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TK75002D Datasheet(PDF) 4 Page - TOKO, Inc |
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TK75002D Datasheet(HTML) 4 Page - TOKO, Inc |
4 / 10 page Page 4 January 1999 TOKO, Inc. TK75002 The TK75002 has a saturation limiter in the feedback loop. For either the opto-coupler or pulse transformer configurations, the error voltage which is transmitted across the isolation device by the TK75002 is limited to less than 5 V. This limiting occurs regardless of the applied V CC (generally, it is V CC which is being regulated by the feedback loop for which the TK75002 transmits the error signal). Thus, when the TK75002 is used in a variable output voltage power supply or in a standard line of various fixed output power supplies, no supplemental signal-limiting circuitry is required in the feedback loop. The characteristic waveforms of the TK75002 driving a pulse transformer are shown in Figure 1. The top trace shows the error voltage (@ 1 V / div.) referenced two divisions below the top; it is equal to approximately 1.3 V. The middle trace shows the DRV Pin voltage with respect to V CC (@ 1 V / div.) referenced three divisions below the top. The bottom trace shows the DRV Pin current (@ 20 mA / div.) referenced one division above the bottom. FIGURE 1: CHARACTERISTIC WAVEFORMS OF THE TK75002 THEORY OF OPERATION The TK75002 can be used in conjunction with either an opto-coupler or a pulse transformer to isolate an error signal developed by its internal op-amp. The op-amp can be externally compensated and features a precision reference voltage at the non-inverting input. When configured to drive a pulse transformer, the TK75002 will automatically oscillate to drive the pulse transformer in a pulse-amplitude-modulation (PAM) mode. When V CC is below the UVLO threshold (~4.1V) the TK75002 does not operate and the DRV pin remains in a high-impedance state. When V CC is above the UVLO threshold, the PAM switch turns on and forces V DRV with respect to V CC to be equal to a gain constant times the op- amp output voltage, V COMP, with respect to 2.5 V. Essentially, then, the inverse of the error voltage (referenced to 2.5 V, a virtual ground) times a small gain constant is what appears between the DRV and V CC pins. Note that this is only valid when V COMP is less than 2.5 V, which also implies that V DRV is less than VCC. If an opto-coupler and series resistor are hooked between the DRV and V CC pins, an error current is transmitted across the opto-coupler which is free from the characteristic of having the zero that is a sort of parasitic effect of the standard configuration of driving an opto-coupler with a TL431. If a pulse transformer is connected between the DRV and V CC pins, the magnetizing current will begin to increase until it reaches a threshold of ~ 55 mA, as detected by the Peak Current Detector. After an internal turnoff delay, the PAM switch turns off and the magnetizing current forces the DRV-pin voltage above V CC. An internal clamp diode between the DRV pin and the V CC pin clamps the voltage and then essentially compares it to a V CC-referenced voltage which corresponds to ~ 1 mA of current flow in the clamp diode (i.e., ~ 650 mV at room temperature). When the magnetizing current has decayed to nearly zero (i.e., ~ 1 mA) and after an internal turn-on delay, the Transformer Reset Detector turns on the PAM switch to initiate the process all over again. By using peak rectification on the secondary side of the pulse transformer, the error signal can be recovered. The time constant of the peak detection circuit is chosen to yield negligible ripple but also an acceptable response time. The magnetizing inductance of the pulse transformer is chosen to yield an acceptable peak current overshoot and/or power dissipation when the switching frequency is at a maximum (maximum switching frequency occurs when the op-amp output is low). |
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