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L6926013TR Datasheet(PDF) 5 Page - STMicroelectronics |
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L6926013TR Datasheet(HTML) 5 Page - STMicroelectronics |
5 / 11 page 5/11 L6926 In this mode of operation, at light load, the device operates discontinuously based on the COMP pin voltage, in order to keep the efficiency very high also in these conditions. While the device is not switching the load dis- charges the output capacitor and the output voltage goes down. When the feedback voltage goes lower than the internal reference, the COMP pin voltage increases and when an internal threshold is reached, the device starts to switch. In these conditions the peak current limit is set approximately in the range of 200mA-400mA, depending on the slope compensation (see related section). Once the device starts to switch the output capacitor is recharged. The feedback pin increases and, when it reaches a value slightly higher than the reference voltage, the output of the error amplifier goes down until a clamp is activated. At this point, the device stops to switch. In this phase, most of the internal circuitries are off, so reducing the device consumption down to a typical value of 25 µA. 4.1.2 Low Noise Mode If for noise reasons, the very low frequencies of the low consumption mode are undesirable, the low noise mode can be selected. In low noise mode, the efficiency is a little bit lower compared with the low consumption mode in very light load conditions but for medium-high load currents the efficiency values are very similar. Basically, the device switches with its internal free running frequency of 600KHz. Obviously, in very light load conditions, the device could skip some cycles in order to keep the output voltage in regulation. 4.1.3 Synchronization The device can also be synchronized with an external signal from 500KHz up to 1.4MHz. In this case the low noise mode is automatically selected. The device will eventually skip some cycles in very light load conditions. The internal synchronization circuit is inhibited in shortcircuit and overvoltage conditions in order to keep the protections effective (see relative sections). 4.2 Short Circuit Protection During the device operation, the inductor current increases during the high side turn on phase and decrease during the high side turn off phase based on the following equations: In strong overcurrent or shortcircuit conditions the VOUT can be very close to zero. In this case ∆ION increases and ∆IOFF decreases. When the inductor peak current reaches the current limit, the high side mosfet turns off and so the TON is reduced down to the minimum value (250ns typ.) in order to reduce as much as possible ∆ION. 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: Where IPEAK is the valley current limit (1.4A typ.) and TON_MIN is the minimum TON of the high side MOSFET. 4.3 Slope Compensation ∆I ON V IN V OUT – () L ----------------------------------- T ON ⋅ = ∆I OFF V OUT () L ------------------- T OFF ⋅ = I PEAK I VALLEY V IN L --------- T ON_MIN ⋅ + = |
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