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LM34910 Folha de dados(PDF) 11 Page - Texas Instruments |
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LM34910 Folha de dados(HTML) 11 Page - Texas Instruments |
11 / 17 page tSS = C6 x 2.5V 11.5 PA C1 = IO x tON 'V LM34910 www.ti.com SNVS297A – OCTOBER 2004 – REVISED FEBRUARY 2005 If the capacitor used for C2 does not have sufficient ESR, R3 is added in series as shown in Typical Application Circuit and Block Diagram. Generally R3 is less than 1 Ω. C2 should generally be no smaller than 3.3 µF, although that is dependent on the frequency and the allowable ripple amplitude at VOUT1. Experimentation is usually necessary to determine the minimum value for C2, as the nature of the load may require a larger value. A load which creates significant transients requires a larger value for C2 than a non-varying load. D1: The important parameters are reverse recovery time and forward voltage. The reverse recovery time determines how long the reverse current surge lasts each time the buck switch is turned on. The forward voltage drop is significant in the event the output is short-circuited as it is mainly this diode’s voltage (plus the voltage across the current limit sense resistor) which forces the inductor current to decrease during the off-time. For this reason, a higher voltage is better, although that affects efficiency. A reverse recovery time of ≊30 ns, and a forward voltage drop of ≊0.75V are preferred. The reverse leakage specification is important as that can significantly affect efficiency. D1’s reverse voltage rating must be at least as great as the maximum VIN, and its current rating must equal or exceed IPK Figure 7. C1 and C5: C1’s purpose is to supply most of the switch current during the on-time, and limit the voltage ripple at VIN, on the assumption that the voltage source feeding VIN has an output impedance greater than zero. If the source’s dynamic impedance is high (effectively a current source), it supplies the average input current, but not the ripple current. At maximum load current, when the buck switch turns on, the current into VIN suddenly increases to the lower peak of the inductor’s ripple current, ramps up to the peak value, then drop to zero at turn-off. The average current during the on-time is the load current. For a worst case calculation, C1 must supply this average load current during the maximum on-time. C1 is calculated from: (15) where Io is the load current, tON is the maximum on-time, and ΔV is the allowable ripple voltage at VIN. C5’s purpose is to help avoid transients and ringing due to long lead inductance at VIN. A low ESR, 0.1 µF ceramic chip capacitor is recommended, located close to the LM34910 . C4: The recommended value for C4 is 0.022 µF. A high quality ceramic capacitor with low ESR is recommended as C4 supplies a surge current to charge the buck switch gate at turn-on. A low ESR also helps ensure a complete recharge during each off-time. C6: The capacitor at the SS pin determines the softstart time, i.e. the time for the reference voltage at the regulation comparator, and the output voltage, to reach their final value. The time is determined from the following: (16) PC BOARD LAYOUT The LM34910 regulation, over-voltage, and current limit comparators are very fast, and respond to short duration noise pulses. Layout considerations are therefore critical for optimum performance. The layout must be as neat and compact as possible, and all of the components must be as close as possible to their associated pins. The current loop formed by D1, L1, C2 and the SGND and ISEN pins should be as small as possible. The ground connection from C2 to C1 should be as short and direct as possible. If it is expected that the internal dissipation of the LM34910 will produce excessive junction temperatures during normal operation, good use of the PC board’s ground plane can help considerably to dissipate heat. The exposed pad on the bottom of the IC package can be soldered to a ground plane, and that plane should extend out from beneath the IC, and be connected to ground plane on the board’s other side with several vias, to help dissipate the heat. The exposed pad is internally connected to the IC substrate. Additionally the use of wide PC board traces, where possible, can help conduct heat away from the IC. Judicious positioning of the PC board within the end product, along with the use of any available air flow (forced or natural convection) can help reduce the junction temperatures. Copyright © 2004–2005, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM34910 |
Nº de peça semelhante - LM34910_14 |
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Descrição semelhante - LM34910_14 |
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