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LMZ10500 Datasheet(PDF) 10 Page - Texas Instruments |
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LMZ10500 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 34 page 100 µs/Div 1V/Div 1V/Div 0.3A/Div 50 mA/Div VCON IL IIN VOUT 10 µs/Div 500 mV/Div 500 mV/Div 200 mA/Div 200 mA/Div VCON IL IOUT VOUT LMZ10500 SNVS723F – OCTOBER 2011 – REVISED FEBRUARY 2015 www.ti.com Feature Description (continued) 7.3.2 Startup Behavior and Softstart The LMZ10500 features a current limit based soft start circuit in order to prevent large in-rush current and output overshoot as VOUT is ramping up. This is achieved by gradually increasing the PFET current limit threshold to the final operating value as the output voltage ramps during startup. The maximum allowed current in the inductor is stepped up in a staircase profile for a fixed number of switching periods in each step. Additionally, the switching frequency in the first step is set at 450kHz and is then increased for each of the following steps until it reaches 2MHz at the final step of current limiting. This current limiting behavior is illustrated in Figure 10 and allows for a smooth VOUT ramp up. Figure 10. Startup Behavior of Current Limit Based softstart The soft start rate is also limited by the VCON ramp up rate. The VCON pin is discharged internally through a pull down device before startup occurs. This is done to deplete any residual charge on the VCON filter capacitor and allow the VCON voltage to ramp up from 0V when the part is started. The events that cause VCON discharge are thermal shutdown, UVLO, EN low, or output short circuit detection. The minimum recommended capacitance on VCON is 220 pF and the maximum is 1 nF. The duration of startup current limiting sequence takes approximately 75 µs. After the sequence is completed, the feedback voltage is monitored for output short circuit events. 7.3.3 Output Short Circuit Protection In addition to cycle by cycle current limit, the LMZ10500 features a second level of short circuit protection. If the load pulls the output voltage down and the feedback voltage falls to 0.375 V, the output short circuit protection will engage. In this mode the internal PFET switch is turned OFF after the current limit comparator trips and the beginning of the next cycle is inhibited for approximately 230 µs. This forces the inductor current to ramp down and limits excessive current draw from the input supply when the output of the regulator is shorted. The synchronous rectifier is always OFF in this mode. After 230 µs of non-switching a new startup sequence is initiated. During this new startup sequence the current limit is gradually stepped up to the nominal value as illustrated in the Startup Behavior and Softstart section. After the startup sequence is completed again, the feedback voltage is monitored for output short circuit. If the short circuit is still persistent after the new startup sequence, switching will be stopped again and there will be another 230 µs off period. A persistent output short condition results in a hiccup behavior where the LMZ10500 goes through the normal startup sequence, then detects the output short at the end of startup, terminates switching for 230 µs, and repeats this cycle until the output short is released. This behavior is illustrated in Figure 11. Figure 11. Hiccup Behavior With Persistent Output Short Circuit 10 Submit Documentation Feedback Copyright © 2011–2015, Texas Instruments Incorporated Product Folder Links: LMZ10500 |
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