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MIC3230 Datasheet(PDF) 10 Page - Micrel Semiconductor |
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MIC3230 Datasheet(HTML) 10 Page - Micrel Semiconductor |
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10 / 20 page  Micrel, Inc. MIC3230/1/2 January 2009 10 M9999-011409-A Dithering (MIC3231 Only) The MIC3231 has a feature which dithers the switching frequency by ±12%. The purpose of this dithering is to help achieve a spread spectrum of the conducted EMI noise. This can allow for an overall reduction in noise emission by approximately 10dB. Internal Gate Driver External FETs are driven by the MIC323x’s internal low impedance gate drivers. These drivers are biased from the VDD and have a source resistance of 2Ω and a sink resistance of 3.5Ω. VDD VDD is an internal linear regulator powered by VIN and VDD is the bias supply for the internal circuitry of the MIC323x. A 10µF ceramic bypass capacitor is required at the VDD pin for proper operation. This pin is for filtering only and should not be utilized for operation. Current Limit The MIC323x family features a current limit protection feature to prevent any current runaway conditions. The current limit circuitry monitors current on a pulse by pulse basis. It limits the current through the inductor by sensing the voltage across RCS. When 0.45V is present at the IS pin, the pulse is truncated. The next pulse continues as normally until the IS pin reaches 0.45V and it is truncated once again. This will continue until the output load is decreased. Select RCS using Equation 5: Eq. (5) () LIMIT PK MIN MAX L SW IN OUT CS I F L D V V R _ 45 . 0 + × × − = Slope Compensation The MIC323x is a peak current mode controller and requires slope compensation. Slope compensation is required to maintain internal stability across all duty cycles and prevent any unstable oscillations. The MIC323x uses slope compensation that is set by an external resistor, RSLC. The ability to set the proper slope compensation through the use of a single external component results in design flexibility. This slope compensation resistor, RSLC, can be calculated using Equation 6: Eq. (6) ( ) SW CS IN OUT SLC F A L R V V R MIN MAX × × × − = μ 250 where VIN_MAX and VOUT_MAX can be selected to system specifications. Current Sense IS The IS pin monitors the rising slope of the inductor current (m1 in Figure 5) and also sources a ramp current (250µA/T) that flows through RSLC that is used for slope compensation. This ramp of 250µA per period, T, generates a ramped voltage across RSLC and is labeled VA in Figure 3. The signal at the IS pin is the sum of VCS + VA (as shown in Figure 3). The current sense circuitry and block diagram is displayed in Figure 4. The IS pin is also used as the current limit (see the previous section on Current Limit). Figure 3. Slope compensation waveforms Soft Start The boost switching convertor features a soft start in order to power up in a controlled manner, thereby limiting the inrush current from the line supply. Without this soft start, the inrush current could be too high for the supply. To prevent this, a soft start delay can be set using the compensation capacitor (CCOMP in Figure 1). For switching to begin, the voltage on the compensation cap must reach about 0.7V. Switching starts with the minimum duty cycle and increases to the final duty cycle. As the duty cycle increases, VOUT will increase from VIN to it’s final value. A 6µA current source charges the compensation capacitor and the soft start time can be calculated in Equation 7: Eq. (7) μA V C T Y_STATE COMP_STEAD COMP SOFTSTART 6 × ≈ VCOMP_STEADY_STATE is usually between 0.7V to 3V, but can be as high as 5V. Eq. (8) ( ) PK A STATE STEADY COMP Vcs V Ai V PK + × = _ _ Where: T D R T I V SLC RAMP APK × × × = and CS PK L CS R I V PK × = _ Ai = 1.4 V/V D = Duty cycle (0 to1) T = period A 10nF ceramic capacitor will make this system stable at all operating conditions. |
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Similar Description - MIC3230 |
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