TEA1791T All information provided in this document is subject to legal disclaimers. © NXP B.V. 2010. All rights reserved.
Product data sheet Rev. 02 — 7 June 2010 3 of 12
NXP Semiconductors
TEA1791T
GreenChip synchronous rectifier controller
7. Functional description
7.1 Introduction
The TEA1791T is the controller for synchronous rectification to be used in discontinuous
conduction mode and quasi-resonant flyback converters.
7.2 Start-up and UnderVoltage LockOut (UVLO)
The IC leaves the undervoltage lockout state and activates the synchronous rectifier
circuitry as soon as the voltage on the V
CC
pin is above 8.5 V (typical). When the voltage
drops below 8.0 V (typical), the undervoltage lockout state is reentered and the SR driver
output is actively kept low.
7.3 Synchronous rectification
After a negative voltage (−310 mV typical) is sensed on the SRSENSE pin, the driver
output voltage is driven HIGH and the external MOSFET is switched on. As soon as the
SRSENSE voltage rises to −55 mV the driver output voltage is regulated to maintain the
−55 mV on the SRSENSE pin. When the SRSENSE voltage is above −12 mV, the driver
output is pulled to ground.
After switch-on of the SR MOSFET, the input signal on the SRSENSE pin is blanked for
2 μs (typical). This eliminates false switch-off due to high frequency ringing at the start of
the secondary stroke.
Because the driver output voltage is reduced when the voltage on the SRSENSE pin is
−55 mV, the external power switch can be switched off quickly when the current through
the switch reaches zero. The zero current switch-off removes the need for a separate
Standby mode to maintain high efficiency during the no-load operation. The zero current is
detected by sensing a −12 mV level on the SRSENSE pin (see Figure 3
).
