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LM34914 Arkusz danych(PDF) 9 Page - Texas Instruments |
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LM34914 Arkusz danych(HTML) 9 Page - Texas Instruments |
9 / 24 page RON = VOUT x (VIN - 1.5) FS x 1.15 x 10 -10 x VIN - 1.4k tON = (VIN - 1.5) 1.15 x 10 -10 x (RON + 1.4k) + 50 ns FS = VOUT 2 x L1 x 1.5 x 1020 RL x RON 2 DC = tON tON + tOFF VOUT VIN = tON x FS = FS = VOUT x (VIN ± 1.5) 1.15 x 10 -10 x (R ON + 1.4k) x VIN LM34914 www.ti.com SNVS453B – MAY 2006 – REVISED MARCH 2013 When in regulation, the LM34914 operates in continuous conduction mode at 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. The approximate operating frequency is calculated as follows: (1) The buck switch duty cycle is equal to: (2) In discontinuous conduction mode, where the inductor’s current reaches zero during the off-time forcing a longer- than-normal 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 decrease with the reduction in load and frequency. The approximate discontinuous operating frequency can be calculated as follows: (3) where RL = the load resistance, and L1 is the circuit’s inductor. The output voltage is set by the two feedback resistors (R1, R2 in the Block Diagram). The regulated output voltage is calculated as follows: VOUT = 2.5 x (R1 + R2) / R2 (4) Output voltage regulation is based on supplying ripple voltage to the feedback input (FB pin), normally obtained from the output voltage ripple through the feedback resistors. The LM34914 requires a minimum of 25 mVp-p of ripple voltage at the FB pin, requiring the ripple voltage at VOUT be higher by the gain factor of the feedback resistor ratio. The output ripple voltage is created by the inductor’s ripple current passing through R3 which is in series with the output capacitor. For applications where reduced ripple is required at VOUT, see Applications Information. If the voltage at FB rises above 2.9V, due to a transient at VOUT or excessive inductor current which creates higher than normal ripple at VOUT, the internal over-voltage comparator immediately shuts off the internal buck switch. The next on-time starts when the voltage FB falls below 2.5V and the inductor current falls below the current limit threshold. ON-Time Timer The on-time for the LM34914 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 (FS), 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 LM34914 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. Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM34914 |
Podobny numer części - LM34914_15 |
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Podobny opis - LM34914_15 |
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