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Si8283CD-IS Scheda tecnica(PDF) 8 Page - Silicon Laboratories |
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Si8283CD-IS Scheda tecnica(HTML) 8 Page - Silicon Laboratories |
8 / 43 page 2.12.4 Thermal Protection A thermal shutdown circuit is implemented to protect the system from over-temperature events. The thermal shutdown is activated at a junction temperature that prevents permanent damage from occurring. 2.12.5 Cycle Skipping Cycle skipping is included to reduce switching power losses at light loads. This feature is transparent to the user and is activated auto- matically at light loads. The product options with integrated power switches (Si8281/83) may never experience cycle skipping during operation, even at light loads, while the external power switch options (Si8282/84) are likely to have cycle skipping start at light loads. 2.12.6 Shutdown (Si8283 and Si8284 Only) This feature allows the operation of the dc-dc converter to be shut down when SH/FC is asserted high. This pin normally has a resistor to ground, the value of which is used in conjunction with the value of the capacitor on the SS pin during startup to determine the dc-dc switching frequency. Therefore, a GPIO pin connected to SH/FC pin to control the shutdown function should be in a high-impedance state during startup to avoid interfering with the internal frequency calculation circuit. During normal operation, this pin should be held low and only taken high to assert dc-dc shutdown. 2.12.7 Soft Start (Si8283 and Si8284 Only) The dc-dc controller has an internal timer that controls the power conversion start-up to limit inrush current. There is also a Soft Start option where users can program the soft start up by an external capacitor connected to the SS pin. The soft start period is the maximum duration of time that the Si8283/84 will try to ramp up the output voltage. If the output voltage fails to reach the targeted voltage level within this soft start period, the Si8283/84 will terminate the dc-dc startup cycle and wait for 40 sec- onds before initiating a new (startup) cycle. The equations for setting the soft start period are as follows: tSS = 200000 × CSS or CSS = tSS 200000 2.12.8 Programmable Frequency (Si8283 and Si8284 Only) The frequency of the PWM modulator is set to a default of 250 kHz for Si828x. Users can program their desired frequency within a given band of 200 kHz to 800 kHz by controlling the time constant of an external RC connected to the SH_FC and SS pins. The equations for setting fSW or RSW are as follows: f SW = 1025.5 (RSW × CSS) or RSW = 1025.5 ( f SW × CSS) The following are the recommended steps for calculating CSS and RSW: 1. Select the maximum soft start duration (typically 40 ms). 2. Calculate Css using Equation A. 3. Select the dc-dc switching frequency. 4. Calculate RSW using the above equation. Si8281/82/83/84 Data Sheet System Overview silabs.com | Smart. Connected. Energy-friendly. Preliminary Rev. 0.5 | 7 |
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