Rev. J 06/13

4

LNK302/304-306

www.powerint.com

Auto-Restart (LNK304-306 Only)

In the event of a fault condition such as output overload, output

short, or an open-loop condition, LinkSwitch-TN enters into

auto-restart operation. An internal counter clocked by the

oscillator gets reset every time the FEEDBACK pin is pulled

high. If the FEEDBACK pin is not pulled high for 50 ms, the

power MOSFET switching is disabled for 800 ms. The auto-

restart alternately enables and disables the switching of the

power MOSFET until the fault condition is removed.

Applications Example

A 1.44 W Universal Input Buck Converter

The circuit shown in Figure 5 is a typical implementation of a

12 V, 120 mA non-isolated power supply used in appliance

control such as rice cookers, dishwashers or other white goods.

This circuit may also be applicable to other applications such as

night-lights, LED drivers, electricity meters, and residential

heating controllers, where a non-isolated supply is acceptable.

The input stage comprises fusible resistor RF1, diodes D3 and

D4, capacitors C4 and C5, and inductor L2. Resistor RF1 is a

flame proof, fusible, wire wound resistor. It accomplishes

several functions: a) Inrush current limitation to safe levels for

rectifiers D3 and D4; b) Differential mode noise attenuation; c)

Input fuse should any other component fail short-circuit

(component fails safely open-circuit without emitting smoke, fire

or incandescent material).

The power processing stage is formed by the LinkSwitch-TN,

freewheeling diode D1, output choke L1, and the output capacitor

C2. The LNK304 was selected such that the power supply

operates in the mostly discontinuous-mode (MDCM). Diode D1

is an ultrafast diode with a reverse recovery time (t

approximately 75 ns, acceptable for MDCM operation. For

continuous conduction mode (CCM) designs, a diode with a t

rr

of ≤35 ns is recommended. Inductor L1 is a standard off-the-

shelf inductor with appropriate RMS current rating (and acceptable

temperature rise). Capacitor C2 is the output filter capacitor; its

primary function is to limit the output voltage ripple. The output

voltage ripple is a stronger function of the ESR of the output

capacitor than the value of the capacitor itself.

To a first order, the forward voltage drops of D1 and D2 are

identical. Therefore, the voltage across C3 tracks the output

voltage. The voltage developed across C3 is sensed and

regulated via the resistor divider R1 and R3 connected to U1’s

FEEDBACK pin. The values of R1 and R3 are selected such

that, at the desired output voltage, the voltage at the

FEEDBACK pin is 1.65 V.

Regulation is maintained by skipping switching cycles. As the

output voltage rises, the current into the FEEDBACK pin will

rise. If this exceeds I

until the current reduces below I

reduced, more cycles will be skipped and if the load increases,

fewer cycles are skipped. To provide overload protection if no

cycles are skipped during a 50 ms period, LinkSwitch-TN will

enter auto-restart (LNK304-306), limiting the average output

power to approximately 6% of the maximum overload power.

Due to tracking errors between the output voltage and the

voltage across C3 at light load or no-load, a small pre-load may

be required (R4). For the design in Figure 5, if regulation to zero

load is required, then this value should be reduced to 2.4 kΩ.

600

020

68 kHz

64 kHz

Time (

µs)

500

400

300

200

100

0

Figure 4.

Frequency Jitter.

RTN

12 V,

120 mA

85-265

VAC

PI-3757-041509

FB

BP

S

D

LinkSwitch-TN

C4

4.7

µF

400 V

C1

100 nF

D4

1N4007

D3

1N4007

D1

UF4005

LNK304

D2

1N4005GP

C2

100

µF

16 V

RF1

8.2

Ω

2 W

R1

13.0 k

Ω

1%

R3

2.05 k

Ω

1%

L2

1 mH

L1

1 mH

280 mA

C5

4.7

µF

400 V

C3

10

µF

35 V

R4

3.3 k

Ω

Figure 5.

Universal Input, 12 V, 120 mA Constant Voltage Power Supply Using LinkSwitch-TN.