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TDA8385 Datasheet(PDF) 8 Page - NXP Semiconductors |
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TDA8385 Datasheet(HTML) 8 Page - NXP Semiconductors |
8 / 21 page March 1994 9 Philips Semiconductors Preliminary specification Control circuit for a Self-Oscillating Power Supply (SOPS) TDA8385 Pulse width modulator (Block IV) The pulse width modulator compares the control voltage Vr with the sawtooth voltage Vsim. If Vsim > Vr output sub-section 8 is HIGH the LED is switched on and then the switching transistor is switched off. In this way the output voltage is controlled. EXAMPLE If the load decreases, VO increases and therefore Vr decreases. This causes the LED to start conducting prematurely, which implies that the switching transistor is turned off sooner. The consequence is that the collector peak current decreases and hence less energy is stored in the transformer and VO will decrease. LED control (Block V) If either output of sub-section 8 or output of sub-section 16 are HIGH the LED is conductive. In order to improve the start-up behaviour of the power supply, the demagnetization signal of sub-section 12 will only activate the LED driver if flip-flop (13) has previously been set. The set signal is generated in the following three ways. 1. Pulse width modulator (sub-section 8) 2. Comparator (18) 3. VP(min) detector Set signal (2.) and (3.) are added as extra security to guarantee a demagnetization pulse in the event of the switching transistor not having enough base current. In that situation e.g. at start-up, no comparator signal, set signal (3.) is generated by sub-section 8. LED driver (Block VI) The LED driver (pin 2) is blocked if the supply voltage VP is in the initialization phase (see Fig.4). The output stage is a push-pull stage, which can sink 5 mA and source 10 mA. Slow-start circuit (Block VII) The slow-start circuit is active at start-up, over voltage protection or after an overload (short-circuited), and stand-by mode. The voltage Vss and therefore the voltage Vmv and the peak current Ic slowly increase at start-up. By means of sub-section 27 the slow start voltage Vss is clamped to the voltage Vfb. If the feedback voltage is reduced, e.g. as overload, the slow-start capacitor is discharged to the level of Vfb. In this way a slow start-up is also guaranteed after an overload, short-circuit situation or after a stand-by mode. The circuit of sub-section 27 is not active during an over voltage protection. When the supply voltage VP is below the reset-level of 5.2 V (sub-section 28) the slow-start capacitor is quickly discharged. The slow-start input (pin 7) can also be used for Ic(max) setting by connecting a resistor to this pin. Over voltage protection (Block VIII) The operation of the over voltage protection circuit is, in the event of the IC being SOPS-supplied, quite different from when the IC is externally supplied. OPERATION WHEN THE IC IS EXTERNALLY SUPPLIED When the voltage on pin 8 exceeds 2.5 V the slow-start capacitor is slowly discharged. During discharge the LED is permanently conducting. Discharge is stopped when Vss is below 115 mV. Flip-flop (23) will then be reset and the circuit is ready again for a new slow-start procedure. During an over voltage sub-section 27 is not active so that the output voltage VO cannot influence the slow-start discharge procedure. OPERATION WHEN IC IS SOPS-SUPPLIED (SEE FIGS 9 AND 10) When the voltage on pin 8 exceeds 2.5 V the slow-start capacitor is slowly discharged. During discharge of Css the supply capacitor CP is also discharged. Because the capacitors CP and Css have almost the same value and the supply current IP ( ≈15 mA) is much larger than the slow discharge current ( ≈50 µA), the LED will be switched off by means of the VP(min) detection circuit (5.2 V). At that moment the switching transistor will be switched on again until the 7.5 V level is reached. During this hysteresis interval the slow-charge capacitor is quickly discharged. At the 7.5 V level the LED will be switched on again because flip-flop (23) output is still HIGH. The same procedure will be repeated several times until the slow-start capacitor reaches the 115 mV reset level. At that moment the slow-start procedure is started again. If there is still an over voltage the procedure will be repeated. Figure 10 is a detailed exposure of Fig.11. |
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