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LNK500G-TL Datasheet(PDF) 3 Page - Power Integrations, Inc. |
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LNK500G-TL Datasheet(HTML) 3 Page - Power Integrations, Inc. |
3 / 20 page 3 LNK500 B 3/03 LinkSwitch Functional Description The duty cycle, current limit and operating frequency relationships with CONTROL pin current are shown in Figure 4. Figure 5 shows a typical power supply outline schematic which is used below to describe the LinkSwitch operation. Power Up During power up, as V IN is first applied (Figure 5), the CONTROL pin capacitor C1 is charged through a switched high voltage current source connected internally between the DRAIN and CONTROL pins (see Figure 2). When the CONTROL pin voltage reaches approximately 5.6 V relative to the SOURCE pin, the high voltage current source is turned off, the internal control circuitry is activated and the high voltage internal MOSFET starts to switch. At this point, the charge stored on C1 is used to supply the internal consumption of the chip. Constant Current (CC) Operation As the output voltage, and therefore the reflected voltage across the primary transformer winding ramp up, the feedback CONTROL current I C increases. As shown in Figure 4, the internal current limit increases with I C and reaches ILIM when IC is equal to I DCT. The internal current limit vs. IC characteristic is designed to provide an approximately constant power supply output current as the power supply output voltage rises. Constant Voltage (CV) Operation When I C exceeds IDCS, typically 2 mA (Figure 4), the maximum duty cycle is reduced. At a value of I C that depends on power supply input voltage, the duty cycle control limits LinkSwitch peak current below the internal current limit value. At this point the power supply transitions from CC to CV operation. With minimum input voltage in a typical universal input design, this transition occurs at approximately 30% duty cycle. R1 (Figure 5) is therefore initially selected to conduct a value of I C approximately equal to I DCT when VOUT is at the desired value at the minimum power supply input voltage. The final choice of R1 is made when the rest of the circuit design is complete. When the duty cycle drops below approximately 4%, the frequency is reduced, which reduces energy consumption under light load conditions. Auto-Restart Operation When a fault condition, such as an output short circuit or open loop, prevents flow of an external current into the CONTROL pin, the capacitor C1 discharges towards 4.7 V. At 4.7 V, auto- restart is activated, which turns the MOSFET off and puts the control circuitry in a low current standby mode. In auto-restart, LinkSwitch periodically restarts the power supply so that normal power supply operation can be restored when the fault is removed. Figure 4. CONTROL Characteristics. Figure 5. Power Supply Outline Schematic. V OUT V IN D2 C4 D S C C2 R1 R2 C1 D1 LinkSwitch PI-2715-112102 PI-2799-112102 Internal Current Limit CONTROL Current I C I DCS CONTROL Current I C I CD1 CONTROL Current I C Duty Cycle Frequency I LIM I DCT 77% 30% 3.8% f OSC f OSC(low) Auto-restart Auto-restart Auto-restart |
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