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TOP260EN Datasheet(PDF) 8 Page - Power Integrations, Inc. |
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TOP260EN Datasheet(HTML) 8 Page - Power Integrations, Inc. |
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8 / 48 page  Rev. F 01/09 8 TOP252-262 www.powerint.com Switching frequency of 132 kHz reduces the transformer size with no noticeable impact on EMI. Frequency jittering reduces EMI in the full frequency mode at high load condition. Hysteretic over-temperature shutdown ensures automatic recovery from thermal fault. Large hysteresis prevents circuit board overheating. Packages with omitted pins and lead forming provide large drain creepage distance. Reduction of the auto-restart duty cycle and frequency to improve the protection of the power supply and load during open loop fault, short circuit, or loss of regulation. Tighter tolerances on I2f power coefficient, current limit reduction, PWM gain and thermal shutdown threshold. The VOLTAGE-MONITOR (V) pin is usually used for line sensing by connecting a 4 M Ω resistor from this pin to the rectified DC high voltage bus to implement line overvoltage (OV), under- voltage (UV) and dual-slope line feed-forward with DC MAX reduction. In this mode, the value of the resistor determines the OV/UV thresholds and the DC MAX is reduced linearly with a dual slope to improve line ripple rejection. In addition, it also provides another threshold to implement the latched and hysteretic output overvoltage protection (OVP). The pin can also be used as a remote ON/OFF using the I UV threshold. The EXTERNAL CURRENT LIMIT (X) pin can be used to reduce the current limit externally to a value close to the operating peak current, by connecting the pin to SOURCE through a resistor. This pin can also be used as a remote ON/OFF input. For the P and G package the VOLTAGE-MONITOR and EXTERNAL CURRENT LIMIT pin functions are combined on one MULTI-FUNCTION (M) pin. However, some of the functions become mutually exclusive. The FREQUENCY (F) pin in the TOP254-258 Y and E/L packages set the switching frequency in the full frequency PWM mode to the default value of 132 kHz when connected to SOURCE pin. A half frequency option of 66 kHz can be chosen by connecting this pin to the CONTROL pin instead. Leaving this pin open is not recommended. In the P, G and M packages and the TOP259-261 Y packages, the frequency is set internally at 66 kHz in the full frequency PWM mode. 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 feedback 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 4. 5. 6. 7. 8. 9. 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 drain peak current and switching frequency from a low starting value to the maximum drain peak current at the full frequency over approximately 17 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 undervoltage lockout threshold). When the externally fed current charges the CONTROL pin to the shunt regulator voltage of 5.8 V, current in excess of the consumption of the chip is shunted to SOURCE through an NMOS current mirror as shown in Figure 3. The output current of that NMOS current mirror 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 undervoltage 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 11. The auto-restart circuit has a divide-by-sixteen counter, which prevents the output MOSFET from turning on again until sixteen discharge/ charge cycles have elapsed. This is accomplished by enabling the output MOSFET only when the divide-by-sixteen counter reaches the full count (S15). The counter effectively limits TOPSwitch-HX power dissipation by reducing the auto-restart duty cycle to typically 2%. 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 triangular waveform for the timing of the pulse width modulator. This oscillator sets the pulse width modulator/current limit latch at the beginning of each cycle. The nominal full switching frequency of 132 kHz was chosen to minimize transformer size while keeping the fundamental EMI frequency below 150 kHz. The FREQUENCY pin (available only in TOP254-258 Y and E, L packages), when shorted to the CONTROL pin, lowers the full switching frequency to 66 kHz |
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