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 filter cost
•
High thermal shutdown temperature (+135
°C
minimum)
Much Higher Performance Over Discrete Buck and
Passive Solutions
•
Supports buck, buck-boost and flyback 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 flyback topology with
external bias at 115/230 VAC input with no-load
•
Meets California Energy Commission (CEC), Energy Star, and
EU requirements
Applications
•
Appliances and timers
•
LED drivers and industrial controls
FB
BP
S
Wide Range LinkSwitch-TN
High-Voltage
DC Input
+
D
DC
Output
+
PI-3492-041509
Figure 1.
Typical Buck Converter Application (See Application Examples Section
for Other Circuit Configurations).
Output Current Table
1
Product
4
LNK302P/G/D
LNK304P/G/D
LNK305P/G/D
LNK306P/G/D
230 VAC ±15%
MDCM
2
63 mA
120 mA
175 mA
225 mA
CCM
3
80 mA
170 mA
280 mA
360 mA
85-265 VAC
MDCM
2
63 mA
120 mA
175 mA
225 mA
CCM
3
80 mA
170 mA
280 mA
360 mA
Table 1. Output Current Table.
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.
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
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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 flyback 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.
January 2019
This Product is Covered by Patents and/or Pending Patent Applications.
LNK302/304-306
BYPASS
(BP)
REGULATOR
5.8 V
DRAIN
(D)
+
BYPASS PIN
UNDERVOLTAGE
CURRENT LIMIT
COMPARATOR
+
-
5.8 V
4.85 V
-
6.3 V
JITTER
CLOCK
DC
MAX
FEEDBACK
(FB)
1.65 V -V
T
S
R
Q
Q
OSCILLATOR
THERMAL
SHUTDOWN
VI
LIMIT
LEADING
EDGE
BLANKING
SOURCE
(S)
PI-3904-032213
Figure 2a. Functional Block Diagram (LNK302).
BYPASS
(BP)
REGULATOR
5.8 V
FAULT
PRESENT
AUTO-
RESTART
COUNTER
CLOCK
RESET
5.8 V
4.85 V
BYPASS PIN
UNDERVOLTAGE
CURRENT LIMIT
COMPARATOR
+
-
DRAIN
(D)
6.3 V
+
-
VI
LIMIT
JITTER
CLOCK
DC
MAX
FEEDBACK
(FB)
1.65 V -V
T
S
R
Q
Q
LEADING
EDGE
BLANKING
OSCILLATOR
THERMAL
SHUTDOWN
SOURCE
(S)
PI-2367-032213
Figure 2b. Functional Block Diagram (LNK304-306).
2
Rev. N 01/19
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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
mF
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
mA
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.
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 FEEDBACK pin consists of a
low impedance source follower output set at 1.65 V. When the
current delivered into this pin exceeds 49
mA,
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 FEEDBACK
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
mF
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 Undervoltage
The BYPASS pin undervoltage 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
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.
P Package (DIP-8B)
G Package (SMD-8B)
S
S
BP
FB
D Package (SO-8C)
BP
FB
1
2
8
7
6
S
S
S
S
1
2
3
4
3a
8
7
S
S
5
D
D
4
5
3b
PI-5422-060613
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
undervoltage 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.
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
3
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Rev. N 01/19
LNK302/304-306
PI-3660-081303
600
500
V
DRAIN
400
300
200
100
0
68 kHz
64 kHz
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
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 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
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 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.
0
Time (µs)
Figure 4.
Frequency Jitter.
20
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
R1
13.0 kΩ
1%
R3
2.05 kΩ
1%
C3
10
µF
35 V
L1
1 mH
280 mA
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
85-265
VAC
C4
4.7
µF
400 V
C5
4.7
µF
400 V
LinkSwitch-TN
LNK304
D1
UF4005
C2
100
µF
16 V
R4
3.3 kΩ
RTN
PI-3757-041509
Figure 5.
Universal Input, 12 V, 120 mA Constant Voltage Power Supply Using LinkSwitch-TN.
4
Rev. N 01/19
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LNK302/304-306
LinkSwitch-TN
RF1
D3
L2
D
FB
BP
R1
D2
+
C3
L1
C2
D1
AC
INPUT
C4
C5
S
S
S
S
C1
R3
DC
OUTPUT
D4
Optimize hatched copper areas (
) for heat sinking and EMI.
PI-3750-061715
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
DC
OUTPUT
L1
+
LinkSwitch-TN
FB
BP
AC
INPUT
C4
C5
C1
R3
R1
D4
Optimize hatched copper areas (
) for heatsinking and EMI.
PI-4546-041509
Figure 6b.
Recommended Printed Circuit Layout for LinkSwitch-TN in a Buck Converter Configuration using D Package to Bottom Side of the Board.
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 tempera-
ture 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 provides the lowest cost and highest efficiency converter.
CCM designs require a larger inductor and ultrafast (t
RR
≤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.
5
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Rev. N 01/19
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