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Zakładka z wyszukiwarką danych komponentów |
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Zakładka z wyszukiwarką danych komponentów |
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SC453 Arkusz danych(PDF) 19 Page - Semtech Corporation |
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SC453 Arkusz danych(HTML) 19 Page - Semtech Corporation |
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19 / 23 page  19 © 2006 Semtech Corp. www.semtech.com POWER MANAGEMENT SC453 Applications Information (Cont.) To calculate the maximum DC value of current we must add the maximum DC current and the maximum ripple value to obtain peak current: S. It is recommended that the current limit be set at 120% of the peak value to allow for inductor current overshoot during load transients: T. The Current Limit Comparator internal to the SC453 moni- tors the output current and turns the high side switch off when the current exceeds the upper current limit thresh- old, I CLMAX and re-enables only if the load current drops be- low the lower current limit threshold, I CLMIN. The current is sensed by monitoring the voltage drop across the current sense resistor R CS. Current limiting will cycle from I CLMAX to ICLMIN for 32 switch- ing cycles to allow for short term transients, then the con- verter is latched off. I CLMAX and ICLMIN are set according to the following equations: U. We set the current limit at I CLIM = ICLMAX, and then solve for R CL, which is R6 in the typical applications circuit. For balance, R8, in series with the CLRF pin is kept the same value as R6. V. RCL ICLIM RHYS ⋅ RCS ⋅ 2.5 VREF ⋅ := RCL 673.59 Ω = R6 681Ω := R8 R6 := R8 681 Ω = Step 7: Small Capacitors/Resistor Selection Several small capacitors are required for signal filtering. Use SMT ceramic capacitors with an X7R or better tem- perature coefficient. COG is preferred. C11, which filters the output voltage feedback, is sized to provide filtering beyond the 5th harmonic of the funda- mental. C11 1 2 Π ⋅ R7 ⋅ FS ⋅ 5 ⋅ := W. C11 9.095 10 11 − × F = In the evaluation board design, we use 100pF, 603, X7R ceramic caps for C11. The DAC output requires a 1nF, X7R or COG capacitor (C23) for high frequency noise fil- tering. The values for C12 and C13 are calculated in a similar manner, though they are returned to the CORE pin because that is the reference point for the current limit comparator. X. C12 1 2 Π ⋅ R6 ⋅ FS ⋅ 5 ⋅ := C13 C12 := C12 1.335 10 10 − × F = C13 1.335 10 10 − × F = IPEAK 23.389 A = IPEAK IMAX_FL IRIPPLE_MAX 2 + := ICLIM 120% IPEAK ⋅ := ICLIM 28.066 A = ICLMAX 3VREF ⋅ RCL RHYS RCS ⋅ ⋅ := ICLMIN 2VREF ⋅ RCL RHYS RCS ⋅ ⋅ := |
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