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LT4430ES6 Datasheet(PDF) 15 Page - Linear Technology |
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LT4430ES6 Datasheet(HTML) 15 Page - Linear Technology |
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15 / 20 page  LT4430 15 4430f S I FOR ATIO APPLICATIO The LT4430’s overshoot control circuitry plus one external capacitor (COC) provide independent control of startup and short-circuit recovery response without compro- mising small-signal frequency compensation. Choosing the optimum COC value is a straightforward laboratory procedure. The following description and set of pictures explain this procedure. Before choosing a value for the OC pin capacitor, complete the remainder of the power supply design. This process includes evaluating the chosen VIN bias generator topology (please consult prior applications information section) and optimizing frequency compensation under all normal operating conditions. During this design phase, set COC to its minimum value of 100pF. This ensures negligible interaction from the overshoot control circuitry. Once these steps are complete, construct a test setup that monitors startup and short-circuit recovery waveforms. Perform this testing with the output lightly loaded. Light load, following full slew operation, is the worst-case as the feedback loop transitions from full to minimal power delivery. As an example, refer to the schematic on the last page illustrating the 5V, 2A isolated flyback converter. All of the following photos are taken with VIN = 48V and ILD = 20mA. Figure 8a demonstrates the power supply startup and short-circuit recovery behavior with no overshoot control compensation (COC = 100pF minimum). The 5V output overshoots by several volts on both startup and short-circuit recovery due to the conservative nature of the small-signal frequency compensation values. Next, increase COC’s value. Either use a capacitor substitu- tion box or solder each new value into the circuit. Monitor the startup and short-circuit recovery waveforms. Note any changes. Figures 8b to 8e illustrate what happens as COC increases. In general, overshoot decreases as COC increases. COC = 0.0168µF in Figure 8b begins to affect loop dynam- ics, but startup still exhibits about 1.5V of overshoot. Short-circuit recovery is considerably more damped. COC = 0.022µF in Figure 8c damps startup overshoot to 0.5V and short-circuit recovery remains similar to that of Figure 8b. COC = 0.033µF in Figure 8d provides under 100mV of overshoot and short-circuit recovery is slightly more damped. COC = 0.047µF in Figure 8e achieves zero over- shoot at the expense of additional damping and delay time in short-circuit recovery. In this example, COC = 0.033µF provides the best value for both startup and short-circuit recovery. Figure 8f provides an expanded scale of the waveforms. After a COC value is selected, check startup and short-circuit recovery over the VIN supply range and with higher output load conditions. Modify the value as necessary. Startup and short-circuit recovery waveforms for various designs will differ from the photos shown in this example. Factors affecting these waveforms include the isolated topology chosen, the primary-side and secondary-side bias circuitry and input/output conditions. For instance, in many isolated power supplies, a winding on the main power transformer bootstraps the supply voltage for the primary-side control circuitry. Under short-circuit condi- tions, the primary-side control circuitry’s supply voltage collapses, generating a restart cycle. Recovery from short-circuit is therefore identical to startup. In the flyback example discussed, the primary-side control circuitry is always active. Switching never stops in short-circuit. The LT4430 error amplifier COMP pin changes from its low clamp level to its higher regulating value during startup and changes from its high clamp level to its lower regulat- ing point during short-circuit recovery. This large-signal behavior explains the observed difference in the startup versus short-circuit recovery waveforms. A final point of discussion involves the chosen COC value. LTC recommends that the designer use a value that con- trols overshoot to the acceptable level, but is not made overly large. The temptation arises to use the overshoot control function as a power supply “soft-start” feature. Larger values of COC, above what is required to control overshoot, do result in smaller dV/dt rates and longer startup times. However, large values of COC may stall the feedback loop during startup or short-circuit recovery, resulting in an extended period of time that the output voltage “flatspots”. This voltage shelf may occur at an intermediate value of output voltage, promoting anomalous behavior with the powered load circuitry. If this situation occurs with the desired COC value, solutions may require circuit modifications. In particular, bias supply holdup times are a prime point of concern as switching stops during these output voltage flatspots. As a reminder, |
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