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LM2695SDX 数据表(PDF) 10 Page - National Semiconductor (TI) |
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LM2695SDX 数据表(HTML) 10 Page - National Semiconductor (TI) |
10 / 14 page ON-Time Timer, and Shutdown (Continued) The LM2695 can be remotely shut down by taking the RON/SD pin below 0.8V. 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 V IN and the RON resistor. Current Limit Current limit detection occurs during the off-time by monitor- ing the recirculating current through the free-wheeling diode (D1). Referring to Figure 3, 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 out- put 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 caus- ing the inductor current to exceed 1.25A during each on- time, that is detected at the beginning of each off-time. The operating frequency is lower due to longer-than-normal off- times. Figure 7 illustrates the inductor current waveform. During normal operation the load current is Io, the average of the ripple waveform. When the load resistance decreases the current ratchets up until the lower peak reaches 1.25A. During the Current Limited portion of Figure 7, the current ramps down to 1.25A during each off-time, initiating the next on-time (assuming the voltage at FB is <2.5V). During each on-time the current ramps up an amount equal to: ∆I=(V IN -VOUT)xtON /L1 During this time the LM2695 is in a constant current mode, with an average load current (I OCL) equal to 1.25A + ∆I/2. The current limit threshold can be increased by connecting an external resistor between SGND and ISEN. The external resistor will typically be less than 1 Ω. The peak current out of SW and ISEN must not exceed 2A. The average current out of SW must be less than 1.5A. N - Channel Buck Switch and Driver The LM2695 integrates an N-Channel buck switch and as- sociated floating high voltage gate driver. The peak current allowed through the buck switch is 2A, and the maximum allowed average current is 1.5A. The gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.022 µF capacitor (C4) connected between BST and SW provides the voltage to the driver during the on-time. During each off-time, the SW pin is at approximately -1V, and C4 charges from V CC through the internal diode. The minimum off-time of 250 ns ensures a minimum time each cycle to recharge the bootstrap capaci- tor. 20170413 FIGURE 6. Shutdown Implementation 20170414 FIGURE 7. Inductor Current - Current Limit Operation www.national.com 10 |
类似零件编号 - LM2695SDX |
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类似说明 - LM2695SDX |
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