Datasheet A7987 (STMicroelectronics) - 9

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A7987 Oscillator and synchronization Figure 5. Switching frequency programmability
1500 1400 1300 1200 1100
z]
1000
H
900
[k
800 700
Fsw
600 500 400 300 10 20 30 40 50 60 70 80 90 100 RFSW [kOhm] 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-forward6a). The slope of the saw-tooth also changes if the oscillator frequency is programmed by the external resistor. In this way a frequency feed-forward is implemented (Figure 6. Feed-forward6b) 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 A7987s can be connected to the same SYNCH pin; however, the clock phase shift from master switching frequency to slaves input clock is 180°. The A7987 can be synchronized to work at a higher frequency, in the range 250 kHz-1500 kHz, providing the SYNCH pin with an external clock signal. The synchronization changes the saw-tooth amplitude, also affecting the PWM gain (Figure 6. Feed-forward6c). 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) A7987 SYNCH pins are tied together, the A7987 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 FSW pin, as suggested above, is required for a proper operation.
DS12928
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Rev 1 page 9/37
Document Outline Cover image Product status link / summary 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 Error amplifier and light-load management 5.4 Low VIN operation 5.5 Overcurrent protection 5.6 Overtemperature protection 6 Application information 6.1 Input capacitor selection 6.2 Output capacitor selection 6.3 Inductor selection 6.4 Compensation network 6.4.1 Type II compensation network 6.4.2 Type III compensation network 6.5 Thermal considerations 6.6 Layout considerations 7 Demonstration board 8 Application ideas 8.1 Positive buck-boost 8.2 Negative buck-boost 9 Package information 9.1 HTSSOP16 package information 10 Ordering information Revision history