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FC3255 Arkusz danych(PDF) 8 Page - First Components International |
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FC3255 Arkusz danych(HTML) 8 Page - First Components International |
8 / 18 page Consequently, the current limit of the internal power transistor current is reduced from its nominal value. Switch Driver and Power Switch The switch driver receives a control signal from the logic section to drive the output power switch. The switch is grounded through emitter resistors (63mW to- tal) to the PGND pin. PGND is not connected to the IC substrate so that switching noise can be isolated from the analog ground. The peak switching current is clamped by an internal circuit. The clamp current is guaranteed to be greater than 1.5A and varies with duty cycle due to slope compensation. The power switch can withstand a maximum voltage of 40V on the collector(V pin). The saturation voltage of the SW switch is typically less than 1V to minimize power dis- sipation. Short Circuit Condition When a short circuit condition happens in a boost circuit, the inductor current will increase during the whole switching cycle, causing excessive current to be drawn from the input power supply. Since control ICs don's have the means to limit load current, an ex- ternal current limit circuit(such as a fuse or relay) has to be implemented to protect the load, power supply and ICs. In other topologies, the frequency shift built into the IC prevents damage to the chip and external compo- nents. This feature reduces the minimum duty cycle and allows the transformer secondary to absorb ex- cess energy before the switch turns back on. 3255 can be activated by either connecting the V pin to a voltage source or by enabling the SS pin. CC When the V voltage is below the minimum supply CC voltage, the V pin is in high impedance. Therefore, SW current conduces directly from the input power source to the output through the inductor and diode. Once V reaches approximately 1.5V, the internal power CC switch briefly turns on. This is a part of nor- mal operation. The turn-on of the power switch ac- counts for the initial current swing. When the V pin voltage rises above the threshold, C the internal power switch starts to switch and a volt- age pulse can be seen at the V pin. Detecting a SW low output voltage at the FB pin, the built-in fre- quency shift feature reduces the switching frequency to a fraction of its nominal value, reducing the mini- mum duty cycle, which is otherwise limited by the min- imum on-time of the switch. The peak current during this phase is clamped by the internal current limit. When the FB pin voltage rises above 0.4V, the fre- quency increases to its nominal value, and the peak current begins to decrease as the output ap- proaches the regulation voltage. The overshoot of the output voltage is prevented by the active pull-on, by which the sink current of the error amplifier is in- creased once an overvoltage condition is detected. The overvoltage condition is defined as when the FB pin voltage is 50mV greater than the reference volt- age. COMPONENT SELECTION Frequency Compensation The goal of frequency compensation is to achieve desirable transient response and DC regulation while ensuring the stability of the system. A typical compensation network, as shown in Figure 9, pro- vides a frequency response of two poles and one zero. This frequency response is further illustrated in the Bode plot shown in Figure 9. The high DC gain in Figure 10 is desirable for achieving DC accuracy over ling and load variations. The DC gain of a transconductance error amplifier can be calculated as follows: Where: The low frequency pole, f , is determined by the er- p1 ror amplifier output resistance and C1 as: The first zero generated C1 and R1 is: GM3255 VC GND R1 C1 C2 Gain = G x R DC M O G = error amplifier transconductance; M R = error amplifier output resistance 1MW O Figure 9. A Typical Compensation Network f = z1 1 2pC1R1 f = P1 1 2pC1RO FC3255 1.5A PWM Switching Regulators |
Podobny numer części - FC3255 |
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Podobny opis - FC3255 |
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