LNK304-306
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
• 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
®
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)
230 VAC
±
15%
PRODUCT
(4)
85-265 VAC
MDCM
(2)
CCM
(3)
MDCM
(2)
CCM
(3)
LNK304P or G 120 mA 170 mA 120 mA 170 mA
LNK305P or G 175 mA 280 mA 175 mA 280 mA
LNK306P or G 225 mA 360 mA 225 mA 360 mA
Table 1.
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. Please see
ordering information.
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 Blue Angel, Energy Star, and EU requirements
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 flyback converters. The fully
integrated auto-restart circuit 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 2004
LNK304-306
BYPASS
(BP)
REGULATOR
5.8 V
FAULT
PRESENT
AUTO-
RESTART
COUNTER
CLOCK
RESET
6.3 V
5.8 V
4.85 V
BYPASS PIN
UNDER-VOLTAGE
DRAIN
(D)
49
µA
+
-
CURRENT LIMIT
COMPARATOR
+
-
VI
LIMIT
JITTER
CLOCK
DC
MAX
OSCILLATOR
FEEDBACK
(FB)
1.63 V -V
TH
S
R
Q
Q
LEADING
EDGE
BLANKING
THERMAL
SHUTDOWN
SOURCE
(S)
PI-2367-112503
Figure 2. Functional Block Diagram.
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 uF 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.
P Package (DIP-8B)
G Package (SMD-8B)
S
S
BP
FB
1
2
3
4
8
7
S
S
5
D
PI-3491-111903
Figure 3. Pin Configuration.
2
D
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LNK304-306
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.
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
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
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.
PI-3660-081303
600
500
400
300
200
100
0
68 kHz
64 kHz
V
DRAIN
0
20
Time (µs)
Figure 4. Frequency Jitter.
D
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3
LNK304-306
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
Figure 5. Universal Input, 12 V, 120 mA Constant Voltage Power Supply Using LinkSwitch-TN.
PI-3757-112103
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 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 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
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, 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.
4
D
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LNK304-306
LinkSwitch-TN
RF1
D1
L2
D
FB
R4
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-3750-112103
Figure 6. Recommended Printed Circuit Layout for LinkSwitch-TN in a Buck Converter Configuration.
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 ultra-fast (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 opto-coupler 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.
Component Selection
Referring to Figure 5, the following considerations may be
helpful in selecting components for a
LinkSwitch-TN
design.
Freewheeling Diode D1
Diode D1 should be an ultra-fast type. For MDCM, reverse
recovery time t
rr
≤75
ns should be used at a temperature of
70
°C
or below. Slower diodes are not acceptable, as continuous
mode operation will always occur during startup, causing high
leading edge current spikes, terminating the switching cycle
prematurely, and preventing the output from reaching regulation.
If the ambient temperature is above 70
°C
then a diode with
t
rr
≤35
ns should be used.
For CCM an ultra-fast diode with reverse recovery time
t
rr
≤35
ns should be used. A slower diode may cause excessive
leading edge current spikes, terminating the switching cycle
prematurely and preventing full power delivery.
Fast and slow diodes should never be used as the large reverse
recovery currents can cause excessive power dissipation in the
diode and/or exceed the maximum drain current specification
of
LinkSwitch-TN.
Feedback Diode D2
Diode D2 can be a low-cost slow diode such as the 1N400X
series, however it should be specified as a glass passivated type
to guarantee a specified reverse recovery time. To a first order,
the forward drops of D1 and D2 should match.
Inductor L1
Choose any standard off-the-shelf inductor that meets the
design requirements. A “drum” or “dog bone” “I” core inductor
is recommended with a single ferrite element due to to its low
cost and very low audible noise properties. The typical
inductance value and RMS current rating can be obtained from
the
LinkSwitch-TN
design spreadsheet available within the
PI Expert
design suite from Power Integrations. Choose L1
greater than or equal to the typical calculated inductance with
RMS current rating greater than or equal to calculated RMS
inductor current.
Capacitor C2
The primary function of capacitor C2 is to smooth the inductor
current. The actual output ripple voltage is a function of this
capacitor’s ESR. To a first order, the ESR of this capacitor
should not exceed the rated ripple voltage divided by the typical
current limit of the chosen
LinkSwitch-TN.
D
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