TDA4605-3 Datasheet, PDF (7/20 Pages) Siemens Semiconductor Group – Control IC for Switched-Mode Power Supplies using MOS-Transistor

TDA 4605-3
Pin 8
The zero detector controlling the logic block recognizes the transformer being discharged by
positive to negative zero crossing of pin 8 voltage and enables the logic for a new pulse. Parasitic
oscillations occurring at the end of a pulse cannot lead to a new pulse (double pulsing), because an
internal circuit inhibits the zero detector for a finite time t_UL after the end of each pulse.
Start-Up Behaviour
The start-up behaviour of the application circuit per sheet 88 is represented an sheet 90 for a line
voltage barely above the lower acceptable limit time t₀ the following voltages built up:
– V₆ corresponding to the half-wave charge current over R₁
– V₂ to V_{2 max} (typically 6.6 V)
– V₃ to the value determined by the divider R ₁₀/R ₁₁ .
The current drawn by the IC in this case is less than 1.6 mA.
If V₆ reaches the threshold V_6E (time point t₁), the IC switches on the internal reference voltage. The
current draw max. rises to 12 mA. The primary current- voltage reproducer regulates V₂ down to V_2B
and the starting impulse generator generates the starting impulses from time point t₅ to t₆ . The
feedback to pin 8 starts the next impulse and so on. All impulses including the starting impulse are
controlled in width by regulating voltage of pin 1. When switching on this corresponds to a short-
circuit event, i.e. V₁ = 0. Hence the IC starts up with "short-circuit impulses" to assume a width
depending on the regulating voltage feedback (the IC operates in the overload range). The IC
operates at the overload point. Thereafter the peak values of V₂ decrease rapidly, as the starting
attempt is aborted (pin 5 is switched to low). As the IC remains switched on, V₆ further decreases
to V₆ . The IC switches off; V₆ can rise again (time point t₄) and a new start-up attempt begins at
time point t₁ . If the rectified alternating Iine voltage (primary voltage) collapses during load, V₃ can
fall below V_3A , as is happening at time point t₃ (switch-on attempt when voltage is too low). The
primary voltage monitor then clamps V₃ to V_3S until the IC switches off (V₆ < V_6A). Then a new start-
up attempt begins at time point t₁ .
Semiconductor Group
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