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FAN5354MPX Fiches technique(PDF) 11 Page - Fairchild Semiconductor |
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FAN5354MPX Fiches technique(HTML) 11 Page - Fairchild Semiconductor |
11 / 14 page © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5354 • Rev. 1.0.4 11 the high-side switch turns off, preventing high currents from causing damage. 16 consecutive PWM cycles in current limit cause the regulator to shut down and stay off for about 1200 μs before attempting a restart. In the event of a short circuit, the soft-start circuit attempts to restart and produces an over-current fault after about 50 μs, which results in a duty cycle of less than 10%, providing current into a short circuit. Thermal Shutdown When the die temperature increases, due to a high load condition and/or a high ambient temperature, the output switching is disabled until the temperature on the die has fallen sufficiently. The junction temperature at which the thermal shutdown activates is nominally 150°C with a 20°C hysteresis. Minimum Off-Time Effect on Switching Frequency tON(MIN) and tOFF(MIN) are both 45ns. This imposes constraints on the maximum VIN VOUT that the FAN5354 can provide, while still maintaining a fixed switching frequency in PWM mode. While regulation is unaffected, the switching frequency will drop when the regulator cannot provide sufficient duty cycle at 3 MHz to maintain regulation. The calculation for switching frequency is given below ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ = ns 3 . 333 1 , t 1 min f ) MAX ( SW SW where ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − • − • + + • = OUT ON OUT IN OFF OUT OUT ) MAX ( SW V R I V R I V 1 ns 45 t OFF R = L N _ DSON DCR R + ON R = L P _ DSON DCR R + (4) Application Information Selecting the Inductor The output inductor must meet both the required inductance and the energy handling capability of the application. The inductor value affects the average current limit, the output voltage ripple, and the efficiency. The ripple current (∆I) of the regulator is: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ • − • ≈ Δ SW OUT IN IN OUT f L V V V V I (5) The maximum average load current, IMAX(LOAD) is related to the peak current limit, ILIM(PK), by the ripple current as: 2 I I I ) PK ( LIM ) LOAD ( MAX Δ − = (6) The FAN5354 is optimized for operation with L=470nH, but is stable with inductances up to 1.2 μH (nominal). The inductor should be rated to maintain at least 80% of its value at ILIM(PK). Failure to do so lowers the amount of DC current the IC can deliver. Efficiency is affected by the inductor DCR and inductance value. Decreasing the inductor value for a given physical size typically decreases the DCR; but since ∆I increases, the RMS current increases, as do core and skin-effect losses. 12 I I I 2 2 ) DC ( OUT RMS Δ + = (7) The increased RMS current produces higher losses through the RDS(ON) of the IC MOSFETs as well as the inductor ESR. Increasing the inductor value produces lower RMS currents, but degrades transient response. For a given physical inductor size, increased inductance usually results in an inductor with lower saturation current. Table 2 shows the effects of inductance higher or lower than the recommended 470nH on regulator performance. Table 2. Effects of Increasing the Inductor Value (from 470nH Recommended) on Regulator Performance IMAX(LOAD) ∆VOUT (EQ. 8) Transient Response Increase Decrease Degraded Inductor Current Rating The FAN5354’s current limit circuit can allow a peak current of 5.5A to flow through L1 under worst-case conditions. If it is possible for the load to draw that much continuous current, the inductor should be capable of sustaining that current or failing in a safe manner. For space-constrained applications, a lower current rating for L1 can be used. The FAN5354 may still protect these inductors in the event of a short circuit, but may not be able to protect the inductor from failure if the load is able to draw higher currents than the DC rating of the inductor. Output Capacitor and VOUT Ripple Note: Table 1 suggests 0805 capacitors, but 0603 capacitors may be used if space is at a premium. Due to voltage effects, the 0603 capacitors have a lower in-circuit capacitance than the 0805 package, which can degrade transient response and output ripple. Increasing COUT has no effect on loop stability and can therefore be increased to reduce output voltage ripple or to improve transient response. Output voltage ripple, ∆VOUT, is: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + • • • Δ = Δ ESR f C 8 1 I V SW OUT OUT (8) where COUT is the effective output capacitance. The capacitance of COUT decreases at higher output voltages, which results in higher ∆VOUT . |
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