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A7986A Datasheet(PDF) 9 Page - STMicroelectronics |
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A7986A Datasheet(HTML) 9 Page - STMicroelectronics |
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9 / 42 page  DocID022801 Rev 5 9/42 A7986A Functional description 5.1 Oscillator and synchronization Figure 4 shows the block diagram of the oscillator circuit. The internal oscillator provides a constant frequency clock. Its frequency depends on the resistor externally connected to the FSW pin. If the FSW pin is left floating, the frequency is 250 kHz; it can be increased as shown in Figure 6 by an external resistor connected to ground. To improve the line transient performance, keeping the PWM gain constant versus the input voltage, the voltage feed-forward is implemented by changing the slope of the sawtooth according to the input voltage change (see Figure 5.a). The slope of the sawtooth also changes if the oscillator frequency is increased by the external resistor. In this way a frequency feed-forward is implemented (Figure 5.b) in order to keep the PWM gain constant versus the switching frequency (see Section 6.4 for PWM gain expression). The synchronization signal is generated on the SYNCH pin. 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 the SYNCH pins are connected, the device with 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 minimizes the RMS current flowing through the input capacitor (see the L5988D datasheet). Figure 4. Oscillator circuit block diagram The device can be synchronized to work at higher frequency feeding an external clock signal. The synchronization changes the sawtooth amplitude, changing the PWM gain (Figure 5.c). This change must be taken into account when the loop stability is studied. To minimize the change of the 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 frequency changes the sawtooth slope in order to render the truncation of sawtooth negligible, due to the external synchronization. Clock Generator Ramp Generator FSW Sawtooth Clock Synchronization SYNCH Clock Generator Ramp Generator FSW Sawtooth Clock Clock Synchronization SYNCH |
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