TDA16846_03 Datasheet, PDF (14/29 Pages) Infineon Technologies AG – Controller for Switch Mode Power Supplies Supporting Low Power Standby and Power Factor

TDA 16846/16846-2
TDA 16847/16847-2
Fixed Frequency and Synchronization Circuit SYN (Pin 7)
Figure 11 shows the Fixed Frequency and Synchronization Circuit. The circuit is
disabled when Pin 7 is not connected or connected to pin 9 (Vref, to avoid noise
sensitivity). With R₇ and C₇ at Pin 7 the circuit is working. C₇ is charged fast with approx.
1 mA and discharged slowly by R₇ (Figure 11). The power transistor is switched on at
beginning of the charge phase. The switching frequency is (charge time ignored)
:
f ≈ ----⁰---^,--⁸-----
R₇ × C₇
When the oscillator circuit is working the Fold Back Point Correction is disabled (not
necessary in fixed frequency mode). “Switch on” is only possible when a “zero crossing”
has occurred at Pin 3, otherwise “switch-on” will be delayed (Figure 12).
Figure 11 Synchronization and Fixed Frequency Circuit
Version 2.0
14
31 Jul 2003