LNK302/304-306
600
500
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).
VDRAIN
400
300
The power processing stage is formed by the LinkSwitch-TN,
freewheeling diode D±, output choke L±, and the output capacitor
C2. The LNK304 was selected such that the power supply
operates in the mostly discontinuous-mode (MDCM). Diode D±
is an ultrafast diode with a reverse recovery time (tRR) of
approximately 71 ns, acceptable for MDCM operation. For
continuous conduction mode (CCM) designs, a diode with a trr
of ≤31 ns is recommended. Inductor L± 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.
200
100
0
68 kHz
64 kHz
0
20
Time (µs)
Figure 4. Frequency Jitter.
Auto-Restart (LNK304-306 Only)
To a first order, the forward voltage drops of D± 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 R± and R3 connected to U±’s
FEEDBACK pin. The values of R± and R3 are selected such
that, at the desired output voltage, the voltage at the
FEEDBACK pin is ±.61 V.
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 10 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.
Regulation is maintained by skipping switching cycles. As the
output voltage rises, the current into the FEEDBACK pin will
rise. If this exceeds IFB then subsequent cycles will be skipped
until the current reduces below IFB. Thus, as the output load is
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 10 ms period, LinkSwitch-TN will
enter auto-restart (LNK304-306), limiting the average output
power to approximately 65 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 1, if regulation to zero
load is required, then this value should be reduced to 2.4 kΩ.
Applications Example
A 1.44 W Universal Input Buck Converter
The circuit shown in Figure 1 is a typical implementation of a
±2 V, ±20 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 RF±, diodes D3 and
D4, capacitors C4 and C1, and inductor L2. Resistor RF± is a
R1
13.0 kΩ
1%
C3
R3
2.05 kΩ
1%
RF1
10 µF
35 V
D2
1N4005GP
8.2 Ω
L2
C1
FB
BP
S
2 W
1 mH
100 nF
12 V,
120 mA
D
L1
D3
1 mH
LinkSwitch-TN
1N4007
C2
280 mA
85-265
VAC
C4
4.7 µF
400 V
C5
4.7 µF
400 V
R4
3.3 kΩ
LNK304
100 µF
D1
UF4005
16 V
D4
1N4007
RTN
PI-3757-041509
Figure 5. Universal Input, 12 V, 120 mA Constant Voltage Power Supply Using LinkSwitch-TN.
4
Rev. J 06/13
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