TDA4605-2 Datasheet, PDF(10/23 Page) Siemens Semiconductor Group – Control IC for Switched-Mode Power Supplies using MOS-Transistors

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TDA4605-2 Datasheet, PDF (10/23 Pages) Siemens Semiconductor Group – Control IC for Switched-Mode Power Supplies using MOS-Transistors

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TDA 4605-2

If the voltage level of the rectified mains voltage is reduced strongly under the influence of the

applied load it can happen that V3 is below the voltage level V3A - please refer to the time t3 in the

diagram. This is because if an attempt is made to start the SMPS with a too low mains voltage. The

internal primary voltage monitor circuit then clamps the voltage V3 to the voltage level V3S until the

IC switches off (V6 < V6A). After this a new attempt to start the SMPS will begin at the time t1 in the

diagram.

Control Range, Overload and No-Load Behaviour

After the IC has started, it is operating in the control range. The voltage level at pin 1 is typically

400 mV. The gain of the control circuit consist of two parts: at first a fixed proportional part which is

internally fixed and an integrating part which can be set by means of the external capacitor at pin 7.

If the load is applied to the output of the SMPS, the control and overload amplifier allows wider

pulses (V5 = "H") .The peak voltage value at pin 2 increases up to V2S max . If the secondary load is

increased further the overload amplifier begins to reduce the pulse width of the output pulse. This

point is referred to as the fold-back point of the power supply. Because of the fact that the IC supply

voltage is directly proportional to the secondary voltage, the supply voltage V6 will be reduced

according to the behaviour of the control circuit under the overload condition. If V6 falls below the

value V6 min , the IC will operate in the burst mode. Because of the large time constant of the start-

up circuit which is operating with half-wave rectification, only a small output power is transferred into

the load during the secondary short-circuit of the SMPS. The overload amplifier reduces the output

pulse width down to the pulse width tpk . 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 is

representing.

If no load is applied to the secondary side, the output pulses (V5 = H) become shorter.

If the pulse width is reduced be low a certain internal limit the IC will suppress some of the output

pulses. If the load is reduced further because of the decreasing duty cycle the measurement error

of the rectifier network (R8 , D3, C6 of the application circuit) is increasing and therefore the

secondary output voltage will increase, too. If the IC is operating with small pulse width of the output

pulse the control amplifier applies an additional current to the control amplifier in order to reduce the

output voltage. The value of the additional current depends on the size of the resistors R 5 , R 8 , R 7 .

This can be used to compensate the increase of the secondary voltages.

Behaviour if the Chip Temperature Exceeds Predefined Limits

An integrated protection circuit against over temperature disables the internal logic if the chip

temperature is too high. The internal logic automatically checks the chip temperature and restart the

SMPS as soon as the temperature decreases to a permissible level.

Version 2.0

1 Jul 2002

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