2-4

Rev. I 11/08

4

LNK302/304-306

Figure 5. Universal Input, 12 V, 120 mA Constant Voltage Power Supply Using LinkSwitch-TN.

power MOSFET is turned off for the remainder of that cycle.

The leading edge blanking circuit inhibits the current limit

comparator for a short time (t

is turned on. This leading edge blanking time has been set so

that current spikes caused by capacitance and rectifier reverse

recovery time will not cause premature termination of the

switching pulse.

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 FB pin is pulled high. If the FB 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

Figure 4. Frequency Jitter.

RTN

12 V,

120 mA

85-265

VAC

PI-3757-112103

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

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 ultra-fast diode with a reverse recovery time (t

of approximately 75 ns, acceptable for MDCM operation. For

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

≤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 FB pin.

The values of R1 and R3 are selected such that, at the desired

output voltage, the voltage at the FB pin is 1.65 V.

Regulation is maintained by skipping switching cycles. As the

output voltage rises, the current into the FB pin will rise. If

this exceeds I

current reduces below I

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

600

020

68 kHz

64 kHz

V

DRAIN

Time (

μs)

500

400

300

200

100

0