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TDA16831G Folha de dados(PDF) 6 Page - Infineon Technologies AG |
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TDA16831G Folha de dados(HTML) 6 Page - Infineon Technologies AG |
6 / 21 page TDA 16831-4 Data Sheet 6 1999-12-10 Circuit Description The TDA 16831-4 is a current mode pulse width modulator with integrated sense CoolMOS transistor. It fulfills the requirements of minimum external control circuitry for a flyback application. Current mode control means that the current through the MOS transistor is compared with a reference signal derived from the output voltage of the flyback application. The result of that comparison determines the on time of the MOS transistor. To minimize external circuitry the sense resistor which gives information about MOS current is integrated. The oscillator resistor and capacitor which determine the switching frequency are integrated, too. Special efforts have been made to compensate temperature dependency and to minimize tolerances of this resistor. The circuit in detail: (see Figure 3) Start Up Circuit (uvlo) Uvlo is monitoring the external supply voltage V CC. When VCC is exceeding the on threshold V CCH = 12 V, the bandgap, the bias circuit and the soft start circuit are switched on. When V CC is falling below the off-threshold V CCL = 9 V the circuit is switched off. During start up the current consumption is about 30 µA. Bandgap (bg) The bandgap generates an internal very accurate reference voltage of 5.5 V to supply the internal circuits. Current Source (bias) The bias circuit provides the internal circuits with constant current. Oscillator (osc) The oscillator is generating a frequency twice the switching frequency f switch = 100 kHz. Resistor, capacitor and current source which determine the frequency are integrated. The charging and discharging current of the implemented oscillator capacitor is internally trimmed, in order to achieve a very accurate switching frequency. Temperature coefficient of switching frequency is very low ( see page 19). Divider Flip Flop (tff) Tff is a flip flop which divides the oscillator frequency by one half to create the switching frequency. The maximum duty cycle is set to Dmax = 0.5. Current Sense Amplifier (pwmop) The positive input of the pwmop is applied to the internal sense resistor. With the internal sense resistor ( R sense) the sensed current coming from the CoolMOS is converted into a sense voltage. The sense voltage is amplified with a gain of 32 dB. The amplified sense voltage is connected to the negative input of the pwm comparator. Each time when the CoolMOS transistor is switched on, a current spike is superposed to the true current information. To eliminate this current spike the sense voltage is smoothed via an internal resistor capacitor network with a time constant of T d1 = 100 ns. This is the first leading edge blanking and only a small spike is left. To reduce this small spike the current sense amplifier is creating a virtual ramp at the output. This is done by a second resistor capacitor network with T d2 = 100 ns and an op-offset of 0.8 V which is seen at the output of the amplifier. When gate drive is |
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