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TOP221Y Datasheet(PDF) 4 Page - Power Integrations, Inc. |
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TOP221Y Datasheet(HTML) 4 Page - Power Integrations, Inc. |
4 / 20 page TOP221-227 C 12/97 4 Control Voltage Supply CONTROL pin voltage V C is the supply or bias voltage for the controller and driver circuitry. An external bypass capacitor closely connected between the CONTROL and SOURCE pins is required to supply the gate drive current. The total amount of capacitance connected to this pin (C T) also sets the auto- restart timing as well as control loop compensation. V C is regulated in either of two modes of operation. Hysteretic regulation is used for initial start-up and overload operation. Shunt regulation is used to separate the duty cycle error signal from the control circuit supply current. During start-up, CONTROL pin current is supplied from a high-voltage switched current source connected internally between the DRAIN and CONTROL pins. The current source provides sufficient current to supply the control circuitry as well as charge the total external capacitance (C T). The first time V C reaches the upper threshold, the high-voltage current source is turned off and the PWM modulator and output transistor are activated, as shown in Figure 5(a). During normal operation (when the output voltage is regulated) feedback control current supplies the V C supply current. The shunt regulator keeps V C at typically 5.7 V by shunting CONTROL pin feedback current exceeding the required DC supply current through the PWM error signal sense resistor R E. The low dynamic impedance of this pin (Z C) sets the gain of the error amplifier when used in a primary feedback configuration. The dynamic impedance of the CONTROL pin together with the external resistance and capacitance determines the control loop compensation of the power system. If the CONTROL pin external capacitance (C T) should discharge to the lower threshold, then the output MOSFET is turned off and the control circuit is placed in a low-current standby mode. The high-voltage current source turns on and charges the external capacitance again. Charging current is shown with a negative polarity and discharging current is shown with a positive polarity in Figure 6. The hysteretic auto-restart comparator keeps V C within a window of typically 4.7 to 5.7 V by turning the high-voltage current source on and off as shown in Figure 5(b). The auto-restart circuit has a divide-by-8 counter which prevents the output MOSFET from turning on again until eight discharge-charge cycles have elapsed. The counter effectively limits TOPSwitch power dissipation by reducing the auto-restart duty cycle to typically 5%. Auto- restart continues to cycle until output voltage regulation is again achieved. Bandgap Reference All critical TOPSwitch internal voltages are derived from a temperature-compensated bandgap reference. This reference is also used to generate a temperature-compensated current source which is trimmed to accurately set the oscillator frequency and MOSFET gate drive current. Oscillator The internal oscillator linearly charges and discharges the internal capacitance between two voltage levels to create a sawtooth waveform for the pulse width modulator. The oscillator sets the pulse width modulator/current limit latch at the beginning of each cycle. The nominal frequency of 100 kHz was chosen to minimize EMI and maximize efficiency in power supply applications. Trimming of the current reference improves oscillator frequency accuracy. Pulse Width Modulator The pulse width modulator implements a voltage-mode control loop by driving the output MOSFET with a duty cycle inversely proportional to the current into the CONTROL pin which generates a voltage error signal across R E. The error signal across R E is filtered by an RC network with a typical corner frequency of 7 kHz to reduce the effect of switching noise. The filtered error signal is compared with the internal oscillator sawtooth waveform to generate the duty cycle waveform. As the control current increases, the duty cycle decreases. A clock signal from the oscillator sets a latch which turns on the output MOSFET. The pulse width modulator resets the latch, turning off the output MOSFET. The maximum duty cycle is set by the symmetry of the internal oscillator. The modulator has a minimum ON-time to keep the current consumption of the TOPSwitch independent of the error signal. Note that a minimum current must be driven into the CONTROL pin before the duty cycle begins to change. Gate Driver The gate driver is designed to turn the output MOSFET on at a controlled rate to minimize common-mode EMI. The gate drive current is trimmed for improved accuracy. Error Amplifier The shunt regulator can also perform the function of an error amplifier in primary feedback applications. The shunt regulator voltage is accurately derived from the temperature compensated bandgap reference. The gain of the error amplifier is set by the CONTROL pin dynamic impedance. The CONTROL pin clamps external circuit signals to the V C voltage level. The CONTROL pin current in excess of the supply current is separated by the shunt regulator and flows through R E as a voltage error signal. Cycle-By-Cycle Current Limit The cycle by cycle peak drain current limit circuit uses the output MOSFET ON-resistance as a sense resistor. A current limit comparator compares the output MOSFET ON-state drain- source voltage, V DS(ON) with a threshold voltage. High drain current causes V DS(ON) to exceed the threshold voltage and turns TOPSwitch Family Functional Description (cont.) |
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