This product is not recommended for new designs.
LNK500
LinkSwitch
Family
Energy Efficient, CV or CV/CC Switcher for
Very Low Cost Chargers and Adapters
Product Highlights
Cost Effective Linear/RCC Replacement
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Lowest cost and component count, constant voltage (CV) or
constant voltage/constant current (CV/CC) solution
Extremely simple circuit configuration
Up to 75% lighter power supply reduces shipping cost
Primary based CV/CC solution eliminates 10 to 20
secondary components for low system cost
Combined primary clamp, feedback, IC supply, and loop
compensation functions – minimizes external components
Fully integrated auto-restart for short circuit and open loop
fault protection – saves external component costs
42 kHz operation simplifies EMI filter design
Universal input range allows worldwide operation
Up to 70% reduction in power dissipation – reduces enclosure
size significantly
CV/CC output characteristic without secondary feedback
System level thermal and current limit protection
Meets all single point failure requirements with only one
additional clamp capacitor
Controlled current in CC region provides inherent soft-start
Optional opto feedback improves output voltage accuracy
Consumes <300 mW at 265 VAC input with no load
Meets California Energy Commission (CEC), Energy Star, and
EU requirements
No current sense resistors – maximizes efficiency
Linear transformer replacement in all ≤3 W applications
Chargers for cell phones, cordless phones, PDAs, digital
cameras, MP3/portable audio devices, shavers, etc.
Home appliances, white goods and consumer electronics
Constant output current LED lighting applications
TV standby and other auxiliary supplies
LinkSwitch
D
C
S
Wide Range
HV DC Input
DC
Output
(V
O
)
(a)
V
O
Example Characteristic
Min Typ
±10%
V
O
±5%
Much Higher Performance Over Linear/RCC
±25%*
For Circuit
Shown Above
I
O
(b)
±25%*
With Optional
Secondary Feedback**
I
O
*Estimated tolerance achievable in high volume production
including transformer and other component tolerances.
**See Optional Secondary Feedback section.
PI-3415-112202
EcoSmart™ – Extremely Energy Efficient
Figure 1. Typical Application – Not a Simplified Circuit (a) and
Output Characteristic Tolerance Envelopes (b).
Output Power Table
1
Product
4
Applications
230 VAC ±15%
Min
2
Typ
2
4W
5.5 W
85-265 VAC
Min
2
2.4 W
Typ
2
3W
No-Load
Input
Power
<300 mW
LNK500
P or G
3.2 W
4.3 W
2.9 W 3.5 W <500 mW
3
Description
LinkSwitch™ is specifically designed to replace low power
linear transformer/RCC chargers and adapters at equal or
lower system cost with much higher performance and energy
efficiency. LNK500 is a lower cost version of the LNK501 with a
wider tolerance output CC characteristic. LinkSwitch introduces
a revolutionary patented topology for the design of low power
switching power supplies that rivals the simplicity and low
cost of linear adapters, and enables a much smaller, lighter,
and attractive package when compared with the traditional
“brick.” With efficiency of up to 75% and <300 mW no-load
Table 1.
Notes: 1. Output power for designs in an enclosed
adapter measured at 50
°C
ambient. 2. See Figure 1 (b) for
Min (CV only designs) and Typ (CV/CC charger designs) power
points identified on output characteristic. 3. Uses higher reflected
voltage transformer designs for increased power capability – see
Key Application Considerations section. 4. For lead-free package
options, see Part Ordering Information.
consumption,
a LinkSwitch solution can save the end user
enough energy over a linear design to completely pay for
the full power supply cost in less than one year.
LinkSwitch
integrates a 700 V power MOSFET, PWM control, high voltage
start-up, current limit, and thermal shutdown circuitry, onto a
monolithic IC.
September 2016
LNK500
CONTROL
VC
ZC
SHUNT REGULATOR/
ERROR AMPLIFIER
-
+
0
DRAIN
INTERNAL
SUPPLY
1
SHUTDOWN/
AUTO-RESTART
+
5.6 V
4.7 V
-
÷
8
CURRENT
LIMIT
ADJUST
CURRENT LIMIT
COMPARATOR
-
+
5.7 V
I
FB
HYSTERETIC
THERMAL
SHUTDOWN
OSCILLATOR
D MAX
CLOCK
SAW
-
+
S
R
Q
PWM
COMPARATOR
IDCS
LOW
FREQUENCY
OPERATION
LEADING
EDGE
EDGE
BLANKING
RE
SOURCE
PI-3416-111802
Figure 2. Block Diagram.
Pin Functional Description
DRAIN (D) Pin:
Power MOSFET drain connection. Provides internal operating
current for start-up. Internal current limit sense point for drain
current.
CONTROL (C) Pin:
Error amplifier and feedback current input pin for duty cycle
and current limit control. Internal shunt regulator connection to
provide internal bias current during normal operation. It is also
used as the connection point for the supply bypass and auto-
restart/compensation capacitor.
SOURCE (S) Pin:
Output MOSFET source connection for high voltage power
return. Primary side control circuit common and reference
point.
S
S
S
S
1
2
3
4
LNK500
8
7
C
S
5
D
P Package (DIP-8B)
G Package (SMD-8B)
Figure 3. Pin Configuration.
PI-3417-111802
2
Rev. E 09/16
www.power.com
LNK500
LinkSwitch Functional Description
The duty cycle, current limit and operating frequency
relationships with CONTROL pin current are shown in
Figure 4. Figure 5 shows a typical power supply outline
schematic which is used below to describe the LinkSwitch
operation.
Power Up
During power up, as V
IN
is first applied (Figure 5), the CONTROL
pin capacitor C1 is charged through a switched high voltage
current source connected internally between the DRAIN and
CONTROL pins (see Figure 2). When the CONTROL pin
voltage reaches approximately 5.6 V relative to the SOURCE
pin, the high voltage current source is turned off, the internal
control circuitry is activated and the high voltage internal
MOSFET starts to switch. At this point, the charge stored on
C1 is used to supply the internal consumption of the chip.
Constant Current (CC) Operation
As the output voltage, and therefore the reflected voltage
across the primary transformer winding ramp up, the feedback
CONTROL current I
C
increases. As shown in Figure 4, the
internal current limit increases with I
C
and reaches I
LIM
when I
C
is equal to I
DCT
. The internal current limit vs. I
C
characteristic is
designed to provide an approximately constant power supply
output current as the power supply output voltage rises.
Constant Voltage (CV) Operation
When I
C
exceeds I
DCS
, typically 2 mA (Figure 4), the maximum
duty cycle is reduced. At a value of I
C
that depends on power
supply input voltage, the duty cycle control limits LinkSwitch
peak current below the internal current limit value. At this point
the power supply transitions from CC to CV operation. With
minimum input voltage in a typical universal input design, this
transition occurs at approximately 30% duty cycle. Resistor R1
(Figure 5) is therefore initially selected to conduct a value of I
C
approximately equal to I
DCT
when V
OUT
is at the desired value
at the minimum power supply input voltage. The final choice
of R1 is made when the rest of the circuit design is complete.
When the duty cycle drops below approximately 4%, the
frequency is reduced, which reduces energy consumption
under light load conditions.
Auto-Restart Operation
When a fault condition, such as an output short circuit or open
loop, prevents flow of an external current into the CONTROL
pin, the capacitor C1 discharges towards 4.7 V. At 4.7 V,
auto-restart is activated, which turns the MOSFET off and puts
the control circuitry in a low current fault protection mode. In
auto-restart, LinkSwitch periodically restarts the power supply
so that normal power supply operation can be restored when
the fault is removed.
Internal Current Limit
Auto-restart
I
LIM
CONTROL Current IC
Duty Cycle
Auto-restart
I
DCT
77%
30%
3.8%
I
CD1
I
DCS
CONTROL Current IC
Frequency
Auto-restart
f
OSC
f
OSC(low)
CONTROL Current IC
PI-2799-112101
Figure 4. CONTROL Characteristics.
LinkSwitch
D
C
S
C1
R1
C2
C4
VOUT
VIN
D1
R2
D2
PI-2715-100801
Figure 5. Power Supply Outline Schematic.
3
www.power.com
Rev. E 09/16
LNK500
The characteristics described above provide an approximate
CV/CC power supply output without the need for secondary-
side voltage or current feedback. The output voltage regulation
is influenced by how well the voltage across C2 tracks the
reflected output voltage. This tracking is influenced by the value
of the transformer leakage inductance which introduces an
error. Resistor R2 and capacitor C2 partially filter the leakage
inductance voltage spike reducing this error. This circuitry, used
with standard transformer construction techniques provides
much better output load regulation than a linear transformer,
making this an ideal power supply solution in many low
power applications. If tighter load regulation is required, an
optocoupler configuration can be used while still employing the
constant output current characteristics provided by LinkSwitch.
Optional Secondary Feedback
Figure 6 shows a typical power supply outline schematic using
LinkSwitch with optocoupler feedback to improve output
voltage regulation. On the primary side, the schematic differs
from Figure 5 by the addition of R3, C3 and optocoupler U1.
R3 forms a potential divider with R1 to limit the U1 collector
emitter voltage.
On the secondary side, the addition of voltage sense circuit
components R4, VR1 and U1 LED provide the voltage
feedback signal. In the example shown, a simple Zener (VR1)
reference is used though a precision TL431 reference is
typically needed to provide ±5% output voltage tolerancing
and cable drop compensation, if required. Resistor R4 provides
biasing for VR1. The regulated output voltage is equal to the
sum of the VR1 Zener voltage plus the forward voltage drop
of the U1 LED. Resistor R5 is an optional low value resistor to
limit U1 LED peak current due to output ripple. Manufacturer’s
specifications for U1 current and VR1 slope resistance should
be consulted to determine whether R5 is required.
U1 is arranged with collector connected to primary ground and
emitter to the anode of D1. This connection keeps the opto
in an electrically “quiet” position in the circuit. If the opto was
instead placed on the cathode side of D1, it would become
LinkSwitch
D
S
C
LNK500
VOUT
C1
C2
U1
R5
R4
R1
85-265
VAC
R2
D1
U1
R3
C3
VR1
RTN
PI-3418-111802
Figure 6. Power Supply Outline Schematic with Optocoupler Feedback.
Output Voltage
Voltage
feedback
threshold
Inherent
CC to CV
transition
point
Tolerance envelope
without optocoupler
Typical inherent
characteristic without
optocoupler
Characteristic with
optocoupler
Load variation
during battery
charging
Output Current
Figure 7. Influence of the Optocoupler on the Power Supply Output Characteristic.
PI-2788-092101
4
Rev. E 09/16
www.power.com
LNK500
a switching node, generating additional common mode EMI
currents through its internal parasitic capacitance.
The feedback configuration in Figure 6 is simply a resistive
divider made up of R1 and R3 with D1, R2, C1 and C2
rectifying, filtering and smoothing the primary winding voltage
signal. The optocoupler therefore effectively adjusts the resistor
divider ratio to control the DC voltage across R1 and therefore,
the feedback current received by the LinkSwitch CONTROL pin.
When the power supply operates in the constant current (CC)
region, for example when charging a battery, the output voltage
is below the voltage feedback threshold defined by U1 and
VR1 and the optocoupler is fully off. In this region, the circuit
behaves exactly as previously described with reference to
Figure 5 where the reflected voltage increases with increasing
output voltage and the LinkSwitch internal current limit is
adjusted to provide an approximate CC output characteristic.
Note that for similar output characteristics in the CC region, the
value of R1 in Figure 5 will be equal to the value of R1 + R3 in
Figure 6.
When the output reaches the voltage feedback threshold set
by U1 and VR1, the optocoupler turns on. Any further increase
in the power supply output voltage results in the U1 transistor
current increasing, which increases the percentage of the
reflected voltage appearing across R1. The resulting increase
in the LinkSwitch CONTROL current reduces the duty cycle
according to Figure 4 and therefore, maintains the output
voltage regulation.
Normally, R1 and R3 are chosen to be equal in value. However,
increasing R3 (while reducing R1 to keep R1 + R3 constant)
increases loop gain in the CV region, improving load regulation.
The extent to which R3 can be increased is limited by opto
transistor voltage and dissipation ratings and should be fully
tested before finalizing a design. The values of C2 and C3 are
less important other than to make sure they are large enough
to have very little influence on the impedance of the voltage
division circuit set up by R1, R3 and U1 at the switching
frequency. Normally, the values of C2 and C3 in Figure 6 are
chosen equal to the value of C2 in Figure 5, though the voltage
rating may be reduced depending on the relative values of R1
and R2 discussed above. See Applications section for typical
values of components.
Figure 7 shows the influence of optocoupler feedback on the
output characteristic. The envelope defined by the dashed
lines represent the worst case power supply DC output voltage
and current tolerances (unit-to-unit and over the input voltage
range) if an optocoupler is not used. A typical example of an
inherent (without optocoupler) output characteristic is shown
dotted. This is the characteristic that would result if U1, R4 and
VR1 were removed. The optocoupler feedback results in the
characteristic shown by the solid line. The load variation arrow
in Figure 7 represents the locus of the output characteristic
normally seen during a battery charging cycle. The two
characteristics are identical as the output voltage rises but then
separate as shown when the voltage feedback threshold is
reached. This is the characteristic seen if the voltage feedback
threshold is above the output voltage at the inherent CC to CV
transition point also indicated in Figure 7.
Figure 8 shows a case where the voltage feedback threshold
is set below the voltage at the inherent CC to CV transition
point. In this case, as the output voltage rises, the secondary
feedback circuit takes control before the inherent CC to CV
transition occurs. In an actual battery charging application, this
simply limits the output voltage to a lower value.
Output Voltage
Inherent
CC to CV
transition
point
V
O(MAX)
Tolerance envelope
without optocoupler
Typical inherent
characteristic without
optocoupler
Characteristic with
optocoupler
Power Supply peak
output power curve
Voltage
feedback
threshold
Load variation
during battery
charging
Characteristic observed with
load variation often applied during
laboratory bench testing
Output Current
Figure 8. Output Characteristic with Optocoupler Regulation (Reduced Voltage Feedback Threshold).
PI-2790-092101
5
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Rev. E 09/16
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