LNK302/304-306
LinkSwitch-TN
Family
®
Lowest Component Count, Energy-Efficient
Off-Line Switcher IC
Product Highlights
Cost Effective Linear/Cap Dropper Replacement
• Lowest cost and component count buck converter solution
• Fully integrated auto-restart for short-circuit and open
loop fault protection – saves external component costs
• LNK302 uses a simplified controller without auto-restart
for very low system cost
• 66 kHz operation with accurate current limit – allows low cost
off-the-shelf 1 mH inductor for up to 120 mA output current
• Tight tolerances and negligible temperature variation
• High breakdown voltage of 700 V provides excellent
input surge withstand
• Frequency jittering dramatically reduces EMI (~10 dB)
– minimizes EMI
fi
lter cost
• High thermal shutdown temperature (+135
°C
minimum)
Much Higher Performance over Discrete Buck and
Passive Solutions
• Supports buck, buck-boost and
fl
yback topologies
• System level thermal overload, output short-circuit and
open control loop protection
• Excellent line and load regulation even with typical
configuration
• High bandwidth provides fast turn-on with no overshoot
• Current limit operation rejects line ripple
• Universal input voltage range (85 VAC to 265 VAC)
• Built-in current limit and hysteretic thermal protection
• Higher efficiency than passive solutions
• Higher power factor than capacitor-fed solutions
• Entirely manufacturable in SMD
EcoSmart
– Extremely Energy Efficient
• Consumes typically only 50/80 mW in self-powered buck
topology at 115/230 VAC input with no load (opto feedback)
• Consumes typically only 7/12 mW in
fl
yback topology
with external bias at 115/230 VAC input with no load
• Meets California Energy Commission (CEC), Energy
Star, and EU requirements
®
FB
BP
S
+
Wide Range
HV DC Input
D
+
DC
Output
LinkSwitch-TN
PI-3492-111903
Figure 1. Typical Buck Converter Application (See Application
Examples Section for Other Circuit Configurations).
OUTPUT CURRENT TABLE
1
PRODUCT
4
LNK302P/G/D
230 VAC ±15%
MDCM
2
63 mA
CCM
3
80 mA
170 mA
280 mA
360 mA
85-265 VAC
MDCM
2
63 mA
CCM
3
80 mA
LNK304P/G/D 120 mA
LNK305P/G/D 175 mA
LNK306P/G/D 225 mA
Table 1. Output Current Table.
120 mA 170 mA
175 mA 280 mA
225 mA 360 mA
Notes:
1. Typical output current in a non-isolated buck converter. Output power
capability depends on respective output voltage. See Key Applications
Considerations Section for complete description of assumptions,
including fully discontinuous conduction mode (DCM) operation.
2. Mostly discontinuous conduction mode.
3. Continuous conduction mode.
4. Packages: P: DIP-8B, G: SMD-8B, D: SO-8C.
Applications
• Appliances and timers
• LED drivers and industrial controls
Description
LinkSwitch-TN
is specifically designed to replace all linear and
capacitor-fed (cap dropper) non-isolated power supplies in the
under 360 mA output current range at equal system cost while
offering much higher performance and energy efficiency.
LinkSwitch-TN
devices integrate a 700 V power MOSFET,
oscillator, simple On/Off control scheme, a high voltage switched
current source, frequency jittering, cycle-by-cycle current limit
and thermal shutdown circuitry onto a monolithic IC. The start-
up and operating power are derived directly from the voltage
on the DRAIN pin, eliminating the need for a bias supply and
associated circuitry in buck or
fl
yback converters. The fully
integrated auto-restart circuit in the LNK304-306 safely limits
output power during fault conditions such as short-circuit or
open loop, reducing component count and system-level load
protection cost. A local supply provided by the IC allows use
of a non-safety graded optocoupler acting as a level shifter to
further enhance line and load regulation performance in buck
and buck-boost converters, if required.
November 2008
LNK302/304-306
BYPASS
(BP)
REGULATOR
5.8 V
DRAIN
(D)
+
BYPASS PIN
UNDER-VOLTAGE
5.8 V
4.85 V
6.3 V
-
CURRENT LIMIT
COMPARATOR
+
-
VI
LIMIT
JITTER
CLOCK
DC
MAX
THERMAL
SHUTDOWN
OSCILLATOR
FEEDBACK
(FB)
1.65 V -V
T
S
R
Q
Q
LEADING
EDGE
BLANKING
SOURCE
(S)
PI-3904-020805
Figure 2a. Functional Block Diagram (LNK302).
BYPASS
(BP)
DRAIN
(D)
REGULATOR
5.8 V
FAULT
PRESENT
AUTO-
RESTART
COUNTER
CLOCK
RESET
6.3 V
5.8 V
4.85 V
BYPASS PIN
UNDER-VOLTAGE
+
-
CURRENT LIMIT
COMPARATOR
+
-
VI
LIMIT
JITTER
CLOCK
DC
MAX
OSCILLATOR
FEEDBACK
(FB)
1.65 V -V
T
S
R
Q
Q
LEADING
EDGE
BLANKING
THERMAL
SHUTDOWN
SOURCE
(S)
PI-2367-021105
Figure 2b. Functional Block Diagram (LNK304-306).
2-2
2
Rev. I 11/08
LNK302/304-306
Pin Functional Description
DRAIN (D) Pin:
Power MOSFET drain connection. Provides internal operating
current for both start-up and steady-state operation.
BYPASS (BP) Pin:
Connection point for a 0.1
μF
external bypass capacitor for the
internally generated 5.8 V supply.
FEEDBACK (FB) Pin:
During normal operation, switching of the power MOSFET is
controlled by this pin. MOSFET switching is terminated when
a current greater than 49
μA
is delivered into this pin.
SOURCE (S) Pin:
This pin is the power MOSFET source connection. It is also the
ground reference for the BYPASS and FEEDBACK pins.
The
LinkSwitch-TN
oscillator incorporates circuitry that
introduces a small amount of frequency jitter, typically 4 kHz
peak-to-peak, to minimize EMI emission. The modulation rate
of the frequency jitter is set to 1 kHz to optimize EMI reduction
for both average and quasi-peak emissions. The frequency
jitter should be measured with the oscilloscope triggered at
the falling edge of the DRAIN waveform. The waveform in
Figure 4 illustrates the frequency jitter of the
LinkSwitch-TN.
Feedback Input Circuit
The feedback input circuit at the FB pin consists of a low
impedance source follower output set at 1.65 V. When the current
delivered into this pin exceeds 49
μA,
a low logic level (disable)
is generated at the output of the feedback circuit. This output
is sampled at the beginning of each cycle on the rising edge of
the clock signal. If high, the power MOSFET is turned on for
that cycle (enabled), otherwise the power MOSFET remains off
(disabled). Since the sampling is done only at the beginning of
each cycle, subsequent changes in the FB pin voltage or current
during the remainder of the cycle are ignored.
5.8 V Regulator and 6.3 V Shunt Voltage Clamp
The 5.8 V regulator charges the bypass capacitor connected to
the BYPASS pin to 5.8 V by drawing a current from the voltage
on the DRAIN, whenever the MOSFET is off. The BYPASS
pin is the internal supply voltage node for the
LinkSwitch-TN.
When the MOSFET is on, the
LinkSwitch-TN
runs off of the
energy stored in the bypass capacitor. Extremely low power
consumption of the internal circuitry allows the
LinkSwitch-TN
to operate continuously from the current drawn from the DRAIN
pin. A bypass capacitor value of 0.1
μF
is sufficient for both
high frequency decoupling and energy storage.
In addition, there is a 6.3 V shunt regulator clamping the
BYPASS pin at 6.3 V when current is provided to the BYPASS
pin through an external resistor. This facilitates powering of
LinkSwitch-TN
externally through a bias winding to decrease
the no-load consumption to about 50 mW.
BYPASS Pin Under-Voltage
The BYPASS pin under-voltage circuitry disables the power
MOSFET when the BYPASS pin voltage drops below 4.85 V.
Once the BYPASS pin voltage drops below 4.85 V, it must rise
back to 5.8 V to enable (turn-on) the power MOSFET.
Over-Temperature Protection
The thermal shutdown circuitry senses the die temperature.
The threshold is set at 142
°C
typical with a 75
°C
hysteresis.
When the die temperature rises above this threshold (142
°C)
the
power MOSFET is disabled and remains disabled until the die
temperature falls by 75
°C,
at which point it is re-enabled.
Current Limit
The current limit circuit senses the current in the power MOSFET.
When this current exceeds the internal threshold (I
LIMIT
), the
2-3
3
Rev. I 11/08
P Package (DIP-8B)
G Package (SMD-8B)
S
S
BP
FB
D Package (SO-8C)
1
2
3
4
3a
8
7
S
S
BP
FB
D
1
2
8
7
6
S
S
S
S
4
5
5
D
3b
PI-3491-120706
Figure 3. Pin Configuration.
LinkSwitch-TN
Functional
Description
LinkSwitch-TN
combines a high voltage power MOSFET switch
with a power supply controller in one device. Unlike conventional
PWM (pulse width modulator) controllers,
LinkSwitch-TN
uses
a simple ON/OFF control to regulate the output voltage. The
LinkSwitch-TN
controller consists of an oscillator, feedback
(sense and logic) circuit, 5.8 V regulator, BYPASS pin under-
voltage circuit, over-temperature protection, frequency jittering,
current limit circuit, leading edge blanking and a 700 V power
MOSFET. The
LinkSwitch-TN
incorporates additional circuitry
for auto-restart.
Oscillator
The typical oscillator frequency is internally set to an average
of 66 kHz. Two signals are generated from the oscillator: the
maximum duty cycle signal (DC
MAX
) and the clock signal that
indicates the beginning of each cycle.
LNK302/304-306
PI-3660-081303
600
500
V
DRAIN
400
300
200
100
0
68 kHz
64 kHz
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
fl
ame 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,
fi
re or incandescent material).
0
Time (μs)
20
Figure 4. Frequency Jitter.
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
LEB
) after the power MOSFET
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.
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
rr
)
of 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
fi
lter 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
fi
rst 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
FB
then subsequent cycles will be skipped until the
current reduces below I
FB
. Thus, as the output load is reduced,
more cycles will be skipped and if the load increases, fewer
Applications Example
A 1.44 W Universal Input Buck Converter
The circuit shown in Figure 5 is a typical implementation of a
R1
13.0 kΩ
1%
R3
2.05 kΩ
1%
C3
10
μF
35 V
L1
1 mH
280 mA
D1
UF4005
RF1
8.2
Ω
2W
D3
1N4007
D4
1N4007
L2
1 mH
FB
D
BP
S
C1
100 nF
D2
1N4005GP
12 V,
120 mA
C2
100
μF
16 V
R4
3.3 kΩ
RTN
85-265
VAC
C4
4.7
μF
400 V
C5
4.7
μF
400 V
LinkSwitch-TN
LNK304
PI-3757-112103
Figure 5. Universal Input, 12 V, 120 mA Constant Voltage Power Supply Using LinkSwitch-TN.
2-4
4
Rev. I 11/08
LNK302/304-306
LinkSwitch-TN
RF1
D3
L2
D
FB
R1
D2
BP
AC
INPUT
C4
C5
+
C1
R3
C3
L1
C2
D1
DC
OUTPUT
S
S
S
S
D4
Optimize hatched copper areas (
) for heatsinking and EMI.
PI-3750-121106
Figure 6a. Recommended Printed Circuit Layout for LinkSwitch-TN in a Buck Converter Configuration using P or G Package.
RF1
D3
L2
D
S
S
S
S
D1
C3
D2
C2
L1
+
LinkSwitch-TN
FB
BP
AC
INPUT
C4
C5
C1
R3
R1
DC
OUTPUT
D4
Optimize hatched copper areas (
) for heatsinking and EMI.
PI-4546-011807
Figure 6b. Recommended Printed Circuit Layout for LinkSwitch-TN in a Buck Converter Configuration using D Package
to Bottom Side of the Board.
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.
Key Application Considerations
LinkSwitch-TN
Design Considerations
Output Current Table
Data sheet maximum output current table (Table 1) represents
the maximum practical continuous output current for both
mostly discontinuous conduction mode (MDCM) and continuous
conduction mode (CCM) of operation that can be delivered from
a given
LinkSwitch-TN
device under the following assumed
conditions:
1) Buck converter topology.
2) The minimum DC input voltage is
≥70
V. The value of
input capacitance should be large enough to meet this
criterion.
3) For CCM operation a KRP* of 0.4.
4) Output voltage of 12 VDC.
5) Efficiency of 75%.
6) A catch/freewheeling diode with t
rr
≤75
ns is used for
MDCM operation and for CCM operation, a diode with
t
rr
≤35
ns is used.
7) The part is board mounted with SOURCE pins soldered
to a sufficient area of copper to keep the SOURCE pin
temperature at or below 100
°C.
*KRP is the ratio of ripple to peak inductor current.
LinkSwitch-TN
Selection and Selection Between
MDCM and CCM Operation
Select the
LinkSwitch-TN
device, freewheeling diode and output
inductor that gives the lowest overall cost. In general, MDCM
2-5
5
Rev. I 11/08
ST - Low Power RF
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