Datasheet ALED6000 (STMicroelectronics) - 9

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ALED6000 Functional description 5 Functional description
The ALED6000 is based on a voltage mode, constant frequency control loop. The LED current, monitored through the voltage drop on the external current sensing resistor, RSNS, is compared to an internal reference (0.25 V) providing an error signal on the COMP pin. The COMP voltage level is then compared to a fixed frequency saw-tooth ramp, which finally controls the on- and off-time of the power switch. The main internal blocks are shown in the block diagram in Figure 2. Block diagram and can be summarized as follows: • The fully integrated oscillator that provides the sawtooth ramp to modulate the duty cycle and the synchronization signal. Its switching frequency can be adjusted by an external resistor. The input voltage feed-forward is implemented • The soft-start circuitry to limit inrush current during the start-up phase • The voltage mode error amplifier • The pulse width modulator and the relative logic circuitry necessary to drive the internal power switch • The high-side driver for embedded N-channel power MOSFET switch and bootstrap circuitry. A dedicated high-resistance low-side MOSFET, for anti-boot discharge management purposes, is also present • The peak current limit sensing block, with programmable threshold, to handle overload including a thermal shutdown block, to prevent thermal runaway • The bias circuitry, which includes a voltage regulator and internal reference, to supply the internal circuitry and provide a fixed internal reference and manages the current dimming feature. The switchover function from VCC to VBIAS can be implemented for higher efficiency. This block also implements a voltage monitor circuitry (UVLO) that checks the input and internal voltages
5.1 Oscillator and synchronization
Figure 4. Oscillator and synchronization shows the block diagram of the oscillator circuit. The internal oscillator provides a constant frequency clock, whose frequency depends on the resistor externally connected between the FSW pin and ground.
Figure 4. Oscillator and synchronization
clock 180° phase shift FSW Clock Generator Synchronization SYNCH Ramp Sawtooth Generator If the FSW pin is left floating, the programmed frequency is 250 kHz (typ.); if FSW pin is connected to an external resistor the programmed switching frequency can be increased up to 1.5 MHz, as shown in Figure 5. Switching frequency programmability . The required RFSW value (expressed in kΩ) is estimated by the equation below : FSW = 250kHz + 12500 R (1) FSW
DS13018
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Rev 2 page 9/45
Document Outline Cover image Features Applications Description 1 Application schematic 2 Block diagram 3 Pin settings 3.1 Pin connection 3.2 Pin description 3.3 Maximum ratings 3.4 Thermal data 3.5 ESD protection 4 Electrical characteristics 5 Functional description 5.1 Oscillator and synchronization 5.2 Soft-start 5.3 Digital dimming 5.4 Error amplifier and light-load management 5.5 Low VIN operation 5.6 Overcurrent protection 5.7 Overtemperature protection 6 Application information 6.1 Input capacitor selection 6.2 Output capacitor and inductor selection 6.3 Compensation strategy 6.4 Thermal considerations 6.5 Layout considerations 7 Demonstration board 8 Application notes – alternative topologies 8.1 Inverting buck-boost 8.2 Positive buck-boost 8.3 Floating boost 8.4 Compensation strategy for alternative topologies 9 Package information 9.1 HTSSOP16 package information Revision history