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IRFBC40 데이터시트(PDF) 3 Page - Texas Instruments |
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IRFBC40 데이터시트(HTML) 3 Page - Texas Instruments |
3 / 20 page TIL5942, TIL5942A CURRENT-MODE-PWM CONTROLLER WITH OPTOISOLATED VOLTAGE REFERENCE AND ERROR AMPLIFIER SOES040 – OCTOBER 1997 3 POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 theory of operation The current through the LED is controlled by the precision reference amplifier according to the voltage error at FB, with respect to the internal 2.5-V reference. The light emitted by the LED is coupled to the PWM controller to change the modulation duty cycle and reduce the error voltage. The PWM controller is in the off state until VCC is greater than the turn-on threshold voltage level. Typically the controller turns on at 16 V. It stays in the on state until the voltage drops below the turn-off threshold of 10 V typical. Upon turning on, the controller generates an internal reference voltage Vref of 5 V, which controls the chip logic and can be used for low-power external applications. A voltage Vreg is also generated to supply current to charge an external capacitor Creg, which supplies instant current for the pulsing output. The oscillator shown in Figure 1 consists of an external resistance-capacitance (RC) network, a voltage comparator, logic, a 125-ns one-shot for the dead time, and a MOSFET used to discharge the external capacitor to ground each cycle. At the end of the reset period, CT charges toward the 5-V reference through timing resistor RT for a time given by one time constant RTCT. The oscillator period is given by: TOSC = RTCT + 0.125 µs. A current IIB(IS) supplied out of IS biases the current-sense resistor on the primary side of the power supply. If the voltage V(IS) at IS exceeds 1 V, the output pulse is reset. An internal current source I(VC) supplies a nominal current of –4 µA as a reference. This current charges a capacitor connected between VC and ground. The photodiode is connected internally in parallel with the capacitor. The light-generated photodiode current tends to discharge the capacitor until a steady-state balance is reached. When I(VC) is greater than the photodiode current, a net current is being sourced and the capacitor is charged. When I(VC) is lower than the photodiode current, a net current is being sinked and the capacitor is discharged. When I(VC) is equal to the photodiode current, the capacitor voltage stays constant. The voltage at VC is monitored by an internal comparator with a threshold voltage given by V(IS) + 2Vd, where Vd is a diode voltage drop of typically 0.7 V. Whenever VC is below this threshold, the output pulse is reset. The output pulse is enabled if V(IS) is less than 1 V and VC is greater than its threshold. The 5-V reference on the PWM chip is brought out on REF and can be used as a reference and/or supply for external circuits as long as the output current is limited to 20 mA and the power dissipation is not exceeded. It is recommended that a 0.1-F ceramic capacitor be connected between REF and PWM_GND. RT (see Note A) CT REF RT/CT PWM_GND NOTE A: tosc = RTCT + 0.125 µs Figure 1. Typical Oscillator Application |
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