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Datenblatt-Suchmaschine für elektronische Bauteile |
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Datenblatt-Suchmaschine für elektronische Bauteile |
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FAN2103EMPX Datenblatt(PDF) 11 Page - Fairchild Semiconductor |
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FAN2103EMPX Datenblatt(HTML) 11 Page - Fairchild Semiconductor |
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11 / 14 page  © 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN2103 Rev. 1.0.5 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 L D) - (1 VOUT • • = Δ L I (5) where f is the oscillator frequency and: f D) - (1 VOUT • Δ • = L I L (6) Setting the Ramp Resistor Value The internal ramp voltage excursion (ΔVRAMP) during tON should be set to 0.6V. RRAMP is approximately: 2 f V 10 x 18 V ) 8 . 1 V ( R IN 6 OUT IN ) K ( RAMP − • • • − = − Ω (7) where frequency (f) is expressed in KHz. Setting the Current Limit There are two levels of current-limit thresholds in FAN2103. The first level of protection is through an internal default limit set at the factory to limit output current beyond normal usage levels. The second level of protection is a flexible one to be set externally by the user. Current-limit protection is enabled whenever the lower of the two thresholds is reached. The FAN2103 uses its internal low-side MOSFET for current-sensing. The current-limit threshold voltage (VILIM) is compared to the voltage drop across the low-side MOSFET, sampled at the end of each PWM off-time/cycle. The internal default threshold (with ILIM open) is temperature compensated. The 10µA current sourced from the ILIM pin can be used to establish a lower, temperature–dependent, current-limit threshold by connecting an external resistor (RILIM) to AGND: 5 . 142 ) I I ( K 4 . 10 L OUT T + Δ − • • = 2 RILIM(KΩ) (8) where: IOUT = desired current limit set point in Amps, KT = the normalized temperature coefficient of the low-side MOSFET (Q2) from Figure 8. 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 Auto-Restart section). The over-current protection fault latch is active during the soft-start cycle. Use a 1% resistor for RILIM. Loop Compensation The loop is compensated using a feedback network around the error amplifier. Figure 22 shows a complete Type-3 compensation network. Type-2 compensation eliminates R3 and C3. Figure 22. Compensation Network Because the FAN2103 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. RRAMP provides feedforward compensation for changes in VIN. With a fixed RRAMP value, the modulator gain increases as VIN is reduced, which could make it difficult to compensate the loop. For designs with low input voltages (3V to 6.5V), it is recommended that a separate RRAMP and the compensation component values are used as compared to designs with VIN between 6.5V and 24V. Protection The converter output is monitored and protected against extreme overload, short-circuit, over-voltage, and under-voltage conditions. An internal “Fault Latch” is set for any fault intended to shut down the IC. When the fault latch is set, the IC discharges VOUT by enhancing the low-side MOSFET until FB<0.25V. The MOSFET is not turned on again unless FB>0.5V. This behavior discharges the output without causing undershoot (negative output voltage). GATE DRIVE PWM 0.25/0.5V FAULT PWM LATCH FB Figure 23. Latched Fault Response |
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