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LNK306PN Datasheet(PDF) 7 Page - Power Integrations, Inc. |
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LNK306PN Datasheet(HTML) 7 Page - Power Integrations, Inc. |
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7 / 20 page  2-7 LNK302/304-306 7 Rev. I 11/08 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 opto - No pre-load required 5. Fail-safe – output is not subjected to input voltage if the internal MOSFET fails provides the lowest cost and highest efficiency converter. CCM designs require a larger inductor and ultra-fast (t rr ≤35 ns) freewheeling diode in all cases. It is lower cost to use a larger LinkSwitch-TN in MDCM than a smaller LinkSwitch-TN in CCM because of the additional external component costs of a CCM design. However, if the highest output current is required, CCM should be employed following the guidelines below. Topology Options LinkSwitch-TN can be used in all common topologies, with or without an optocoupler and reference to improve output voltage tolerance and regulation. Table 2 provide a summary of these configurations. For more information see the Application Note – LinkSwitch-TN Design Guide. 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 ultra-fast 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 ultra-fast 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 FB pin. It is recommended that R3 be chosen as a standard 1% resistor of 2 k Ω. Thisensuresgoodnoiseimmunitybybiasingthefeedback 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 μF to 22 μF; smaller values cause poorer regulation at light load conditions. LinkSwitch-TN PI-3756-111903 + BP FB D S V O V IN + |
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