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LM25085QMYE bảng dữ liệu(PDF) 10 Page - Texas Instruments |
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10 / 30 page FS = VOUT x (VIN - 1.56V + RT/3167) VIN x [(1.45 x 10 -7 x (RT + 1.4)) + (tD x (VIN - 1.56V + RT/3167))] tON = (VIN - 1.56V + RT/3167) 1.45 x 10 -7 x (RT + 1.4) + 50 ns LM25085, LM25085-Q1 SNVS593H – OCTOBER 2008 – REVISED MARCH 2013 www.ti.com The LM25085 is available in both an 8 pin HVSSOP-PowerPAD package and an 8 pin WSON package with an exposed pad to aid in heat dissipation. An 8 pin VSSOP package without an exposed pad is also available. REGULATION CONTROL CIRCUIT The LM25085 buck DC-DC controller employs a control scheme based on a comparator and a one-shot on- timer, with the output voltage feedback compared to an internal reference voltage (1.25V). When the FB pin voltage falls below the feedback reference, Q1 is switched on for a time period determined by the input voltage and a programming resistor (RT). Following the on-time Q1 remains off until the FB voltage falls below the reference. Q1 is then switched on for another on-time period. The output voltage is set by the feedback resistors (RFB1, RFB2 in Block Diagram). The regulated output voltage is calculated as follows: VOUT = 1.25V x (RFB2+ RFB1)/ RFB1 (2) The feedback voltage supplied to the FB pin is applied to a comparator rather than a linear amplifier. For proper operation sufficient ripple amplitude is necessary at the FB pin to switch the comparator at regular intervals with minimum delay and noise susceptibility. This ripple is normally obtained from the output voltage ripple attenuated through the feedback resistors. The output voltage ripple is a result of the inductor’s ripple current passing through the output capacitor’s ESR, or through a resistor in series with the output capacitor. Multiple methods are available to ensure sufficient ripple is supplied to the FB pin, and three different configurations are discussed in Applications Information. When in regulation, the LM25085 operates in continuous conduction mode at medium to heavy load currents and discontinuous conduction mode at light load currents. In continuous conduction mode the inductor’s current is always greater than zero, and the operating frequency remains relatively constant with load and line variations. The minimum load current for continuous conduction mode is one-half the inductor’s ripple current amplitude. In discontinuous conduction mode, where the inductor’s current reaches zero during the off-time, the operating frequency is lower than in continuous conduction mode and varies with load current. Conversion efficiency is maintained at light loads since the switching losses are reduced with the reduction in load and frequency. If the voltage at the FB pin exceeds 1.6V due to a transient overshoot or excessive ripple at VOUT the internal over-voltage comparator immediately switches off Q1. The next on-time period starts when the voltage at FB falls below the feedback reference voltage. ON-TIME TIMER The on-time of the PFET gate drive output (PGATE pin) is determined by the resistor (RT) and the input voltage (VIN), and is calculated from: where • RT is in kΩ (3) The minimum on-time, which occurs at maximum VIN, should not be set less than 150ns (see CURRENT LIMITING). The buck regulator effective on-time, measured at the SW node (junction of Q1, L1, and D1) is typically longer than that calculated in Equation 3 due to the asymmetric delay of the PFET. The on-time difference caused by the PFET switching delay can be estimated as the difference of the turn-off and turn-on delays listed in the PFET data sheet. Measuring the difference between the on-time at the PGATE pin versus the SW node in the actual application circuit is also recommended. In continuous conduction mode, the inverse relationship of tON with VIN results in a nearly constant switching frequency as VIN is varied. The operating frequency can be calculated from: where • RT is in kΩ • tD is equal to 50ns plus the PFET’s delay difference (4) 10 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated Product Folder Links: LM25085 LM25085-Q1 |
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