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LM3503SQ-25 数据表(PDF) 8 Page - National Semiconductor (TI) |
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LM3503SQ-25 数据表(HTML) 8 Page - National Semiconductor (TI) |
8 / 20 page 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 1 for the following operational explanation. At the start of each cycle, the oscil- lator 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 cur- rent 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 volt- age 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 opera- tional, 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 in- crease. The LM3503 has two control pins, En1 and En2, used for selecting which segment of a single white LED string net- work is active for dual display applications. En1 controls the main display (MAIN) segment of the single string white LED network between pins V OUT1 and VOUT2. En2 controls the sub display (SUB) segment of the single string white LED network between the V OUT2 and Fb. If both VEn1 and VEn2 are ≤ 0.3V, the LM3503 will shutdown, for further description of the En1 and En2 operation, see Figure 3. During shut- down the output capacitor discharges through the string of white LEDs and feedback resistor to ground. The LED cur- rent can be dynamically controlled by a DC voltage on the Cntrl pin. When V Cntrl = 0V the white LED current may not be equal to zero because of offsets within the LM3503 internal circuitry. To guarantee zero white LED current the LM3503 must be in shutdown mode operation. The LM3503 has dedicated protection circuitry active during normal operation to protect the integrated circuit (IC) and external components. Soft start circuitry is present in the LM3503 to allow for slowly increasing the current limit to its steady-state value to prevent undesired high inrush current during start up. Thermal shutdown circuitry turns off the internal NMOS power device, N1, when the internal semi- conductor junction temperature reaches excessive levels. The LM3503 has a under-voltage protection (UVP) compara- tor that disables the internal NMOS power device when battery voltages are too low, thus preventing an on state where the internal NMOS power device conducts large amounts of current. The over-voltage protection (OVP) com- parator prevents the output voltage from increasing beyond the protection limit when the white LED string network is removed or if there is a white LED failure. OVP allows for the use of low profile ceramic capacitors at the output. The current through the internal NMOS power device, N1, is monitored to prevent peak inductor currents from damaging the IC. If during a cycle (cycle=1/switching frequency) the peak inductor current exceeds the current limit for the LM3503, the internal NMOS power device will be turned off for the remaining duration of that cycle. 20128605 FIGURE 2. Operational Characteristics Table www.national.com 8 |
类似零件编号 - LM3503SQ-25 |
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类似说明 - LM3503SQ-25 |
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