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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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SC5014 датащи(PDF) 12 Page - Semtech Corporation |
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SC5014 датащи(HTML) 12 Page - Semtech Corporation |
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12 / 34 page  SC5014 2 General Description The SC504 contains a high frequency, current-mode, internally compensated boost controller with 4 constant current sinks for driving LED strings. The LED current for all strings is programmed by an external resistor. The boost converter operates to maintain minimal required output voltage for regulating the LED current to the programmed value. A typical backlight application uses 3 to 4 LEDs per each string, with current driven up to 20mA. The unique control loop of the SC504 allows fast transient response in dealing with line and load disturbances. The SC504, operating with an external power MOSFET, regu- lates the boost converter output voltage based on the instantaneous requirement of the 4 string current sources. This provides power to the entire lighting subsystem with increased efficiency and reduced component count. It supports PWM dimming frequencies from 00Hz to 30kHz and the supply current is reduced to 2mA typical when all LED strings are off. Start-Up When the EN pin is pulled up high (>2.V), the device is enabled and the UVLO and VCC pin voltages are checked. The VCC voltage has fixed under-voltage rising and falling trip points. If the VCC pin is higher than 4.2V and the UVLO pin voltage is greater than .23V, the SC504 goes into a start-up sequence. The UVLO pin voltage can be used to program the input power source voltage VIN turn-on threshold and its hysteresis (refer to the detailed applica- tion circuit on page 23, Figure 2) as shown by the follow- ing equations: V IN_TurnOn [V] = .23 X (R + R2) / R V IN_Hysteresis [V] = 0 -5 X R 2 [Ω] In the next phase, the SC504 checks each IO pin to deter- mine if the respective LED string is enabled. Each IO pin is pulled up with a 00µA current source. If any IO pin is con- nected to ground, it will be detected as an unused string, and will be turned off. This unused string checking proce- dure typically takes ms. After this, the SC504 enters into a soft-start sequence. The soft-start function helps to prevent excess inrush current through the input rail during start-up. In the SC504, the soft-start is implemented by slowly ramping up the reference voltage fed to the error amplifier. This closed loop start-up method allows the output voltage to ramp up without any overshoot. The duration of the soft-start in the SC504 is controlled by an internal timing circuit, which is used during start-up and is based on the boost converter switching frequency. For example, with switching frequency at MHz, it is 8ms typical and becomes 4ms typical when the switching frequency is 2MHz. If the PWM voltage goes low while the SC504 is in soft- start operation, the SC504 switches to standby mode, where the external power MOSFET and the LED current sources will be turned off immediately. The internal soft- start timer is turned off and the soft-start value is saved. When the PWM voltage goes high again, the soft-start resumes from the previously saved value. Each LED current source (IO to IO4) tries to regulate the LED current to its set point. The control loop will regulate the output voltage such that all the IO pin voltages are at least 0.9V typical. Shutdown When the EN pin is pulled down below 0.8V, the device enters into shutdown mode. In this mode, all the internal circuitry is turned off and the supply current is less than µA (max). In the scenario where the EN pin voltage is high, but VCC voltage falls below the respective UVLO threshold, the SC504 goes into a suspend mode. In this mode, all the internal circuitry except the reference and the oscillator are turned off. Thermal Shutdown (TSD) If the thermal shutdown temperature of typical 50°C is reached, the boost converter and all IO current sources are turned off. The FLT pin is forced low in this condition. When the temperature falls below the TSD trip point by 0°C, the SC504 will restart following the start-up sequence as described before. The FLT pin is latched and will stay low, it is reset by cycling the EN, VCC or UVLO. Applications Information |
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