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.

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

practice this only occurs when the mains input voltage has been disconnected for a short

period of time

C

OVP

n

I

ch OVPFCAP

()

t

p OVPFCAP

()

×

V

OVPFCAP

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×

=