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ML4801CP Folha de dados(PDF) 9 Page - Micro Linear Corporation |
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ML4801CP Folha de dados(HTML) 9 Page - Micro Linear Corporation |
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9 / 14 page  ML4801 9 operating frequency can typically be approximated by: f t OSC RAMP = 1 (5) EXAMPLE: For the application circuit shown in the data sheet, with the oscillator running at: fkHz t OSC RAMP == 100 1 tR C RAMP T T =´ ´ = ´ - 051 1 10 5 . Solving for RT x CT yields 2 x 10-4. Selecting standard components values, CT = 270pF, and RT = 36.5kΩ. PWM SECTION The PWM section of the ML4801 is straightforward, but there are several points which should be noted. Foremost among these is its inherent synchronization to the PFC section of the device, and that the PWM stage is optimized for current-mode operation. In the ML4801, the operating frequency of the PFC section is fixed at 1/2 of the PWM's operating frequency. This is done through the use of a 2:1 digital frequency divider ("T" flip-flop) linking the two functional sections of the IC. No voltage error amplifier is included in the PWM stage of the ML4801, as this function is generally performed on the output side of the PWM’s isolation boundary. To facilitate the design of optocoupler feedback circuitry, an offset has been built into the PWM’s RAMP 2 input which allows VDC to command a zero percent duty cycle for input voltages below 1.25V. PWM Current Limit The RAMP 2 pin provides a direct input to the cycle-by- cycle current limiter for the PWM section. Should the input voltage at this pin ever exceed 1.5V, the output of the PWM will be disabled until the output flip-flop is reset by the clock pulse at the start of the next PWM power cycle. VIN OK Comparator The VIN OK comparator monitors the DC output of the PFC and inhibits the PWM if this voltage on VFB is less than its nominal 2.5V. Once this voltage reaches 2.5V, which corresponds to the PFC output capacitor being charged to its rated boost voltage, the soft-start commences. PWM Control (RAMP 2) In addition to its PWM current limit function, RAMP 2 is used as the sampling point for a voltage representing the current in the primary of the PWM’s output transformer. This voltage may be derived either by a current sensing resistor or a current transformer. Soft Start Start-up of the PWM is controlled by the selection of the external capacitor at SS. A current source of 25µA supplies the charging current for the capacitor, and start- up of the PWM begins at 1.25V. Start-up delay can be programmed by the following equation: Ct A V SS DELAY =× 25 125 µ . (6) where CSS is the required soft start capacitance, and tDELAY is the desired start-up delay. It is important that the time constant of the PWM soft-start allow the PFC time to generate sufficient output power for the PWM section. The PWM start-up delay should be at least 5ms. Solving for the minimum value of CSS: Cms A V nF SS =× = 5 25 125 100 µ . Generating VCC The ML4801 is a voltage-fed part. It requires an external 15V±10% or better Zener shunt voltage regulator, or some other VCC regulator, to maintain the voltage supplied to the part at 15V nominal. This allows a low power dissipation while at the same time delivering 13V nominal of gate drive at the PWM OUT and PFC OUT outputs. If using a Zener diode, it is important to limit the current through the Zener to avoid overheating or destroying it. This can be easily done with a single resistor in series with the Vcc pin, returned to a bias supply of typically 18V to 20V. The resistor’s value must be chosen to meet the operating current requirement of the ML4801 itself (8.5mA max.) plus the current required by the two gate driver outputs. EXAMPLE: With a VBIAS of 20V, a VCC limit of 16.5V (max) and driving a total gate charge of 110nC at 100kHz (1 IRF840 MOSFET and 2 IRF830 MOSFETs), the gate driver current required is: IkHz nC mA GATEDRIVE =´ = 100 110 11 R VV mA mA BIAS = - + = 20 16 5 75 11 180 . . Ω The ML4801 should be locally bypassed with a 10nF and a 1 µF ceramic capacitor. In most applications, an electrolytic capacitor of between 33µF and 100µF is also required across the part, both for filtering and as part of the start-up bootstrap circuitry. FUNCTIONAL DESCRIPTION (Continued) |
Nº de peça semelhante - ML4801CP |
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Descrição semelhante - ML4801CP |
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