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TEA1553T Datasheet(PDF) 10 Page - NXP Semiconductors |
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TEA1553T Datasheet(HTML) 10 Page - NXP Semiconductors |
10 / 27 page TEA1553T_1 © NXP B.V. 2007. All rights reserved. Product data sheet Rev. 01 — 3 July 2007 10 of 27 NXP Semiconductors TEA1553T GreenChip II SMPS control IC 7.8 Demagnetization The system will be in discontinuous conduction mode all the time. The oscillator will not start a new primary stroke until the secondary stroke has ended. Demagnetization features a cycle-by-cycle output short-circuit protection by immediately lowering the frequency (longer off-time), thereby reducing the power level. Demagnetization recognition is suppressed during the first tsup(xfmr_ring) time. This suppression may be necessary in applications where the transformer has a large leakage inductance and at low output voltages/start-up. 7.9 Overvoltage protection An OVP mode is implemented in the GreenChip series. For the TEA1553T, this works by sensing the auxiliary voltage via the current flowing into pin DEM during the secondary stroke. The auxiliary winding voltage is a well-defined replica of the output voltage. Any voltage spikes are averaged by an internal filter. Pin OVPFCAP is used to program the OVP function as follows: 1. Pin grounded: OVP is disabled. 2. Pin at 5 V (e.g. connected to pin 12, the VCC5V pin): the internal OVP circuit is enabled. 3. A capacitor is connected from the pin to the ground: this capacitor is used to set the number of OVP events that can occur before the logic determines that an actual OVP condition exists. The minimum timing is the internal OVP timing. In the last case, the number of OVP events can be set by an external capacitor connected to pin OVPFCAP: Where n is the number of OVP counts which are allowed to occur before an actual OVP state is detected. If the output voltage exceeds the OVP trip level, an internal counter starts counting subsequent OVP events. The counter has been added to prevent incorrect OVP detections which could occur during ESD / lightning events. If the output voltage exceeds the OVP trip level a few times, and then does not exceed it in the next cycle, the internal counter will count down twice as fast as it counted up. However, when typically 10 cycles of subsequent OVP events are detected, the IC assumes a true OVP state exists and the OVP circuit switches the MOSFET off. Next, the controller waits until the Vth(UVLO) level is reached on pin VCC, and then capacitor CVCC is recharged to the Vstartup level. Operation only recommences when the VCC voltage drops below a level of about 4.5 V, in practice this only occurs when the mains input voltage has been disconnected for a short period of time The output voltage at which the OVP function trips, Vtrip(OVPFCAP), can be set by the demagnetization resistor, RDEM: C OVP n I ch OVPFCAP () t p OVPFCAP () × V OVPFCAP ---------------------------------------------------------------------- × = |
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