TEA1762T_2
?NXP B.V. 2007. All rights reserved.
Product data sheet
Rev. 02 25 April 2007
4 of 15
NXP Semiconductors
TEA1762T
GreenChip synchronous rectier controller
7.1 Start-up and undervoltage lock-out
The IC leaves the under-voltage lock-out state and activates the synchronous rectier
circuitry and also the voltage/current sense circuitry as soon as the voltage on the V
CC
pin
is above 8.6 V (typical). As soon as the voltage drops below 8.1 V (typical), the
under-voltage lock-out state is re-entered and the SR driver output is actively kept low and
also the opto driver outputs are disabled.
7.2 Synchronous rectication
After a negative voltage (310 mV typical) is sensed on the SRSENSE pin, the driver
output voltage is made 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. As soon as 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 祍 (typical). This will eliminate false switch-off due to
high frequency ringing at the start of the secondary stroke.
Because the driver output voltage is reduced as soon as the voltage on the SRSENSE pin
is 55 mV, the external power switch can be switched off fast when the current through the
switch reaches zero. With this zero-current switch off, no separate standby mode is
needed to maintain high efciency during no-load operation. The zero current is detected
by sensing a 12 mV level on the SRSENSE pin. See
Figure
3.
If the secondary stroke of the yback converter is shorter than 2 祍 (typical), the driver
output is disabled. This will guarantee stable operation for very low duty cycles. When the
secondary stroke increases above 2.2 祍 (typical), the driver output is again enabled.
Fig 3. Synchronous rectication
014aaa057
0 V
12 mV
55 mV
310 mV
0 A
0 A
0 V
V
SRSENSE
V
DRIVER
Primary
current
Secondary
current
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