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LM34925 Datasheet(PDF) 10 Page - Texas Instruments |
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LM34925 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 27 page 0.07 x VIN TOFF(ILIM) = VFB + 0.2V 2s 10 -10 x R ON TON = VIN LM34925 SNVS846F – JUNE 2012 – REVISED NOVEMBER 2014 www.ti.com Feature Description (continued) At high-input voltages, the power dissipated in the high voltage regulator is significant and can limit the overall achievable output power. As an example, with the input at 48 V and switching at high frequency, the VCC regulator may supply up to 7 mA of current resulting in 48 V × 7 mA = 336 mW of power dissipation. If the VCC voltage is driven externally by an alternate voltage source, between 8.55 V and 14 V, the internal regulator is disabled. This reduces the power dissipation in the IC. 7.3.3 Regulation Comparator The feedback voltage at FB is compared to an internal 1.225-V reference. In normal operation, when the output voltage is in regulation, an on-time period is initiated when the voltage at FB falls below 1.225 V. The high-side switch will stay on for the on-time, causing the FB voltage to rise above 1.225 V. After the on-time period, the high-side switch will stay off until the FB voltage again falls below 1.225 V. During start-up, the FB voltage will be below 1.225 V at the end of each on-time causing the high-side switch to turn on immediately after the minimum forced off-time of 144 ns. The high-side switch can be turned off before the on-time is over, if peak current in the inductor reaches the current limit threshold. 7.3.4 Overvoltage Comparator The feedback voltage at FB is compared to an internal 1.62-V reference. If the voltage at FB rises above 1.62 V the on-time pulse is immediately terminated. This condition can occur if the input voltage and/or the output load changes suddenly. The high-side switch will not turn on again until the voltage at FB falls below 1.225 V. 7.3.5 On-Time Generator The on-time for the LM34925 device is determined by the RON resistor, and is inversely proportional to the input voltage (VIN), resulting in a nearly constant frequency as VIN is varied over its range. The on-time equation for the LM34925 can is determined by Equation 3. (3) See Figure 5. RON should be selected for a minimum on-time (at maximum VIN) greater than 100 ns, for proper operation. This requirement limits the maximum frequency for high VIN. 7.3.6 Current Limit The LM34925 contains an intelligent current limit off-timer. If the current in the buck switch exceeds 270 mA, the present cycle is immediately terminated, and a non-resetable off-timer is initiated. The length of off-time is controlled by the FB voltage and the input voltage VIN. As an example, when FB = 0 V and VIN = 48 V, a maximum off-time is set to 16 μs. This condition occurs when the output is shorted, and during the initial part of start-up. This amount of time ensures safe short circuit operation up to the maximum input voltage of 100 V. In cases of overload where the FB voltage is above zero volts (not a short circuit) the current limit off-time is reduced. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery time, and start-up time. The off-time is calculated from Equation 4. (4) The current limit protection feature is peak limited, the maximum average output will be less than the peak. 7.3.7 N-Channel Buck Switch and Driver The LM34925 device integrates an N-Channel Buck switch and associated floating high voltage gate driver. The gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.01-uF ceramic capacitor connected between the BST pin and SW pin provides the voltage to the driver during the on-time. During each off-time, the SW pin is at approximately 0 V, and the bootstrap capacitor charges from VCC through the internal diode. The minimum off-timer, set to 144 ns, ensures a minimum time each cycle to recharge the bootstrap capacitor. 10 Submit Documentation Feedback Copyright © 2012–2014, Texas Instruments Incorporated Product Folder Links: LM34925 |
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