Datasheet LNK3202, LNK3204-7, LNK3294, LNK3296 (Power Integrations) - 6

LinkSwitch-TN2 LinkSwitch-TN2 Selection and Selection Between
tee a specified reverse recovery time. To a first order, the forward
MDCM and CCM Operation
drops of D1 and D2 should match. Select the LinkSwitch-TN2 device, freewheeling diode and output
Inductor L1
inductor that gives the lowest overall cost. In general, MDCM Choose any standard off-the-shelf inductor that meets the design provides the lowest cost and highest efficiency converter. CCM requirements. A “drum” or “dog bone” “I” core inductor is recom- designs require a larger inductor and ultrafast (t ≤35 ns) freewheel- mended with a single ferrite element due to its low-cost and very low RR ing diode in all cases. It is lower cost to use a larger LinkSwitch-TN2 audible noise properties. However, the inductor should be selected in MDCM than a smal er LinkSwitch-TN2 in CCM because of the as varnished type in order to get low audible noise. The typical additional external component costs of a CCM design. However, if inductance value and RMS current rating can be obtained from the the highest output current is required, CCM should be employed LinkSwitch-TN2 design spreadsheet available within the PI Expert fol owing the guidelines below. design suite from Power Integrations. Choose L1 greater than or equal to the typical calculated inductance with RMS current rating
Topology Options
greater than or equal to calculated RMS inductor current. Care LinkSwitch-TN2 can be used in all common topologies, with or without should be taken to ensure that the inductor has sufficient voltage an optocoupler and reference to improve output voltage tolerance rating as this is a high-voltage application. and regulation. Table 2 provides a summary of these configurations.
Capacitor C2
For more information see the Application Note – LinkSwitch-TN2 The primary function of capacitor C2 is to smooth the inductor current. Design Guide. The actual output ripple voltage is a function of this capacitor’s ESR.
Component Selection
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 Referring to Figure 8, the fol owing considerations may be helpful in LinkSwitch-TN2. selecting components for a LinkSwitch-TN2 design.
Feedback Resistors R1 and R3 BYPASS Pin Capacitor C1
The values of the resistors in the resistor divider formed by R1 and Capacitor connected from the BYAPSS pin provides decoupling for the R3 are selected to maintain 2.00 V at the FEEDBACK pin. It is control er and also selects current limit. A 0.1 μF or 1 μF capacitor may recommended that R3 be chosen as a standard 1% resistor of 2.49 kΩ. be used as indicated in the data sheet. Though electrolytic capaci- This ensures good noise immunity by biasing the feedback network tors can be used, often surface mount multi-layer ceramic capacitors with a current of approximately 0.8 mA. are preferred for use as they enable placement of capacitors close to the IC and design of compact switching power supplies. 16 V, 25 V or
External Bias Resistor R5
higher X7R dielectric capacitors are recommended to ensure To reduce the no-load input power of the power supply, resistor R5, minimum capacitance change under DC bias and temperature. connected from the feedback capacitor C3 to the BYPASS pin, is recommended. This is applicable to the power supply whose output
Freewheeling Diode D1
voltage is higher than V . To achieve lowest no-load power BP(SHUNT) Diode D1 should be an ultrafast type. For MDCM, reverse recovery consumption, the current fed into the BYPASS pin should be slightly time t ≤75 ns should be used at a temperature of 70 °C or below. higher than 120 μA. For the best full load efficiency and thermal RR Slower diodes are not acceptable, as continuous mode operation wil performance, the current fed into the BYPASS pin should be slightly always occur during startup, causing high leading edge current higher than the I Max value. S2 spikes, terminating the switching cycle prematurely, and preventing the output from reaching regulation. If the ambient temperature is
Feedback Capacitor C3
above 70 °C then a diode with t ≤35 ns should be used. Capacitor C3 can be a low cost general purpose capacitor. It provides RR a “sample and hold” function, charging to the output voltage during For CCM an ultrafast diode with reverse recovery time t ≤35 ns the off time of LinkSwitch-TN2. Its value should be 10 μF to 22 μF; RR should be used. A slower diode may cause excessive leading edge smal er values cause poorer regulation at light load conditions. current spikes, terminating the switching cycle prematurely and preventing full power delivery.
Pre-Load Resistor R4
In high-side, direct feedback designs where the minimum load is Fast recovery and slow recovery diodes should never be used as the <3 mA, a pre-load resistor is required to maintain output regulation. large reverse recovery currents can cause excessive power dissipa- This ensures sufficient inductor energy to pull the inductor side of the tion in the diode and/or exceed the maximum drain current specifica- feedback capacitor C3 to input return via D2. The value of R4 should tion of LinkSwitch-TN2. be selected to provide a minimum output load of 3 mA.
Feedback Diode D2
In designs with an optocoupler a Zener diode or reference bias Diode D2 can be a low-cost slow diode such as the 1N400X series, current provides a 1 mA to 2 mA minimum load, preventing “pulse however it should be specified as a glass passivated type to guaran- bunching” and increased output ripple at zero load.
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Rev. M 10/20 www.power.com Document Outline Product Highlights Description Output Current Table Pin Functional Description LinkSwitch-TN2 Functional Description Applications Example Key Application Considerations Quick Design Checklist Absolute Maximum Ratings Thermal Resistance Key Electrical Characteristics Typical Performance Characteristics PDIP-8C (P Package) SMD-8C (G Package) SO-8C (D Package) PDIP-8C (P) and SMD-8C Package Marking SO-8C (D) Package Marking MSL Table ESD and Latch-Up Part Ordering Information