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ML4812IP Datasheet(PDF) 9 Page - Micro Linear Corporation |
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ML4812IP Datasheet(HTML) 9 Page - Micro Linear Corporation |
9 / 16 page ML4812 9 CURRENT SENSE AND SLOPE (RAMP) COMPENSATION COMPONENT SELECTION Slope compensation in the ML4812 is provided internally. Rather than adding slope to the noninverting input of the PWM comparator, it is actually subtracted from the voltage present at the inverting input of the PWM comparator. The amount of slope compensation should be at least 50% of the downslope of the inductor current during the off time, as reflected to the inverting input of the PWM comparator. Note that slope compensation is required only when the inductor current is continuous and the duty cycle is more than 50%. The downslope of the inductor current at the verge of discontinuity can be found using the expression given below: di dt VV L VV mH As L OUT INDRY = - = - =m 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 OUT INDRY S C = - ´ (10) S V mH Vs PWM = - ´= m 380 20 2 100 80 0 225 ./ Where RS is the current sense resistor and 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. Sense FETs or resistive sensing can also be used to sense the switch current. The sensed signal has to be amplified to the proper level before it is applied to the ML4812. The value of the ramp compensation (SCPWM) as seen at the inverting terminal of the PWM comparator is: SC R RC R PWM M TT SC = ´ ´´ 25 . (11) The required value for RSC can therefore be found by equating: SCPWM = ASC × SPWM, where ASC is the amount of slope compensation and solving for RSC. The value of GM OUT depends on the selection of RAMP COMP. R V ImA k P IN PEAK SINE PEAK == ´ = (max) . . () 260 1414 05 750 Ω (12) R VR V k k M CLAMP P IN PEAK = ´ = ´ ´ = () . . . 49 750 90 1414 288 Ω Ω (13) The peak of the inductor current can be found approximately by: I P V A LPEAK OUT IN RMS = ´ = ´ = 1414 1414 200 90 314 . . . () (14) Selection of NC which depends on the maximum switch current, assume 4A for this example is 80 turns. R VN I S CLAMP C LPEAK = ´ = ´ =W 49 80 4 100 . (15) Where RS is the sense resistor, and VCLAMP is the current clamp at the inverting input of the PWM comparator. This clamp is internally set to 5V. In actual application it is a good idea to assume a value less than 5V to avoid unwanted current limiting action due to component tolerances. In this application, VCLAMP was chosen as 4.9V. Having calculated RS, the value SPWM and of RSC can now be calculated: R R AS R C R k KnF k SC M SC PWM T T SC = ´ ´´ ´ = ´ ´´ ´ ´ = 25 25 288 07 0 225 10 14 1 33 6 . .. .( . ) Ω Ω (16) The following values were used in the calculation: RM = 28.8kΩ ASC = 0.7 RT = 14kΩ CT = 1nF VOLTAGE REGULATION COMPONENTS The values of the voltage regulation loop components are calculated based on the operating output voltage. Note that voltage safety regulations require the use of sense resistors that have adequate voltage rating. As a rule of thumb if 1/4W resistors are chosen, two of them should be used in series. The input bias current of the error amplifier is approximately 0.5µA, therefore the current available from the voltage sense resistors should be significantly higher than this value. Since two 1/4W resistors have to be used the total power rating is 1/2W. The operating power is set to be 0.4W then with 380V output voltage the value can be calculated as follows: RV W k 1 2 380 0 4 360 == () / . Ω (17) Choose two 178k Ω, 1% connected in series. Then R2 can be calculated using the formula below: R VR VV Vk VV k REF OUT REF 2 1 5356 380 5 4747 = ´ - = ´ - = Ω Ω . (18) TYPICAL APPLICATIONS (Continued) |
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