/DC converter behaves as a controlled current source for white LED applications. The operation can best be understood by referring to the block diagram in Figure 3 for the following operational explanation. At the start of each cycle, the oscillator sets the driver logic and turns on the internal NMOS power device, N1, conducting current through the inductor and reverse biasing the external diode. The white LED current is supplied by the output capacitor when the internal NMOS power device, N1, is turned on. The sum of the error amplifier’s output voltage and an internal voltage ramp are compared with the sensed power NMOS, N1, switch voltage. Once these voltages are equal, the PWM comparator will then reset the driver logic, thus turning off the internal NMOS power device, N1, and forward biasing the external diode. The inductor current then flows through the diode to the white LED load and output capacitor. The inductor current recharges the output capacitor and supplies the current for the white LED load. The oscillator then sets the driver logic again repeating the process. The output voltage of the error amplifier 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 and minimizing EMI radiation. The duty limit comparator is always operational, it prevents the internal NMOS power switch, N1, from being on for more than one oscillator cycle and conducting large amounts of current. The light load comparator allows the LM3503 to properly regulate light/small white LED load currents, where regulation becomes difficult for the LM3503’s primary control loop. Under light load conditions, the LM3503 will enter into a pulse skipping pulse-frequency-mode (PFM) of operation where the operational frequency will vary with the load. As a result of PFM mode operation, the output voltage ripple magnitude will significantly increase. 6 Submit Documentation Feedback Copyright © 2005–2006, Texas Instruments Incorporated Product Folder Links: LM3503 |
类似零件编号 - LM3503SQ-44 |
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类似说明 - LM3503SQ-44 |
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