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ML4826IS-1 Datasheet(PDF) 11 Page - Micro Linear Corporation |
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ML4826IS-1 Datasheet(HTML) 11 Page - Micro Linear Corporation |
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11 / 16 page  ML4826 11 Volts peak. The composite waveform feeding the RAMP2 pin for the PWM consists of the reflected output current signal along with the transformer magnetizing current and the slope compensation signal. Equation 8 describes the composite signal feeding RAMP2, consisting of the primary current of the main transformer and the slope compensation. Equation 9 solves for the required slope compensation peak voltage. VI V L N N T n VV RAMP PRI OUT S P S CT FB 2 1 2 1 15 =+ × × × ×≤ –. (8) V V L N N T R n V H V SC OUT S P S SENSE CT =× × × ×= × × × = 1 2 1 2 48 20 14 90 5 471 200 22 µ µsec . Ω (9) Soft Start Start-up of the PWM is controlled by the selection of the external capacitor at SS. A current source of 50 µA supplies the charging current for the capacitor, and start-up of the PWM begins at 1.5V. Start-up delay can be programmed by the following equation: Ct A V SS DELAY =× 50 15 µ . (10) 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 50 15 167 µ . (11) VCC The ML4826 is a current-fed part. It has an internal shunt voltage regulator, which is designed to regulate the voltage internal to the part at 13.5V. This allows a low power dissipation while at the same time delivering 10V of gate drive at the PWM OUT and PFC OUT outputs. It is important to limit the current through the part to avoid overheating or destroying the part. There are a number of different ways to supply VCC to the ML2826. The method suggested in Figure 5, is one which keeps the ML4826 ICC current to a minimum, and allows for a loosely regulated bootstrap winding. By feeding external gate drive components from the base of Q1, the constant current source does not have to account for variations in the gate drive current. This helps to keep the maximum ICC of the ML4826 to a minimum. Also, the current available to charge the bootstrap capacitor from the bootstrap winding is not limited by the constant – + – + VCC VREF PWM CMP DC ILIMIT 1V 1.5V RTCT R40 47.0k Ω R38 10.0k Ω R13 2.2k Ω RAMP2 AGND DC ILIMIT VDC U2 R21 8.63k Ω Q14 2N2222 D1 R16 471 Ω C11 1000pF C26 220pF T3 200:1 ISENSE x Former 4 x IN4148 6 10 11 9 7 18 17 Figure 4. Slope Compensation and Current Sense FUNCTIONAL DESCRIPTION (Continued) |
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