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FAN5234MTC Datasheet(PDF) 10 Page - Fairchild Semiconductor |
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FAN5234MTC Datasheet(HTML) 10 Page - Fairchild Semiconductor |
10 / 15 page PRODUCT SPECIFICATION FAN5234 10 REV. 1.0.10 5/3/04 Setting the Output Voltage The internal reference is 0.9V. The output is divided down by a voltage divider to the VSEN pin (for example, R1 and R2 in Figure 1). The output voltage therefore is: To minimize noise pickup on this node, keep the resistor to GND (R2) below 2K. We selected R2 at 1.82K. Then choose R5: Output Inductor Selection The minimum practical output inductor value is the one that keeps inductor current just on the boundary of continuous conduction at some minimum load. The industry standard practice is to choose the ripple current to be somewhere from 15% to 35% of the nominal current. At light load, the ripple current also determines the point where the converter will automatically switch to hysteretic mode of operation (IMIN) to sustain high efficiency. The following equations help to choose the proper value of the output filter inductor. where ∆I is the inductor ripple current, which we will choose for 20% of the full load current and ∆V OUT is the maximum output ripple voltage allowed. for this example we'll use: VIN = 20V, VOUT = 1.8V ∆I = 20% * 3.5A = 0.7A FSW = 300KHz. therefore L ≈ 8µH Output Capacitor Selection The output capacitor serves two major functions in a switch- ing power supply. Along with the inductor it filters the sequence of pulses produced by the switcher, and it supplies the load transient currents. The output capacitor require- ments are usually dictated by ESR, Inductor ripple current ( ∆I) and the allowable ripple voltage (∆V). For our example, In addition, the capacitor's ESR must be low enough to allow the converter to stay in regulation during a load step. The rip- ple voltage due to ESR for the converter in Figure 1 is 100mV P-P. Some additional ripple will appear due to the capacitance value itself: which is only about 1.5mV for the converter in Figure 1 and can be ignored. The capacitor must also be rated to withstand the RMS cur- rent which is approximately 0.3 X ( ∆I), or about 210mA for our example. High frequency decoupling capacitors should be placed as close to the loads as physically possible. Input Capacitor Selection The input capacitor should be selected by its ripple current rating. The input RMS current at maximum load current (IL) is: where the converter duty cycle; , which for the circuit in Figure 1, with VIN=6 calculates to: Power MOSFET Selection Losses in a MOSFET are the sum of its switching (P SW) and conduction (P COND ) losses. In typical applications, the FAN5234 converter's output volt- age is low with respect to its input voltage, therefore the Lower MOSFET (Q2) is conducting the full load current for most of the cycle. Q2 should be therefore be selected to min- imize conduction losses, thereby selecting a MOSFET with low RDS(ON). In contrast, the high-side MOSFET (Q1) has a much shorter duty cycle, and it's conduction loss will therefore have less of an impact. Q1, however, sees most of the switching losses, so Q1's primary selection criteria should be gate charge. High-Side Losses: Figure 8 shows a MOSFET's switching interval, with the upper graph being the voltage and current on the Drain to Source and the lower graph detailing VGS vs. time with a constant current charging the gate. The x-axis therefore is also representative of gate charge (QG) . CISS = CGD + CGS, and it controls t1, t2, and t4 timing. CGD receives the current from the gate driver during t3 (as VDS is falling). The gate charge (QG) parameters on the lower graph are either speci- fied or can be derived from MOSFET datasheets. 0.9V R2 ------------ V OUT 0.9V – R1 --------------------------------- = (9a) R5 1.82K () 1.8V 0.9 – () • 0.9 -------------------------------------------------------- 1.82K == (9b) ∆I2 I MIN – ∆V OUT ESR ------------------ == (10) L V IN V OUT – F SW ∆I × ------------------------------ V OUT V IN -------------- × = (11) ESR ∆V ∆I -------- < (12) ESR MAX () ∆V ∆I -------- 0.1V 0.7A ------------ 142m Ω == = ∆V ∆I C OUT 8 × F SW × ----------------------------------------- = (13) I RMS I L DD 2 – = (14) D V OUT V IN -------------- = I RMS 1.6A = |
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