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SC4612HSTRT Datasheet(PDF) 11 Page - Semtech Corporation |
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SC4612HSTRT Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 18 page  SC4612H 0dB at a rate of -20dB/dec. 4. Cancel w ESR with compensation pole wP (wP=wESR). 5. Place a high frequency compensation pole w P2 at half the switching frequency to get the maximum attenuation of the switching ripple and the high frequency noise with adequate phase lag at w C. Fig2. Power stage and compensated loop gain. COMPONENT SELECTION: SWITCHING SECTION OUTPUT CAPACITORS - Selection begins with the most critical component. Because of fast transient load current requirements in modern microprocessor core supplies, the output capacitors must supply all transient load current requirements until the current in the output inductor ramps up to the new level. Output capacitor ESR is therefore one of the most important criteria. The maximum ESR can be simply calculated from. . . . . . . . . . (4) step current Transient I excursion voltage transient Maximum V , where I V R t t t t ESR = = ≤ For example, to meet a 00mV transient limit with a 0A load step, the output capacitor ESR must be less than 0mW. To meet this kind of ESR level, there are four available capacitor technologies. Applications Information (continued) Technology Each Capacitor Qty Rqd. Total C (mF) ESR (mW) C (mF) ESR (mW) Ceramic 22 2-0 22 2-0 SP Cap 220 7 220 7.0 POS-CAP 680 8 2 360 9.0 Low ESR Aluminum 500 44 5 7500 8.8 The choice of which to use is simply a cost/ performance issue, with low ESR Aluminum being the cheapest, but taking up the most space. INDUCTOR - Having decided on a suitable type and value of output capacitor, the maximum allowable value of inductor can be calculated. Too large an inductor will produce a slow current ramp rate and will cause the output capacitor to supply more of the transient load current for longer - leading to an output voltage sag below the ESR excursion calculated above. The maximum inductor value may be calculated from: . . . . . . . . . (5) ( ) O IN t ESR V V I C R L − ⋅ ≤ The calculated maximum inductor value assumes 00% duty cycle, so some allowance must be made. Choosing an inductor value of 50 to 75% of the calculated maximum will guarantee that the inductor current will ramp fast enough to reduce the voltage dropped across the ESR at a faster rate than the capacitor sags, hence ensuring a good recovery from transient with no additional excursions. We must also be concerned with ripple current in the output inductor and a general rule of thumb has been to allow 0%-20% of maximum output current as ripple current. Note that most of the output voltage ripple is produced by the inductor ripple current flowing in the output capacitor ESR. Ripple current can be calculated from: . . . . . . . . . (6) OSC IN L f L 4 V I RIPPLE ⋅ ⋅ = Ripple current allowance will define the minimum permitted inductor value. |
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