Datasheet ALED6000 (STMicroelectronics) - 10

link to page 11 link to page 11 link to page 11
ALED6000 Oscillator and synchronization Figure 5. Switching frequency programmability
1500 1400 1300 1200 1100 z] 1000 H 900 [k 800 w 700 Fs 600 500 400 300 10 20 30 40 50 60 70 80 90 100 RFSW [kΩ] To improve the line transient performance, keeping the PWM gain constant versus the input voltage, the input voltage feed-forward is implemented by changing the slope of the saw-tooth ramp, according to the input voltage change (Figure 6. Feed-forward a). The slope of the sawtooth also changes if the oscillator frequency is programmed by the external resistor. In this way a frequency feed-forward is implemented (Figure 6. Feed-forwardb) in order to keep the PWM modulator gain constant versus the switching frequency. On the SYNCH pin the synchronization signal is generated. This signal has a phase shift of 180° with respect to the clock. This delay is useful when two devices are synchronized connecting the SYNCH pins together. When SYNCH pins are connected, the device with a higher oscillator frequency works as master, so the slave device switches at the frequency of the master but with a delay of half a period. This helps reducing the RMS current flowing through the input capacitor. Up to five ALED6000s can be connected to the same SYNCH pin; however, the clock phase-shift from master switching frequency to slave input clock is 180°. The ALED6000 device can be synchronized to work at a higher frequency, in the range 250 kHz-1500 kHz, providing an external clock signal on SYNCH pin. The synchronization changes the saw-tooth amplitude, also affecting the PWM gain (Figure 6. Feed-forwardc). This change must be taken into account when the loop stability is studied. In order to minimize the change of PWM gain, the free-running frequency should be set (with a resistor on the FSW pin) only slightly lower than the external clock frequency. This pre-adjusting of the slave IC switching frequency keeps the truncation of the ramp saw-tooth negligible. In case two or more (up to five) ALED6000 SYNCH pins are tied together, the ALED6000 IC with higher programmed switching frequency is typically the master device; however, the SYNCH circuit is also able to synchronize with a slightly lower external frequency, so the frequency pre-adjustment with the same resistor on the FSW pin, as suggested above, is required for a proper operation. The SYNCH signal is provided as soon as EN is asserted high; however, if DIM is kept low for more than TDIMTO timeout, the SYNCH signal is no more available until DIM re-assertion high.
DS13018
-
Rev 2 page 10/45
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