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ML4804CS Scheda tecnica(PDF) 11 Page - Micro Linear Corporation |
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ML4804CS Scheda tecnica(HTML) 11 Page - Micro Linear Corporation |
11 / 14 page ML4804 11 No voltage error amplifier is included in the PWM stage of the ML4804, 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 DC ILIMIT pin is a direct input to the cycle-by-cycle current limiter for the PWM section. Should the input voltage at this pin ever exceed 1V, 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 2.45V. Once this voltage reaches 2.45V, which corresponds to the PFC output capacitor being charged to its rated boost voltage, the soft-start begins. PWM Control (RAMP 2) When the PWM section is used in current mode, RAMP 2 is generally used as the sampling point for a voltage representing the current in the primary of the PWM’s output transformer, derived either by a current sensing resistor or a current transformer. In voltage mode, it is the input for a ramp voltage generated by a second set of timing components (RRAMP2, CRAMP2), that will have a minimum value of zero volts and should have a peak value of approximately 5V. In voltage mode operation, feedforward from the PFC output buss is an excellent way to derive the timing ramp for the PWM stage. 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 . (6a) Generating VCC The ML4804 is a voltage-fed part. It requires an external 15V, ±10% (or better) shunt voltage regulator, or some other VCC regulator, to regulate the voltage supplied to the part at 15V nominal. This allows low power dissipation while at the same time delivering 13V nominal gate drive at the PWM OUT and PFC OUT outputs. If using a Zener Figure 4. Typical Trailing Edge Control Scheme FUNCTIONAL DESCRIPTION (Continued) RAMP VEAO TIME VSW1 TIME REF EA – + – + OSC DFF R D Q Q CLK U1 RAMP CLK U4 U3 C1 RL I4 SW2 SW1 + DC I1 I2 I3 VIN L1 U2 |
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