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LMZ14203EXT Datasheet(PDF) 11 Page - Texas Instruments |
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LMZ14203EXT Datasheet(HTML) 11 Page - Texas Instruments |
11 / 31 page 0.47 éF 6.8 éH Co CIN Cvcc FB EN SS Vin Linear reg RON Timer Css RON RFBT RFBB CFF Regulator IC VO Internal Passives VOUT GND VIN 1 2 3 4 5 6 7 LMZ14203EXT www.ti.com SNVS666H – JUNE 2010 – REVISED OCTOBER 2015 7 Detailed Description 7.1 Overview 7.1.1 COT Control Circuit Overview Constant ON-Time control is based on a comparator and an ON-time one-shot, with the output voltage feedback compared with an internal 0.8-V reference. If the feedback voltage is below the reference, the main MOSFET is turned on for a fixed ON-time determined by a programming resistor RON. RON is connected to VIN such that ON- time is reduced with increasing input supply voltage. Following this ON-time, the main MOSFET remains off for a minimum of 260 ns. If the voltage on the feedback pin falls below the reference level again the ON-time cycle is repeated. Regulation is achieved in this manner. 7.2 Functional Block Diagram 7.3 Feature Description 7.3.1 Output Overvoltage Comparator The voltage at FB is compared to a 0.92-V internal reference. If FB rises above 0.92 V the ON-time is immediately terminated. This condition is known as overvoltage protection (OVP). It can occur if the input voltage is increased very suddenly or if the output load is decreased very suddenly. When OVP is activated, the top MOSFET ON-times will be inhibited until the condition clears. Additionally, the synchronous MOSFET will remain on until inductor current falls to zero. 7.3.2 Current Limit Current limit detection is carried out during the OFF-time by monitoring the current in the synchronous MOSFET. Referring to the Functional Block Diagram, when the top MOSFET is turned off, the inductor current flows through the load, the PGND pin and the internal synchronous MOSFET. If this current exceeds 4.2 A (typical) the current limit comparator disables the start of the next ON-time period. The next switching cycle will occur only if the FB input is less than 0.8 V and the inductor current has decreased below 4.2 A. Inductor current is monitored during the period of time the synchronous MOSFET is conducting. So long as inductor current exceeds 4.2 A, further ON-time intervals for the top MOSFET will not occur. Switching frequency is lower during current limit due to the longer OFF-time. It must also be noted that current limit is dependent on both duty cycle and temperature. Copyright © 2010–2015, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LMZ14203EXT |
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