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LM3502SQX-25 Folha de dados(PDF) 7 Page - National Semiconductor (TI) |
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LM3502SQX-25 Folha de dados(HTML) 7 Page - National Semiconductor (TI) |
7 / 18 page Detailed Description of Operation The LM3502 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 2 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 resets 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 LM3502 to properly regulate light/small white LED load currents, where regulation becomes difficult for the LM3502’s primary control loop. Under light load conditions, the LM3502 will enter into a pulse skipping pulse-frequency- mode (PFM) of operation where the switching frequency will vary with the load. The LM3502 has 2 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. For a quick review of the LM3502 control pin operational characteristics, see Fig- ure 3. When the Cntrl pin is ≥ 1.4V, the LM3502 will enter in low I Q state if both En1 and En2 ≤ 0.3V. At this time, both the P1 and N2 FETs will turn off. The output voltage will be a diode drop below the supply voltage and the soft-start will be reset limiting the peak inductor current at the next start-up. The LM3502 is designed to control the LED current with a PWM signal without the use of an external RC filter. Utilizing special circuitry, the LM3502 can operate over a large range of PWM frequencies without restarting the soft-start allowing for fast recovery at high PWM frequencies. Figure 4 reprsents a PWM signal driving the Cntrl pin where t L is defined as the low time of the signal. The following is true: • If t L < 12ms (typical): The device will stop switching during this time and the soft-start will not be reset allow- ing LED current PWM control. • If t L > 12ms (typical): The device will shutdown and the soft-start will reset to prevent high peak currents at the next startup. Both the N2 and P1 switches will turn off. The LM3502 has dedicated protection circuitry active during normal operation to protect the integrated circuit (IC) and external components. The thermal shutdown circuitry turns off the internal NMOS power device, N1, when the internal semiconductor junction temperature reaches excessive lev- els. The LM3502 has a under-voltage protection (UVP) com- parator 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 though 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 LM3502, the internal NMOS power device will be turned off for the remaining duration of that cycle. www.national.com 7 |
Nº de peça semelhante - LM3502SQX-25 |
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Descrição semelhante - LM3502SQX-25 |
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