TDA4605-2 Datasheet, PDF (9/23 Pages) Siemens Semiconductor Group – Control IC for Switched-Mode Power Supplies using MOS-Transistors

TDA 4605-2
Pin 7
By means of a resistor pin 7 is connected with the output of the control amplifier. If V₁ is approx.
equal to the control voltage V_R the control amplifier has a proportional integrating control
characteristics. The response of the control loop is derived from the capacitor connected to pin 7. If
V₁ is equal to 0 V the control and overload amplifier is generating a ramp-up function using the
capacitor connected to pin 7.
Pin 8
The zero crossing detector controlling the logic block recognizes the complete discharge of the
energy stored in the transformator core by detecting the zero crossing the positive to negative
voltage transition of the voltage at pin 8. This enables the logic for a new pulse. Parasitic oscillations
occurring at the end of a pulse cannot lead to a new pulse because of an internal circuit which
inhibits the zero detector for a certain dead time t_UL after the end of each pulse.
Start-Up Behaviour
The start-up behaviour of the application circuit (which is given on page 68) is explained in the
diagrams on page 70 for a line voltage barely above the lower acceptable lower limit of the mains
voltage. After applying the mains voltage at the time t₀ the following built up of different voltages can
be seen:
– 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 R₁₀/R₁₁
The current drawn by the IC in this case is less than 0.8 mA.
If V₆ reaches the threshold (at the time t₁ in the diagram), the IC internal reference voltage is
switched on. The supply current drawn by the IC rises to 12 mA max.. The primary current/voltage
converter reduces V₂ down to V_2B and the start pulse generator generates the start pulses from time
point t₅ to t₆ in the diagram. The feedback to pin 8 starts the next pulse and so on. The width of all
pulses including the start pulse are controlled by means of the control and overload amplifier. After
turn on the IC is generating a signal at pin 7 which slowly ramping up. This signal is used to increase
the duration of the output pulses slowly (soft-start function). The max. output pulse width is limited
by means of the overload amplifier. If the feedback control voltage V₁ is increasing, the overload
amplifier allows the generation of output pulses with a wider pulse width. The max. pulse width is
reached at time t₂ in the diagram (V₂ = V_{2S max}). The IC is then operating at the fold-back point.
Thereafter the peak values of V₂ decrease rapidly, because of the IC control range. The control loop
is in a steady, operational state.
If the voltage V₆ falls below the switch-off threshold V_{6 min} before the foldback point is reached, the
attempt to start the SMPS is aborted (pin 5 is switched to low). As the internal circuits of the IC
remain switched on, V₆ further decreases to V_6A . The IC switches off; V₆ can rise again (time t₄ in
the diagram) and new start-up attempt begins at time t₁ .
Version 2.0
9
1 Jul 2002