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L6926Q1TR Datasheet(PDF) 9 Page - STMicroelectronics |
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L6926Q1TR Datasheet(HTML) 9 Page - STMicroelectronics |
9 / 16 page L6926 Operation description Doc ID 9302 Rev 9 9/16 current limit, the high side MOSFET turns off and so the TON is reduced down to the minimum value (250 ns typ.) in order to reduce as much as possible ΔI ON. Anyway, if VOUT is low enough it can be that the inductor peak current further increases because during the TOFF the current decays very slowly. Due to this reason a second protection that fixes the maximum inductor valley current has been introduced. This protection doesn't allow the high side MOSFET to turn on if the current flowing through the inductor is higher that a specified threshold (valley current limit). Basically the TOFF is increased as much as required to bring the inductor current down to this threshold. So, the maximum peak current in worst case conditions will be: Equation 3 Where IPEAK is the valley current limit (1.4 A typ.) and TON_MIN is the minimum TON of the high side MOSFET. 4.3 Slope compensation In current mode architectures, when the duty cycle of the application is higher than approximately 50%, a pulse-by-pulse instability (the so called sub harmonic oscillation) can occur. To allow loop stability also in these conditions a slope compensation is present. This is realized by reducing the current flowing through the inductor necessary to trigger the COMP comparator (with a fixed value for the COMP pin voltage). With a given duty cycle higher than 50%, the stability problem is particularly present with an higher input voltage (due to the increased current ripple across the inductor), so the slope compensation effect increases as the input voltage increases. From an application point of view, the final effect is that the peak current limit depends both on the duty cycle (if higher than approximately 40%) and on the input voltage. 4.4 Loop stability Since the device is realized with a current mode architecture, the loop stability is usually not a big issue. For most of the application a 220 pF connected between the COMP pin and ground is enough to guarantee the stability. In case very low ESR capacitors are used for the output filter, such as multilayer ceramic capacitors, the zero introduced by the capacitor itself can shift at very high frequency and the transient loop response could be affected. Adding a series resistor to the 220 pF capacitor can solve this problem. The right value for the resistor (in the range of 50 k) can be determined by checking the load transient response of the device. Basically, the output voltage has to be checked at the scope after the load steps required by the application. In case of stability problems, the output voltage could oscillates before to reach the regulated value after a load step. I PEAK I VALLEY V IN L --------- T ON_MIN ⋅ + = |
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