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LCS701HG Datasheet(PDF) 14 Page - Power Integrations, Inc. |
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LCS701HG Datasheet(HTML) 14 Page - Power Integrations, Inc. |
14 / 26 page Rev. B 062011 14 LCS700-708 www.powerint.com For example, if BT2 is selected, and f MAX is 800 kHz, then fSTART = 300 kHz, and f STOP = 350 kHz. If during normal operation the load is reduced and the frequency rises to 350 kHz, the switching will stop. This causes the output voltage to drop and the feedback loop to decrease the FEEDBACK pin current. When the current decreases to a value which corresponds to 300 kHz, switching will commence, and the cycle will repeat. During start-up mode, however, the outputs can switch at a frequency between f STOP and fMAX (250 kHz and 800 kHz in the above example). Start-up mode is exited once the switching frequency drops below f STOP, and the HiperLCS will subsequently enter burst mode if the feedback loop attempts to produce a switching frequency >f STOP. f MAX is the frequency at which the internal counters run when the HiperLCS is in the off-state of the auto-restart cycle, or in the power-up delay before switching. The minimum recommended dead-time is 275 ns, and thus the maximum f MAX setting is 1 MHz. To simplify the selection of R FMAX, see the selection curves in Figure 17. Figure 16. FEEDBACK Pin and DT/BF Pin Current vs. Frequency. 600 400 200 0 800 1000 0 50 150 100 200 250 300 350 400 450 Frequency (kHz) 250 300 350 400 500 450 Dead-Time (ns) 13.0 12.0 11.0 9.0 10.0 8.0 7.0 6.0 5.0 BT1 BT2 BT3 Figure 17. R FMAX vs. Dead-Time, for the 3 Different Burst Threshold Settings. Figure 18. f START (Lower Burst Threshold Frequency) vs. Dead-Time Setting for Different Burst Threshold Settings (BT1, BT2, BT3). 250 300 350 400 500 450 Dead-Time (ns) 500 450 350 400 300 250 200 150 BT1 BT2 BT3 The f STOP to fSTART ratio is fixed, and dependent on the Burst Threshold setting (see Table 5). Table 5. Ratio of f STOP /fSTART vs. Burst Threshold Selection. As a first approximation, during burst mode, the frequency ramps from f START to fSTOP; then switching stops, and then the cycle repeats. FEEDBACK Pin The FEEDBACK pin is the voltage regulation FEEDBACK pin. It has a nominal Thevenin equivalent circuit of 0.65 V and 2.5 kW. In normal operation, it sinks current. During the off-period of auto-restart, and during the clocked delay before start-up, it pulls up internally to V REF in order to discharge the soft-start capacitor. The current entering the pin determines switching frequency. Higher current yields higher frequency and thus reduces LLC output voltage. In a typical application an optocoupler connected to the VREF pin pulls up on the FEEDBACK pin, via a resistor network. The optocoupler is configured to source increasing FEEDBACK pin current, as the output rises. The resistor network between the optocoupler, FEEDBACK pin, and VREF pin, determine the minimum and maximum FEEDBACK pin current (and thus the minimum and maximum operating frequency), that the optocoupler can command as it goes from cutoff to saturation. This network also contains the soft-start timing capacitor, C START (Figure 19). The minimum frequency as set by this network must be lower than the frequency required by the powertrain at minimum input voltage. In Figure 19 this is determined by the sum of R FMIN and R START. The FEEDBACK pin current is determined by these two resistors when the optocoupler is cut off. C START can be ignored during normal operation. Do not confuse R START, which determines Burst Threshold Setting f STOP / fSTART 1 1.14 2 1.17 3 1.20 |
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