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LM34910 Datasheet(PDF) 8 Page - Texas Instruments |
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LM34910 Datasheet(HTML) 8 Page - Texas Instruments |
8 / 17 page STOP RUN RON/SD Input Voltage LM34910 VIN RON FMAX = VOUT VINMAX x 200 ns tON = VIN 1.3 x 10 -10 x R ON LM34910 SNVS297A – OCTOBER 2004 – REVISED FEBRUARY 2005 www.ti.com Regulation Comparator The feedback voltage at FB is compared to the voltage at the Softstart pin (2.5V). In normal operation (the output voltage is regulated), an on-time period is initiated when the voltage at FB falls below 2.5V. The buck switch stays on for the on-time, causing the FB voltage to rise above 2.5V. After the on-time period, the buck switch stays off until the FB voltage falls below 2.5V. Bias current at the FB pin is nominally 100 nA. Over-Voltage Comparator The voltage at FB is compared to an internal 2.875V reference. If the voltage at FB rises above 2.875V the on- time pulse is immediately terminated. This condition can occur if the input voltage or the output load changes suddenly, or if the inductor (L1) saturates. The buck switch remains off until the voltage at FB falls below 2.5V. ON-Time Timer, and Shutdown The on-time for the LM34910 is determined by the RON resistor and the input voltage (VIN), and is calculated from: (5) See Figure 3. The inverse relationship with VIN results in a nearly constant frequency as VIN is varied. RON should be selected for a minimum on-time (at maximum VIN) greater than 200 ns. This requirement limits the maximum frequency for each application, depending on VIN and VOUT, calculated from the following: (6) The LM34910 can be remotely shut down by taking the RON/SD pin below 0.65V. See Figure 6. 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 normal operation to resume. The voltage at the RON/SD pin is between 1.5V and 3.0V, depending on VIN and the RON resistor. Figure 6. Shutdown Implementation Current Limit Current limit detection occurs during the off-time by monitoring the recirculating current through the free-wheeling diode (D1). Referring to Typical Application Circuit and Block Diagram, when the buck switch is turned off the inductor current flows through the load, into SGND, through the sense resistor, out of ISEN and through D1. If that current exceeds 1.25A the current limit comparator output switches to delay the start of the next on-time period if the voltage at FB is below 2.5V. The next on-time starts when the current out of ISEN is below 1.25A and the voltage at FB is below 2.5V. If the overload condition persists causing the inductor current to exceed 1.25A during each on-time, that is detected at the beginning of each off-time. The operating frequency may be lower due to longer-than-normal off-times. 8 Submit Documentation Feedback Copyright © 2004–2005, Texas Instruments Incorporated Product Folder Links: LM34910 |
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