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FAN2108EMPX Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FAN2108EMPX Datasheet(HTML) 11 Page - Fairchild Semiconductor |
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11 / 14 page  © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN2108 • Rev. 1.0.2 11 Calculating the Inductor Value Typically the inductor is set for a ripple current ( ΔIL) of 10% to 35% of the maximum DC load. Regulators requiring fast transient response use a value on the high side of this range; while regulators that require very low output ripple and/or use high-ESR capacitors restrict allowable ripple current. f ) V - (1 V OUT OUT • Δ • = L IN I V L (4) where f is the oscillator frequency. Setting the Ramp Resistor Value The internal ramp voltage excursion (∆VRAMP) during tON should be set to 0.6 V at nominal operating point. RRAMP is approximately: 2 10 18 ) 8 . 1 ( 6 ) ( − • • • − = − Ω f V x V V R IN OUT IN K RAMP (5) where frequency (f) is expressed in KHz. Setting the Current Limit The current limit system involves two comparators. The MAX ILIMIT comparator is used with a VILIM fixed-voltage reference and represents the maximum current limit allowable. This reference voltage is temperature compensated to reflect the RDSON variation of the low- side MOSFET. The ADJUST ILIMIT comparator is used where the current limit needs to be set lower than the VILIM fixed reference. The 10 µA current source does not track the RDSON changes over temperature, so change is added into the equations for calculating the ADJUST ILIMIT comparator reference voltage, as is shown below. Figure 22 shows a simplified schematic of the over- current system. Figure 22. Current-Limit System Schematic Since the ILIM voltage is set by a 10 µA current source into the RILIM resistor, the basic equation for setting the reference voltage is: VRILIM = 10µA*RILIM (6) To calculate RILIM: RILIM = VRILIM/ 10µA (7) The voltage VRILIM is made up of two components, VBOT (which relates to the current through the low-side MOSFET) and VRMPEAK (which relates to the peak current through the inductor). Combining those two voltage terms results in: RILIM = (VBOT + VRMPEAK)/ 10µA (8) RILIM = {0.96 + (ILOAD * RDSON *KT*8)} + {D*(VIN – 1.8)/(fSW*0.03*RRAMP)}/10µA (9) where: VBOT = 0.96 + (ILOAD * RDSON *KT*8); VRMPEAK = D*(VIN – 1.8)/(fSW*0.03*RRAMP); ILOAD = the desired maximum load current; RDSON = the nominal RDSON of the low-side MOSFET; KT = the normalized temperature coefficient for the low-side MOSFET (on datasheet graph); D = VOUT/VIN duty cycle; fSW = Clock frequency in kHz; and RRAMP = chosen ramp resistor value in k Ω. After 16 consecutive, pulse-by-pulse, current-limit cycles, the fault latch is set and the regulator shuts down. Cycling VCC or EN restores operation after a normal soft-start cycle (refer to the Auto-Restart section) . The over-current protection fault latch is active during the soft-start cycle. Use 1% resistor for RILIM. Loop Compensation The loop is compensated using a feedback network around the error amplifier. Figure 23 shows a complete type-3 compensation network. For type-2 compensation, eliminate R3 and C3. Figure 23. Compensation Network Since the FAN2108 employs summing current-mode architecture, type-2 compensation can be used for many applications. For applications that require wide loop bandwidth and/or use very low-ESR output capacitors, type-3 compensation may be required. Protection + _ VCC 10µA ILIMIT ILIM RILIM + _ ILIMIT ADJUST MAX + _ COMP PWM VERR PWM ILIMTRIP VILIM RAMP |
Similar Part No. - FAN2108EMPX_12 |
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Similar Description - FAN2108EMPX_12 |
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