TDA4605 Datasheet, PDF (8/21 Pages) STMicroelectronics – CONTROL CIRCUIT FOR SWITCH MODE POWER SUPPLIES USING MOS TRANSISTORS

TDA 4605
Regulation, Overload and No-Load Behaviour
When the IC has started up, it is operating in the regulation range. The potential at pin 1 typically is
400 mV. If the output is loaded, the regulation amplifier allows broader impulses (V₅ = H). The peak
voltage value at pin 2 increases up to V_{2S max} . If the secondary load is further increased, the
overload amplifier begins to regulate the pulse width downward. This point is referred to as the
overload point of the power supply. As the IC supply voltage V₆ is directly proportional to the
secondary voltage, it goes down in accordance with the overload regulation behaviour. If V₆ falls
below the value V_{6 min} , the IC goes into burst operation. As the time constant of the half-wave
charge-up is relatively large, the short-circuit power remains small. The overload amplifier cuts back
to the pulse width t_pk . This pulse width must remain possible, in order to permit the IC to start-up
without problems from the virtual short circuit, which every switching on with V₁ = 0 represents. If
the secondary side is unloaded, the loading impulses (V₅ = H) become shorter. The frequency
increases up to the resonance frequency of the system. If the load is further reduced, the secondary
voltages and V₆ increase. When V₆ = V_{6 max} , the logic is blocked. The IC converts to burst
operation. This renders the circuit absolutely safe under no-load conditions.
Behaviour when Temperature Exceeds Limit
An integrated temperature protection disables the logic when the chip temperature becomes too
high. The IC automatically interrogates the temperature and starts as soon as the temperature
decreases to permissible values.
Semiconductor Group
40