tea1781 NXP Semiconductors, tea1781 Datasheet - Page 12

• Manufacturer Part Number: tea1781
• Description: Greenchip Pc Secondary Control Ics
• Manufacturer: NXP Semiconductors

NXP Semiconductors
TEA1781_82_1
Preliminary data sheet
7.7 Output drivers
Figure 10
system.
The freewheel switch SF is turned on and off between the VDD voltage (12 V) and ground
level as the source of SF is connected to ground. The source of switch SB varies between
zero (actually a small negative voltage when the SF or its back gate diode is conducting)
and the maximum secondary transformer voltage. Therefore the driver of SB requires a
bootstrap circuit.
When V
the VDD level. Switch SB can then be turned on via this charged capacitor, as it ensures a
VDD voltage with respect to DF_x (pins DF_3V3, DF_5V, or DF_12V). As this capacitor
will be discharged when charging the gate source capacitance of SB, the driving voltage
will be slightly less than the VDD voltage.
The source of the rectifying switch SR is connected to the transformer, which can be
positive or negative. Therefore the FSR capacitor is charged via a boost circuit with a
special voltage source that is about 12 V higher than the secondary transformer voltage at
the appropriate time.
As the source of the rectifying switch SR is connected to the transformer, which can be
either negative or positive, the TEA1781T and TEA1782T are developed in a special
process (ABCD - SOI) which is capable of handling these (high) negative and positive
voltages.
Fig 10. Output driver circuit
(1) WND_SEC_x: The “x” stands for either 1 or 2
(2) FSR_x,, GSR_x, GSB_x, DF_x, FSB_x:
(3) VOUT_x: The “x” stands for either 3V3, 5V, 12V1 or 12V2.
V
V
WND_SEC_x
WND_SEC_x
DF_x
The “x” for these pins and symbols stands for either 3V3, 5V or 12V
shows a part of the secondary side output circuitry and the applied gate drive
(V
SR
DF_3V3
+ 12 V
Drive
SB
, V
Rev. 01 — 13 February 2009
DF_5V
VDD
, or V
SF
DF_12V
V
DF_x
WND_SEC_x
V
V
) is (about) zero, the FSB capacitor is charged to
INDC
INDC
TEA1781T; TEA1782T
/ N
/ N
0
0
V
GreenChip PC secondary control ICs
VOUT_x
V
FSB_x
V
DF_x
© NXP B.V. 2009. All rights reserved.
014aaa190
12 of 32