LNK454/456-458/460
LinkSwitch-PL
Family
™
LED Driver IC with TRIAC Dimming, Single-Stage PFC and
Constant Current Control for Non-Isolated Applications
Product Highlights
Dramatically Simplifies Off-line LED Drivers
•
Flicker-free phase-controlled TRIAC dimming
•
Single-stage power factor correction and accurate constant
current (CC) output
•
Very low component count with small non-electrolytic bulk
capacitor for compact replacement lamp designs
•
Compact SO8, eSOP, and eDIP packages
•
Completely eliminates control loop compensation
Advanced Performance Features
•
Optimized for non-isolated flyback designs
•
Frequency jitter greatly reduces EMI filter size and costs
•
Low dissipation direct sensing of LED current
Advanced Protection and Safety Features
•
Cycle skipping regulation for abnormally low output power to
clamp peak output current delivered
•
725 V integrated power MOSFET allows small bulk capacitance
and maximizes power capability
•
Short-circuit, overload, open feedback and output overvoltage
protection
•
Hysteretic thermal shutdown
•
Meets high-voltage creepage between DRAIN and all other pins
both on PCB and at package
EcoSmart
™
- Energy Efficient
•
High power factor optimizes system lumen per input VA
•
Control algorithm balances switching and conduction losses
over line and load to maintain optimum efficiency
Figure 1.
AC
IN
D
LinkSwitch-PL
CONTROL
BP
S
FB
PI-5835-060710
Basic Application Schematic.
Output Power Table
Product
2
LNK454D
LNK456D
LNK457D/K/V
LNK458K/V
LNK460K/V
85-265 VAC
Minimum Output
Maximum Output
Power
Power
1
1.5 W
3W
3W
6W
4W
8W
6W
11.5 W
8W
16 W
Description
The LinkSwitch-PL family enables a very small and low cost
single-stage power factor corrected constant current driver for
solid state lighting. Optimized for direct LED current sensing, the
LinkSwitch-PL operates over a wide input voltage range delivering
an output power of up to 16 W. The LinkSwitch-PL control
algorithm provides flicker-free TRIAC dimming with minimal
external components.
Each device incorporates a 725 V rated power MOSFET, a novel
discontinuous mode variable frequency variable on-time controller,
frequency jitter, cycle-by-cycle current limit and hysteretic thermal
shutdown in a monolithic 4-pin IC, available in SO-8C, eSOP-12,
and eDIP-12 packages.
Table 1. Output Power Table.
Notes:
1. Maximum practical continuous power in an open frame design with adequate
heat sinking, measured at +50 °C ambient (see Key Applications Considerations
for more information).
2. Packages: D: SO-8C, K: eSOP-12, V: eDIP-12.
Output Current
Number of
Serial LEDs
1
2
3
4
5
6
7
8
9
10
11
12
Figure 2.
350 mA
LNK454
LNK454
LNK456
LNK456
LNK457
LNK457
LNK458
LNK458
LNK458
LNK460
LNK460
LNK460
500 mA
LNK454
LNK456
LNK456
LNK457
LNK458
LNK458
LNK460
LNK460
LNK460
700 mA
LNK454
LNK456
LNK457
LNK458
LNK460
LNK460
1000 mA
LNK456
LNK457
LNK458
LNK460
Device Selection Based on Length of Output LED Series String and
Current. A Typical Voltage Drop of 3.5 V per LED is Assumed.
www.powerint.com
October 2011
LNK454/456-458/460
BYPASS (BP)
UV
4.9 V
I
LIM
REGULATOR
5.85 V
DRAIN (D)
I
LIM
+
V_I
LIM
SOA
Q
V
FB(SK)
+
SET
S
CURRENT LIMIT
SOA
STATE MACHINE
Q
CLR
R
UV
V
FB(LO)
+
I
FB
1
µA
FEEDBACK (FB)
+
V
REF
DAC
PHASE
MEASUREMENT
Zero Crossing
+
V_Z
LIM
FILTER
Update
CLK
DIGITAL
INTEGRATOR
INC/DEC
FREQUENCY/
DUTY CYCLE
CONTROLLER
AUTO-RESTART
S
SET
Q
R
ON-TIME
EXTENSION
CLR
Q
PI-5893-091010
SOURCE (S)
Figure 2.
Functional Block Diagram.
Pin Functional Description
DRAIN (D) Pin:
High-voltage power MOSFET drain connection. The internal
start-up bias current is drawn from this pin through a switched
high-voltage current source. Drain current sensing and
associated controller functions are also performed using this pin.
SOURCE (S) Pin:
Power MOSFET source connection. Ground reference for
BYPASS and FEEDBACK pins.
BYPASS (BP) Pin:
Connection point for the external bypass capacitor for the
internally generated 5.85 V supply.
FEEDBACK (FB) Pin:
LED current sensing pin. During normal operation the 290 mV
threshold determines the average value of the current flowing
through the load sense resistor.
A second threshold clamps excessive output current ripple.
A third higher threshold is used to protect against output
short-circuit and overvoltage conditions (see Figure 5).
D Package (SO-8C)
FB
BP
1
2
8
7
6
D
4
5
S
S
S
S
Exposed Pad (On Bottom)
Internally Connected to
SOURCE Pin
K Package
(eSOP-12)
NC 1
FB 2
BP 3
NC 4
12 S
11 S
10 S
9S
8S
7S
Exposed Pad Internally
Connected to SOURCE Pin
D6
V Package (eDIP-12)
S 12
S 11
S 10
S 9
S 8
S 7
Figure 3.
Pin Configuration (Top View).
1 NC
2 FB
3 BP
4 NC
6 D
PI-5836a-061311
2
Rev. C 10/11
www.powerint.com
LNK454/456-458/460
D
ES
DZ
OV
AC
IN
R
ES
D
R
OV
LinkSwitch-PL
CONTROL
BP
S
FB
C
F
R
SENSE
R
F
PI-5837-060710
Figure 4.
Typical Application Schematic.
Functional Description
The LinkSwitch-PL combines a high-voltage power MOSFET
switch with a power supply controller in one device. The IC
provides a single-stage power factor correction plus LED
current control. The LinkSwitch-PL controller consists of an
oscillator, feedback (sense and logic) circuit, 5.85 V regulator,
hysteretic over-temperature protection, frequency jittering,
cycle-by-cycle current limit, loop compensation circuitry, auto-
restart, switching on-time extension, power factor and constant
current control.
In a direct LED current sensing configuration, the average
FEEDBACK pin voltage is a replica of the LED current, scaled
by the sense resistor (R
SENSE
in Figure 4). A small low-pass filter
(R
F
and C
F
in Figure 4) reduces high frequency noise at the
FEEDBACK pin.
Figure 5 illustrates the operating regions of the FEEDBACK pin
voltage. The LinkSwitch-PL sets its operating point such that
the average FEEDBACK pin voltage in steady-state operation is
290 mV. This threshold is set low to minimize the sensing
resistor dissipation. The internal MOSFET switching frequency
and on-time are updated once every input AC half-cycle to
regulate the output current and maintain high power factor.
If the FEEDBACK pin peak voltage exceeds 550 mV, cycle
skipping mode is triggered and the power processed by the
integrated power MOSFET is clamped on a cycle-by-cycle
basis. Switching frequency may vary during an input voltage
half-cycle to reduce thermal stress on the output LEDs.
Auto-restart protection is triggered by a FEEDBACK pin voltage
in excess of 2 V. This feature can be used to provide output
overvoltage protection (via DZ
OV
and R
OV
, in Figure 4), which
triggers the IC to enter auto-restart.
Auto-Restart
2V
Cycle Skipping
Mode
550 mV
Normal Operation
290 mV
PI-5838-060111
Figure 5.
FEEDBACK Pin Operational Voltage Thresholds.
3
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Rev. C 10/11
LNK454/456-458/460
V
FB
ϕ
OS
ϕ
OL
ϕ
OL
ϕ
OS
V
FB(
ϕ
)
V
FB(
ϕ
)
Phase
Angle
0°
V
LINE
V
TRIAC
180°
0°
180°
Phase
Angle
ϕ
0°
Phase
Angle
Conduction
Angle
Phase
Angle
0°
Conduction
Angle
Trailing Edge
Dimmers
ϕ
Phase
Angle
Phase
Angle
Leading Edge
TRIAC Dimmers
PI-5894a-091010
Figure 6.
Feedback Voltage vs. Phase Angle Dimming Characteristics.
TRIAC (Phase-Controlled) Dimming
The LinkSwitch-PL integrates several features to improve
dimming range and reduce external circuit complexity when
using a phase-controlled TRIAC dimmer. The output LED
current is controlled by the FEEDBACK pin voltage which
changes proportionally to the TRIAC dimmer conduction angle.
When the conduction angle decreases, the voltage at the
FEEDBACK pin decreases causing the average LED current to
decrease.
The FEEDBACK pin reference voltage adjustment is initiated at
approximately 25% of the AC line half-cycle duration. When this
(
j
OS) threshold is exceeded, V
FB
and the output LED current
are reduced until a second phase angle threshold is reached.
At this point, with the TRIAC conduction angle being very
limited, the IC runs open loop at constant frequency and duty
cycle (
j
OL region) and the integrated power MOSFET processes
as much power as the heavily chopped input voltage will allow
creating a light output that is deeply dimmed.
The 520 mV clamping feedback threshold is also linearly reduced
during dimming to control LED current ripple.
4
Rev. C 10/11
www.powerint.com
LNK454/456-458/460
IC Supply and BYPASS Pin
The internal 5.85 V regulator charges the bypass capacitor
connected to the BYPASS pin to 5.85 V by drawing current
from the voltage on the DRAIN pin whenever the power MOSFET
is off. The BYPASS pin is the internal supply voltage node.
When the power MOSFET is on, the device operates from the
energy stored in the bypass capacitor. Extremely low power
consumption of the internal circuitry allows LinkSwitch-PL to
operate continuously from current it takes from the DRAIN pin.
A bypass capacitor value of 1
µF
is sufficient for both high
frequency decoupling and energy storage. Dimming
applications may require a higher bypass capacitor value.
During phase angle dimming when the conduction angle is
small the AC input voltage is present for only short periods of
time. In that case the IC should not rely on the integrated
high-voltage current source, but instead external bias circuitry
should be used to supply the IC from the output (D
ES
and R
ES
in
Figure 4). If the output voltage is less than 7 V, external bias
circuitry should be implemented. This is accomplished by
adding an auxiliary winding on the transformer, which is then
rectified and filtered via a diode (ultrafast) and capacitor. The
winding voltage (turns) should be selected such that the maximum
IC consumption can be supported at the lowest operating
output current.
Start-up, Switching Frequency and On-time Range
At start-up the controller uses an initial switching frequency f
MIN
and minimum on-time t
ON(MIN)
. The charging of the output
capacitor together with the energy delivery to the output LEDs
determines a step-by-step increase of the power MOSFET
switching frequency and on-time updated every half-cycle of the
AC input voltage.
The steady-state switching frequency and on-time are
determined by the line voltage, voltage drop across the LEDs
and converter efficiency.
At light load when the device reaches the minimum frequency
f
MIN
and on-time t
ON(MIN)
, the controller regulates by skipping
cycles. In this mode of operation the input current is not power
factor corrected and the average output current is not guaranteed
to fall within the normal range. A properly designed supply will
not operate in this mode under normal load conditions.
A power supply designed correctly will operate within the
switching frequency range [f
MIN
… f
MAX
], with an on-time falling
between t
ON(MIN)
and t
ON(MAX)
when connected to a normal load.
Overload Protection
In case of overload, the system will increase the operating
frequency and on-time each AC half-cycle until the maximum
frequency and maximum on-time are reached. When this state
is reached, the controller enters auto-restart protection, thus
inhibiting the gate of the power MOSFET for approximately
1.28 s if the main line frequency is 50 Hz, 1.02 s if it is 60 Hz.
After this auto-restart off-time expires, the power MOSFET is
re-enabled and a normal start-up is initiated, i.e. at f
MIN
and
t
ON(MIN)
, stepping up until regulation is achieved again. In case of
a persistent overload condition, the auto-restart duty cycle DC
AR
is ~33%.
Overload protection is inhibited during phase dimming when the
TRIAC conduction duty cycle is less than 60%.
Output Overvoltage Protection
If a no-load condition is present on the output of the supply, the
output overvoltage Zener (DZ
OV
in Figure 4) will conduct once its
threshold is reached. A voltage V
OV
in excess of V
FB(AR)
= 2 V will
appear across the FEEDBACK pin and the IC will enter auto-
restart.
Output Short-Circuit
If the output of the supply (i.e. the LED load) is short-circuited,
then a large amount of energy will be delivered to the sense
resistor, generating a high-voltage at the FEEDBACK pin. If this
condition develops more than 2 V on the FEEDBACK pin, then
the IC will interpret this event as an output short-circuit and will
enter auto-restart.
Safe Operating Area (SOA) Protection
If 3 consecutive cycles of the power MOSFET are prematurely
terminated due to the power MOSFET current exceeding the
current limit after the leading edge blanking time, SOA protection
mode is triggered and the IC will enter auto-restart.
Hysteretic Thermal Shutdown
The thermal shutdown circuitry senses the die junction
temperature. The thermal shutdown threshold is set to 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 the power MOSFET is re-enabled.
5
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Rev. C 10/11
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