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LA5774MP Folha de dados(PDF) 4 Page - ON Semiconductor |
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LA5774MP Folha de dados(HTML) 4 Page - ON Semiconductor |
4 / 5 page LA5774MP No.A0490-4/5 1.5V 0.62V PWM wave Description of Functional Settings 1. Calculation equation to set the output voltage This IC controls the switching output so that the VOS pin voltage becomes 1.26V (typ). The equation to set the output voltage is as follows: The VOS pin has the inrush current of 1 μA (typ). Therefore, the error becomes larger when R1 and R2 resistance values are large. 2. Start delay function The SS pin has the internally-connected 22 μA (typ) constant-current supply. When the voltage of SS pin exceeds the threshold voltage, the regulator starts operation. As the threshold voltage is 0.62V (typ), the start delay time can be calculated as follows: ex. For setting at 1 μF 3. Soft start function The internal PWM waveform has the voltage value as shown in the right. If down-conversion from the voltage of VIN = 15 V to VIN = 3.3V is to be made, for example, the PWM-ON duty has the value as shown below. (Note that calculation is made with Vsat = 1V and VF = 0.2V) The output voltage of error amplifier, which is 3.3 V, is the value with PWM = 23%, as calculated in the above equation, so that this voltage is determined as follows: Ver = ( ΔVPWM) × PWMduty + VPWML = 0.88V × 0.23 + 0.62V = 0.82V ( ΔVPWM is the PWM amplitude value or 0.88V(typ) while VPWML is the lower limit voltage of PWM waveform or 0.62V(typ)) SS pin and error amplifier output voltages are designed to prefer the lower voltages, so that VOUT will reach the designed regulation voltage in timing when the SS pin voltage exceeds the error amplifier output. Therefore, the soft start time is calculated as follows: For the set conditions of C = 1 μF and PWMduty = 23%: VO = 1+ × 1.26V(typ) R2 R1 PWMduty = = 23 % VOUT VIN − Vsat + VF Tss = = C × ΔVPWM × PWMduty i C × 0.88 × PWMduty 22 μA Td = = = 28.2 ms C × V i 1 μF × 0.62 22 μA Tss = = 9.2ms 1 μF × 0.88V × 0.23 22 μA |
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