LNK3205PSLASHGSLASHD datasheet(6/28 Pages) POWERINT

6 / 28 page

Rev. Q 06/23

LinkSwitch-TN2

www.power.com

LinkSwitch-TN2 Selection and Selection Between

MDCM and CCM Operation

Select the LinkSwitch-TN2 device, freewheeling diode and output

inductor that gives the lowest overall cost. In general, MDCM

provides the lowest cost and highest efficiency converter. CCM

designs require a larger inductor and ultrafast (t

RR ≤35 ns) freewheel-

ing diode in all cases. It is lower cost to use a larger LinkSwitch-TN2

in MDCM than a smaller LinkSwitch-TN2 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-TN2 can be used in all common topologies, with or without

an optocoupler and reference to improve output voltage tolerance

and regulation. Table 2 provides a summary of these configurations.

For more information see the Application Note – LinkSwitch-TN2

Design Guide.

Component Selection

Referring to Figure 8, the following considerations may be helpful in

selecting components for a LinkSwitch-TN2 design.

BYPASS Pin Capacitor C1

Capacitor connected from the BYAPSS pin provides decoupling for the

controller and also selects current limit. A 0.1 μF or 1 μF capacitor may

be used as indicated in the data sheet. Though electrolytic capaci-

tors can be used, often surface mount multi-layer ceramic capacitors

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

higher X7R dielectric capacitors are recommended to ensure

minimum capacitance change under DC bias and temperature.

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 recovery and slow recovery diodes should never be used as the

large reverse recovery currents can cause excessive power dissipa-

tion in the diode and/or exceed the maximum drain current specifica-

tion of LinkSwitch-TN2.

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 recom-

mended with a single ferrite element due to its low-cost and very low

audible noise properties. However, the inductor should be selected

as varnished type in order to get low audible noise. The typical

inductance value and RMS current rating can be obtained from the

LinkSwitch-TN2 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. Care

should be taken to ensure that the inductor has sufficient voltage

rating as this is a high-voltage application.

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-TN2.

Feedback Resistors R1 and R3

The values of the resistors in the resistor divider formed by R1 and

R3 are selected to maintain 2.00 V at the FEEDBACK pin. It is

recommended that R3 be chosen as a standard 1% resistor of 2.49 kΩ.

This ensures good noise immunity by biasing the feedback network

with a current of approximately 0.8 mA.

External Bias Resistor R5

To reduce the no-load input power of the power supply, resistor R5,

connected from the feedback capacitor C3 to the BYPASS pin, is

recommended. This is applicable to the power supply whose output

voltage is higher than V

BP(SHUNT). To achieve lowest no-load power

consumption, the current fed into the BYPASS pin should be slightly

higher than 120 μA. For the best full load efficiency and thermal

performance, the current fed into the BYPASS pin should be slightly

higher than the I

S2 Max value.

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-TN2. Its value should be 10 μF to 22 μF;

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 provide a minimum output load of 3 mA.

In designs with an optocoupler a Zener diode or reference bias

current provides a 1 mA to 2 mA minimum load, preventing “pulse

bunching” and increased output ripple at zero load.

LNK3205P/G/D Datenblatt(PDF) 6 Page - Power Integrations, Inc.

LNK3205P/G/D Datenblatt(HTML) 6 Page - Power Integrations, Inc.

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datenblatt Download

Link URL