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SMP402 Datasheet(PDF) 4 Page - Power Integrations, Inc. |
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SMP402 Datasheet(HTML) 4 Page - Power Integrations, Inc. |
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4 / 16 page  SMP402 D 1/96 4 Oscillator The oscillator frequency can be adjusted by changing the external C EXT capacitor. This capacitor is charged and discharged by switched constant current sources. The voltage switch points are determined by hysteresis built into a comparator. The period of the waveform is determined by values of the current sources which set the rising and falling slopes of the sawtooth waveform. Maximum duty cycle is equal to the ratio of the charge time to the period. Clock and blanking signals are synthesized from the comparator output for use by the modulator. Error Amplifier The error amplifier consists of a high performance operational amplifier with the non-inverting input connected to the internal bandgap reference voltage. The output of the error amplifier directly controls the duty cycle of the power switch. Pulse Width Modulator The pulse width modulator implements a voltage-mode control loop, and generates the digital driver signal which controls the power switch. The duty cycle of the driver signal will change as a function of input voltage and load. Increasing the duty cycle causes the power supply output voltage to go up. Conversely, decreasing the duty cycle causes the output voltage to go down. The pulse width modulator compares the control voltage (error amplifier output) with the sawtooth voltage generated by the oscillator to produce the required duty cycle. Thermal Shutdown Temperature protection is provided by a precision analog circuit that turns the power switch off when the junction gets too hot (typically 140 °C). The device will automatically reset and turn back on again when the junction has cooled past the hysteresis temperature level. Functional Description (cont.) SENSE+ and SENSE- Inputs SENSE+ and SENSE-are both current mirror inputs consisting of N-channel MOSFETs connected as diodes. The threshold voltage of each transistor is typically 1.7 V. An input current which exceeds the indicated threshold current will turn on the mirror transistor for an active-low signal. POLARITY and LEVEL Outputs The LEVEL output is high when the input current to either SENSE+ or SENSE- exceeds the LEVEL current threshold and the output voltage is in regulation. During normal operation, the POLARITY output is high when the input current to SENSE+ is above the POLARITY current threshold and the output voltage is in regulation. During emergency operation (when the DC input voltage is inverted), the POLARITY output is low when the input current to SENSE- exceeds the INPUT 5 V OUT LEVEL POLARITY 48 V 5 V 0 V 5 V 0 V 5 V 0 V 0 -48 V PI-960A-032293 32 V 30 V 10 V 12 V -10 V -30 V -32 V Figure 4. Turn-on and Turn-off Waveforms of the SMP402. |
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