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TOP248Y Datasheet(PDF) 6 Page - Power Integrations, Inc. |
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TOP248Y Datasheet(HTML) 6 Page - Power Integrations, Inc. |
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6 / 52 page  TOP242-250 6 G 1/02 PI-2545-082299 S1 S2 S6 S7 S1 S2 S6 S7 S0 S1 S7 S0 S0 5.8 V 4.8 V S7 0 V 0 V 0 V V LINE V C V DRAIN V OUT Note: S0 through S7 are the output states of the auto-restart counter 2 1 2 3 4 0 V S6 S7 V UV S2 CONTROL (C) Pin Operation The CONTROL pin is a low impedance node that is capable of receiving a combined supply and feedback current. During normal operation, a shunt regulator is used to separate the feed- back signal from the supply current. CONTROL pin voltage V C is the supply voltage for the control circuitry including the MOSFET gate driver. An external bypass capacitor closely connected between the CONTROL and SOURCE pins is required to supply the instantaneous gate drive current. The total amount of capacitance connected to this pin also sets the auto-restart timing as well as control loop compensation. When rectified DC high voltage is applied to the DRAIN pin during start-up, the MOSFET is initially off, and the CONTROL pin capacitor is charged through a switched high voltage current source connected internally between the DRAIN and CONTROL pins. When the CONTROL pin voltage V C reaches approximately 5.8 V, the control circuitry is activated and the soft-start begins. The soft-start circuit gradually increases the duty cycle of the MOSFET from zero to the maxi- mum value over approximately 10 ms. If no external feedback/ supply current is fed into the CONTROL pin by the end of the soft-start, the high voltage current source is turned off and the CONTROL pin will start discharging in response to the supply current drawn by the control circuitry. If the power supply is designed properly, and no fault condition such as open loop or shorted output exists, the feedback loop will close, providing external CONTROL pin current, before the CONTROL pin voltage has had a chance to discharge to the lower threshold voltage of approximately 4.8 V (internal supply under-voltage lockout threshold). When the externally fed current charges the CONTROL pin to the shunt regulator voltage of 5.8 V, cur- rent in excess of the consumption of the chip is shunted to SOURCE through resistor R E as shown in Figure 2. This current flowing through R E controls the duty cycle of the power MOSFET to provide closed loop regulation. The shunt regulator has a finite low output impedance Z C that sets the gain of the error amplifier when used in a primary feedback configuration. The dynamic impedance Z C of the CONTROL pin together with the external CONTROL pin capacitance sets the dominant pole for the control loop. When a fault condition such as an open loop or shorted output prevents the flow of an external current into the CONTROL pin, the capacitor on the CONTROL pin discharges towards 4.8 V. At 4.8 V, auto-restart is activated which turns the output MOSFET off and puts the control circuitry in a low current standby mode. The high-voltage current source turns on and charges the external capacitance again. A hysteretic internal supply under-voltage comparator keeps V C within a window of typically 4.8 V to 5.8 V by turning the high-voltage current source on and off as shown in Figure 8. The auto-restart circuit has a divide-by-eight counter which prevents the out- put MOSFET from turning on again until eight discharge/charge cycles have elapsed. This is accomplished by enabling the output MOSFET only when the divide-by-eight counter reaches full count (S7). The counter effectively limits TOPSwitch-GX power dissipation by reducing the auto-restart duty cycle to typically 4%. Auto-restart mode continues until output voltage regulation is again achieved through closure of the feedback loop. Oscillator and Switching Frequency The internal oscillator linearly charges and discharges an internal capacitance between two voltage levels to create a sawtooth waveform for the pulse width modulator. This Figure 8. Typical Waveforms for (1) Power Up (2) Normal Operation (3) Auto-restart (4) Power Down. |
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