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UC1548 Scheda tecnica(PDF) 5 Page - Texas Instruments |
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UC1548 Scheda tecnica(HTML) 5 Page - Texas Instruments |
5 / 9 page Figure 3: Error Amplifier Gain and Phase Response over Frequency A capacitor from the CT pin to GND programs oscillator frequency, as shown in Figure 2. Frequency is determined by: F = 1 / (10k • CT). The sawtooth wave shape is generated by a charging cur- rent of 200 µA and a discharge current of 1800µA. The discharge time of the sawtooth is guaranteed dead time for the output driver. If the maximum duty cycle control is defeated by connecting DMAX to VREF, the maximum duty cycle is limited by the oscillator to 90%. If an adjust- ment is required, an additional trim resistor RT from CT to ground can be used to adjust the oscillator frequency. RT should not be less than 40kohms. This will allow up to a 22% decrease in frequency. UC1548 UC2548 UC3548 UDG-95040 OSCILLATOR INDUCTOR CURRENT WAVEFORM SYNTHESIZER Average current mode control is a very useful technique to control the value of any current within a switching con- verter. Input current, output inductor current, switch current, diode current or almost any other current can be controlled. In order to implement average current mode control, the value of the current must be explicitly known at all times. To control output inductor current (IL) in a buck derived isolated converter, switch current provides inductor current information, but only during the on time of the switch. During the off time, switch current drops abruptly to zero, but the inductor current actually dimin- ishes with a slope dIL/dt = −Vo/L. This down slope must be synthesized in some manner on the primary side to provide the entire inductor current waveform for the con- trol circuit. The patented current waveform synthesizer (Figure 4) consists of a unidirectional voltage follower which forces the voltage on capacitor CI to follow the on time switch current waveform. A programmable discharge current synthesizes the off time portion of the waveform. ION is the input to the follower. The discharge current is pro- grammed at IOFF. The follower has a one volt offset, so that zero current corresponds to one volt at CI. The best utilization of the UC3548 is to translate maximum average inductor current to a 4 volt signal level. Given N and Ns (the turns ratio of the power and current sense transformers respectively), proper scaling of IL to V(CI) requires a sense resistor Rs as calculated from: Rs = 4V • Ns • N / IL(max). Restated, the maximum average inductor current will be limited to: IL(max) = 4V • Ns • N/Rs. IOFF and CI need to be chosen so that the ratio of dV(CI)/dt to dIL/dt is the same during switch off time as on time. Recommended nominal off current is 100 µA. This requires CI = (100 µA • N • Ns • L) / (Rs • Vo(nom)) where L is the output inductor value and Vo(nom) is the converter regulated output voltage. 5 |
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