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DPA425 Datasheet(PDF) 8 Page - Power Integrations, Inc. |
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DPA425 Datasheet(HTML) 8 Page - Power Integrations, Inc. |
8 / 36 page DPA423-426 8 K 1/04 (typical), the CONTROL pin goes into the hysteretic mode of operation. In this mode, the CONTROL pin goes through alternate charge and discharge cycles between 4.8 V and 5.8 V (see CONTROL Pin Operation section above) and the IC runs entirely off the high voltage DC input, but with very low power consumption (30 mW typical at 48 V with LINE-SENSE and EXTERNAL CURRENT LIMIT pins open). When the DPA-Switch is remotely turned on after entering this mode, it will initiate a normal start-up sequence with soft-start the next time the CONTROL pin reaches 5.8 V. In the worst case, the delay from remote on to start-up can be equal to the full discharge/charge cycle time of the CONTROL pin, which is approximately 36 ms for a 22 µF CONTROL pin capacitor. This reduced-consumption remote-off mode can eliminate expensive and unreliable in-line mechanical switches. It also allows for microprocessor-controlled turn-on and turn-off sequences that may be required in certain applications. Synchronization In addition to sensing incoming current for OV, UV and remote ON/OFF, the LINE-SENSE pin also monitors its pin voltage through a 1 V threshold comparator. A pin voltage below 1 V turns on DPA-Switch. When the voltage at LINE-SENSE pin rises beyond 1 V to disable the output, DPA-Switch completes its current switching cycle before the output is forced off (similar to remote ON/OFF operation). The internal oscillator is stopped at the end of the current cycle awaiting the LINE-SENSE pin voltage to go low to start the next cycle. This allows the use of the 1 V threshold to synchronize DPA-Switch to an external signal with a frequency lower than its internal switching frequency. A transistor or an optocoupler output connected between the LINE-SENSE pin and the SOURCE pin implements this function (see Figure 24). Please refer to Figure 6 for the timing waveforms of synchronization operation. In order to be recognized as a synchronization pulse, the LINE-SENSE pin needs to stay low (on-time) for at least 120 ns but no more than 2250 ns for 400 kHz (or 3080 ns for 300 kHz) internal switching frequency. In addition, the off-time must be kept below 7.7 µs which is a limitation set by the lowest synchronization frequency of 128 kHz allowed by the chip. The effective DC MAX for synchronization operation can be calculated as 0.75 x f SYNC/fOSC. An off-time longer than 7.7 µs may force the CONTROL pin to go into the hysteretic mode and initiate a soft- start cycle at next turn-on. Soft-Start Two on-chip soft-start functions are activated at start-up with a duration of 5 ms (typical). Maximum duty cycle starts from 0% and linearly increases to the default maximum of 75% at the end of the 5 ms duration and the current limit starts from about 85% and linearly increases to 100% at the end of the 5 ms duration. In addition to start-up, soft-start is also activated at each restart attempt during auto-restart and when restarting after being in hysteretic regulation of CONTROL pin voltage (V C), due to remote off or thermal shutdown conditions. This effectively minimizes current and voltage stresses on the output MOSFET, the clamp circuit and the output rectifier during start-up. This feature also helps minimize output overshoot and prevents saturation of the transformer during start-up. Shutdown/Auto-Restart To minimize DPA-Switch power dissipation under fault conditions, the shutdown/auto-restart circuit turns the power supply on and off at an auto-restart duty cycle of typically 4% if an out of regulation condition persists. Loss of regulation interrupts the external current into the CONTROL pin. V C regulation changes from shunt mode to the hysteretic auto- restart mode as described in CONTROL pin operation section. When the fault condition is removed, the power supply output becomes regulated, V C regulation returns to shunt mode, and normal operation of the power supply resumes. Hysteretic Over-Temperature Protection Over temperature protection is provided by a precision analog circuit that turns the output MOSFET off when the junction temperature exceeds the thermal shutdown temperature (137 °C typical). When the junction temperature cools to below the hysteretic temperature (110 °C typical), normal operation resumes providing automatic recovery. V C is regulated in hysteretic mode and a 4.8 V to 5.8 V (typical) sawtooth waveform is present on the CONTROL pin while in thermal shutdown. Bandgap Reference All critical DPA-Switch internal voltages are derived from a temperature-compensated bandgap reference. This reference is also used to generate a temperature-compensated current reference that is trimmed to accurately set the switching frequency, current limit, and the line OV/UV thresholds. DPA-Switch has improved circuitry to maintain all of the above critical parameters within very tight absolute and temperature tolerances. High-Voltage Bias Current Source This current source biases DPA-Switch from the DRAIN pin and charges the CONTROL pin external capacitance during start-up or hysteretic operation. Hysteretic operation occurs during auto-restart, remote off and over-temperature shutdown. In this mode of operation, the current source is switched on and off with an effective duty cycle of approximately 20%. This duty cycle is determined by the ratio of CONTROL pin charge (I C(CH)) and discharge currents (ICD1 and ICD2). This current source is turned off during normal operation when the output MOSFET is switching. The effect of the current source switching may be seen on the DRAIN voltage waveform as small disturbances which is normal. |
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