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MAX16974 Datasheet(PDF) 14 Page  Maxim Integrated Products 

MAX16974 Datasheet(HTML) 14 Page  Maxim Integrated Products 
14 / 19 page HighVoltage, 2.2MHz, 2A Automotive Step Down Converter with Low Operating Current 14_ _ ______________________________________________________________________________________ Table 1 shows a comparison between small and large inductor sizes. The inductor value must be chosen so the maximum induc tor current does not reach the minimum current limit of the device. The optimum operating point is usually found between 10% and 30% ripple current. When pulse skip ping (light loads), the inductor value also determines the loadcurrent value at which PFM/PWM switchover occurs. Find a lowloss inductor having the lowest possible DC resistance that fits in the allotted dimensions. Most inductor manufacturers provide inductors in standard values, such as 1.0FH, 1.5FH, 2.2FH, 3.3FH, etc. Also look for nonstandard values, which can provide a bet ter compromise in LIR across the input voltage range. If using a swinging inductor (where the noload inductance decreases linearly with increasing current), evaluate the LIR with properly scaled inductance values. For the selected inductance value, the actual peaktopeak inductor ripple current (DIINDUCTOR) is defined by: OUT SUP OUT INDUCTOR SUP SW V (V V ) I V f L − ∆ = × × where DIINDUCTOR is in A, L is in H, and fSW is in Hz. Ferrite cores are often the best choices, although pow dered iron is inexpensive and can work well at 220kHz. The core must be large enough not to saturate at the peak inductor current (IPEAK): INDUCTOR PEAK LOAD(MAX) I I I 2 ∆ = + Input Capacitor The input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on the input caused by the circuit’s switching. The input capacitor RMS current requirement (IRMS) is defined by the following equation: OUT SUP OUT RMS LOAD(MAX) SUP V (V V ) I I V − = IRMS has a maximum value when the input voltage equals twice the output voltage (VSUP = 2VOUT), so IRMS(MAX) = ILOAD(MAX)/2. Choose an input capacitor that exhibits less than +10NC selfheating temperature rise at the RMS input current for optimal longterm reliability. The inputvoltage ripple is comprised of DVQ (caused by the capacitor discharge) and DVESR (caused by the equivalent series resistance (ESR) of the capacitor). Use lowESR ceramic capacitors with high ripplecurrent capability at the input. Assume the contribution from the ESR and capacitor discharge equal to 50%. Calculate the input capacitance and ESR required for a specified inputvoltage ripple using the following equations: ESR IN L OUT V ESR I I 2 ∆ = ∆ + where SUP OUT OUT L SUP SW (V V ) V I V f L − × ∆ = × × and OUT OUT IN Q SW SUPSW I D(1 D) V C and D V f V × − = = ∆ × where IOUT is the maximum output current, and D is the duty cycle. Output Capacitor The output filter capacitor must have low enough ESR to meet output ripple and loadtransient requirements, yet have high enough ESR to satisfy stability requirements. The output capacitance must be high enough to absorb the inductor energy while transitioning from fullload to noload conditions without tripping the overvoltage fault protection. When using highcapacitance, lowESR capacitors, the filter capacitor’s ESR dominates the output voltage ripple. So the size of the output capaci tor depends on the maximum ESR required to meet the output voltage ripple (VRIPPLE(PP)) specifications: RIPPLE(P P) LOAD(MAX) V ESR I LIR − = × × The actual capacitance value required relates to the physical size needed to achieve low ESR, as well as to the chemistry of the capacitor technology. Thus, the capacitor is usually selected by ESR and voltage rating rather than by capacitance value. Table_1._Inductor_Size_Comparison INDUCTOR_SIZE SMALLER LARGER Lower price Smaller ripple Smaller form factor Higher efficiency Faster load response Larger fixedfrequency range in skip mode 
