LNK306 Ver la hoja de datos (PDF) - Unspecified

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componentes Descripción

Fabricante

LNK306

Lowest Component Count, Energy-Efficient Off-Line Switcher IC

Unspecified 

LNK302/304-306

TOPOLOGY

Low-Side

Buck Boost –

Optocoupler

Feedback

BASIC CIRCUIT SCHEMATIC

+

LinkSwitch-TN

VIN

BP FB

S

D

VO

PI-3756-111903

KEY FEATURES

1. Output referenced to input

2. Positive output (VO) with respect to +VIN

3. Step up/down – VO > 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

Table 2 (cont). Common Circuit Configurations Using LinkSwitch-TN.

provides the lowest cost and highest efficiency converter. CCM

designs require a larger inductor and ultra-fast (trr ≤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 trr ≤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

trr ≤35 ns should be used.

For CCM an ultra-fast diode with reverse recovery time

trr ≤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 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 μF to 22 μF; smaller values cause poorer regulation at

light load conditions.

27-7

Rev. H 12/06

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