Datenblatt-Suchmaschine für elektronische Bauteile |
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LM34917A Datenblatt(PDF) 10 Page - Texas Instruments |
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LM34917A Datenblatt(HTML) 10 Page - Texas Instruments |
10 / 22 page STOP RUN RON/SD Input Voltage LM34917A VIN RON RON = VOUT x (VIN - 1.35V) VIN x 1.16 x 10 -10 x f SW -1.4k: tON = VIN - 1.35V 1.16 x 10 -10 x (RON + 1.4 k:) + 100 ns LM34917A SNOSAX7D – DECEMBER 2007 – REVISED MARCH 2013 www.ti.com ON-Time Timer The on-time for the LM34917A is determined by the RON resistor and the input voltage (VIN), calculated from: (5) The inverse relationship with VIN results in a nearly constant frequency as VIN is varied. To set a specific continuous conduction mode switching frequency (fSW), the RON resistor is determined from the following: (6) Equation 1, Equation 5 and Equation 6 are valid only during normal operation - i.e., the circuit is not in current limit. When the LM34917A operates in current limit, the on-time is reduced by approximately 50%. This feature reduces the peak inductor current which may be excessively high if the load current and the input voltage are simultaneously high. This feature operates on a cycle-by-cycle basis until the load current is reduced and the output voltage resumes its normal regulated value. Equation 1, Equation 5 and Equation 6 have a ±25% tolerance. Remote Shutdown The LM34917A can be remotely shut down by taking the RON/SD pin below 0.65V. See Figure 13. In this mode the SS pin is internally grounded, the on-timer is disabled, and bias currents are reduced. Releasing the RON/SD pin allows the circuit to resume operation after the SS pin voltage is below 0.18V. The voltage at the RON/SD pin is normally between 1.4V and 3.5V, depending on VIN and the RON resistor. Figure 13. Remote Shutdown Input Over-Voltage Shutdown If the input voltage at VIN increases above 34.8V an internal comparator disables the buck switch and the on- timer, and grounds the soft-start pin. Normal operation resumes when the VIN voltage reduces below 34.8V, and when the soft-start voltage (at the SS pin) has reduced below 0.18V. Current Limit Current limit detection occurs during the off-time by monitoring the recirculating current flowing out of the ISEN pin. Referring to the Block Diagram, during the off-time the inductor current flows through the load, into SGND, through the internal sense resistor, out of ISEN and through D1 to the inductor. If that current exceeds the current limit threshold the current limit comparator output delays the start of the next on-time period. The next on- time starts when the current out of ISEN is below the threshold and the voltage at FB falls below 2.5V. The operating frequency is typically lower due to longer-than-normal off-times. The valley current limit threshold is a function of the input voltage (VIN) and the output voltage sensed at FB, as shown in the graph “Valley Current Limit Threshold vs. VFB and VIN”. This feature reduces the inductor current’s peak value at high line and load. To further reduce the inductor’s peak current, the next cycle’s on-time is reduced by approximately 50% if the voltage at FB is below its threshold when the inductor current reduces to the current limit threshold (VOUT is low due to current limiting). 10 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated Product Folder Links: LM34917A |
Ähnliche Teilenummer - LM34917A_15 |
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Ähnliche Beschreibung - LM34917A_15 |
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