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LNK305P Datasheet(PDF) 7 Page - Power Integrations, Inc. |
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LNK305P Datasheet(HTML) 7 Page - Power Integrations, Inc. |
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7 / 18 page  Rev. J 06/13 7 LNK302/304-306 www.powerint.com Table 2 (cont). Common Circuit Configurations Using LinkSwitch-TN. Topology Basic Circuit Schematic Key Features Low-Side Buck-Boost – Optocoupler Feedback 1. Output referenced to input 2. Positive output (V O) with respect to +VIN 3. Step up/down – V O > VIN or VO < VIN 4. Optocoupler feedback - Accuracy only limited by reference choice - Low cost non-safety rated optocoupler - No pre-load required 5. Fail-safe – output is not subjected to input voltage if the internal power MOSFET fails 6. Minimum no-load consumption LinkSwitch-TN PI-3756-041509 + BP FB D S VO VIN + Component Selection Referring to Figure 5, the following considerations may be helpful in selecting components for a LinkSwitch-TN design. Freewheeling Diode D1 Diode D1 should be an ultrafast type. For MDCM, reverse recovery time t RR ≤75 ns should be used at a temperature of 70 °C or below. Slower diodes are not acceptable, as continuous mode operation will always occur during startup, causing high leading edge current spikes, terminating the switching cycle prematurely, and preventing the output from reaching regulation. If the ambient temperature is above 70 °C then a diode with t RR ≤35 ns should be used. For CCM an ultrafast diode with reverse recovery time t RR ≤35 ns should be used. A slower diode may cause excessive leading edge current spikes, terminating the switching cycle prematurely and preventing full power delivery. Fast and slow diodes should never be used as the large reverse recovery currents can cause excessive power dissipation in the diode and/or exceed the maximum drain current specification of LinkSwitch-TN. Feedback Diode D2 Diode D2 can be a low-cost slow diode such as the 1N400X series, however it should be specified as a glass passivated type to guarantee a specified reverse recovery time. To a first order, the forward drops of D1 and D2 should match. Inductor L1 Choose any standard off-the-shelf inductor that meets the design requirements. A “drum” or “dog bone” “I” core inductor is recommended with a single ferrite element due to its low cost and very low audible noise properties. The typical inductance value and RMS current rating can be obtained from the LinkSwitch-TN design spreadsheet available within the PI Expert design suite from Power Integrations. Choose L1 greater than or equal to the typical calculated inductance with RMS current rating greater than or equal to calculated RMS inductor current. Capacitor C2 The primary function of capacitor C2 is to smooth the inductor current. The actual output ripple voltage is a function of this capacitor’s ESR. To a first order, the ESR of this capacitor should not exceed the rated ripple voltage divided by the typical current limit of the chosen LinkSwitch-TN. Feedback Resistors R1 and R3 The values of the resistors in the resistor divider formed by R1 and R3 are selected to maintain 1.65 V at the FEEDBACK pin. It is recommended that R3 be chosen as a standard 1% resistor of 2 kΩ. This ensures good noise immunity by biasing the feedback network with a current of approximately 0.8 mA. Feedback Capacitor C3 Capacitor C3 can be a low cost general purpose capacitor. It provides a “sample and hold” function, charging to the output voltage during the off time of LinkSwitch-TN. Its value should be 10 mF to 22 mF; smaller values cause poorer regulation at light load conditions. Pre-Load Resistor R4 In high-side, direct feedback designs where the minimum load is <3 mA, a pre-load resistor is required to maintain output regulation. This ensures sufficient inductor energy to pull the inductor side of the feedback capacitor C3 to input return via D2. The value of R4 should be selected to give a minimum output load of 3 mA. In designs with an optocoupler the Zener or reference bias current provides a 1 mA to 2 mA minimum load, preventing “pulse bunching” and increased output ripple at zero load. LinkSwitch-TN Layout Considerations In the buck or buck-boost converter configuration, since the SOURCE pins in LinkSwitch-TN are switching nodes, the copper area connected to SOURCE should be minimized to minimize EMI within the thermal constraints of the design. In the boost configuration, since the SOURCE pins are tied to DC return, the copper area connected to SOURCE can be maximized to improve heat sinking. The loop formed between the LinkSwitch-TN, inductor (L1), freewheeling diode (D1), and output capacitor (C2) should be kept as small as possible. The BYPASS pin capacitor C1 (Figure 6) should be located physically close to the SOURCE (S) and BYPASS (BP) pins. To minimize direct coupling from switching nodes, the LinkSwitch-TN should be placed away |
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