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ML4819CP 데이터시트(PDF) 10 Page - Fairchild Semiconductor |
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ML4819CP 데이터시트(HTML) 10 Page - Fairchild Semiconductor |
10 / 15 page ML4819 10 REV. 1.0 10/10/2000 then: ILDRY = 100mA Step 3: The value of the inductance can now be found using previously calculated data. L VD If V mA kHz mH INDRY ON MAX LDRY OSC 1 20 0 95 100 100 2 = × × = × × = () () . (5) The inductor can be allowed to decrease in value when the current sweeps from minimum to maximum value. This allows the use of smaller core sizes. The only requirement is that the ramp compensation must be adequate for the lower inductance value of the core so that there is adequate compensation at high current. Step 4: The presence of the ramp compensation will change the dry out point, but the value found above can be considered a good starting point. Based on the amount of power factor correction the value of L1 can be optimized after a few iterations. Gapped Ferrites, Molypermalloy, and Powdered Iron cores are typical choices for core material. The core material selected should have a high saturation point and acceptable losses at the operating frequency. One ferrite core that is suitable at around 200W is the #4229PL00-3C8 made by Ferroxcube. This ungapped core will require a total gap of 0.180" for this application. OSCILLATOR COMPONENT SELECTION The oscillator timing components can be calculated by using the following expression: f RC OSC TT = × 136 . (6) For example: Step 1: At 100kHz with 95% duty cycle TOFF = 500ns calculate CT using the following formula: C tI V pF T OFF DIS OSC = × = 1000 (7) Step 2: Calculate the required value of the timing resistor. R f C kHz pF k Choose R k T OSC T T = × = × =Ω = Ω 136 1 36 100 1000 13 6 14 .. . . . (8) CURRENT SENSE AND SLOPE (RAMP) COMPENSATION COMPONENT SELECTION Slope compensation in the ML4819 is provided internally. A current equal to VCT/2(R18) is added to ISENSE A (pin 1). this is converted to a voltage by R10, adding slope to the sensed current through T1. The amount of slope compensation should be at least 50% of the downslope of the inductor current during the off time as reflected on pin 1. Note that slope compensation is a requirement only if the inductor current is continuous and the duty cycle is more than 50%. The highest inductor downslope is found at the point of inductor discontinuity: di dt VV L VV mH As L BIN DRY = − = − =µ 380 20 2 018 ./ (9) The downslope as reflected to the input of the PWM comparator is given by: S VV L R N PWM BIN DRY C = − × 1 11 (10) Where NC is the turns ratio of the current transformer (T1) used. In general, current transformers simplify the sensing of switch currents, especially at high power levels where the use of sense resistors is complicated by the amount of power they have to dissipate. Normally the primary side of the transformer consists of a single turn and the secondary consists of several turns of either enameled magnet wire or insulated wire. The diameter of the ferrite core used in this example is 0.5" (SPANG/Magnetics F41206-TC). The rectifying diode at the output of the current transformer can be a 1N4148 for secondary currents up to 75mA average. Current-sensing MOSFETs or resistive sensing can also be used to sense the switch current. In these cases, the sensed signal has to be amplified to the proper level before it is applied to the ML4819. The value of the ramp compensation (SCPWM) as seen at pin 1 is: SC R RC R PWM = × ×× 25 9 16 6 18 . (11) The required value for R18 can therefore be found by equating: SC A S PWM SC PWM =× where ASC is the amount of slope compensation and solving for R18. |
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