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LYT4221-4228/4321-4328
LYTSwitch-4 High Power LED Driver IC Family
Single-Stage Accurate Primary-Side Constant Current (CC) Controller with
PFC for High-Line Applications with TRIAC Dimming and Non-Dimming Options
Product Highlights
Better than ±5% CC regulation
TRIAC dimmable to less than 5% output
Fast start-up
<250 ms at full brightness
<1s at 10% brightness
High power factor >0.9
Easily meets EN61000-3-2
Less than 10% THD in optimized designs
Up to 92% efficient
132 kHz switching frequency for small magnetics
High Performance, Combined Driver, Controller, Switch
The LYTSwitch-4 family enables off-line LED drivers with high
power factor which easily meet international requirements for
THD and harmonics. Output current is tightly regulated with
better than ±5% CC tolerance1. Efficiency of up to 92% is easily
achieved in typical applications.
Supports a Wide Selection of TRIAC Dimmers
The LYTSwitch-4 family provides excellent turn-on characteristics
for leading-edge and trailing-edge TRIAC dimming applications.
This results in drivers with a wide dimming range and fast
start-up, even when turning on from a low conduction angle –
large dimming ratio and low “pop-on” current.
Low Solution Cost and Long Lifetime
LYTSwitch-4 ICs are highly integrated and employ a primary-side
control technique that eliminates the optoisolator and reduces
component count. This allows the use of low-cost single-sided
printed circuit boards. Combining PFC and CC functions into a
single-stage also helps reduce cost and increase efficiency.
The 132 kHz switching frequency permits the use of small,
low-cost magnetics.
LED drivers using the LYTSwitch-4 family do not use primary-
side aluminum electrolytic bulk capacitors. This means greatly
extended driver lifetime, especially in bulb and other high
temperature applications.
eSIP-7C (E Package)
Figure 2. Package Options.
AC
IN D V LYTSwitch-4
CONTROL
BP
S R FB
Figure 1. Typical Schematic.
PI-6800-050913
Optimized for Different Applications and Power Levels
Part Number
LYT4221-LYT4228
LYT4321-LYT4328
Input Voltage Range
160-308 VAC
160-308 VAC
TRIAC Dimmable
No
Yes
Output Power Table1,2
Product 6 Minimum Output Power 3 Maximum Output Power 4
LYT4x21E5
6W
12 W
LYT4x22E
6W
15 W
LYT4x23E
8W
18 W
LYT4x24E
9W
22 W
LYT4x25E
11 W
25 W
LYT4x26E
14 W
35 W
LYT4x27E
19 W
50 W
LYT4x28E
33 W
78 W
Table 1. Output Power Table.
Notes:
1. Performance for typical design. See Application Note.
2. Continuous power in an open frame design with adequate heat sinking; device
local ambient of 70 °C. Power level calculated assuming a typical LED string
voltage and efficiency >80%.
3.
4.
5.
6.
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This Product is Covered by Patents and/or Pending Patent Applications.
November 2014

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LYT4221-4228/4321-4328
Topology
Isolated Flyback
Buck
Tapped Buck
Buck-Boost
Isolation
Yes
No
No
No
Efficiency
88%
92%
89%
90%
Cost
High
Low
Middle
Low
Table 2. Performance of Different Topologies in a Typical Non-Dimmable 10 W High-Line Design.
THD
Best
Good
Best
Best
Output Voltage
Any
Limited
Any
High-Voltage
Typical Circuit Schematic
AC
IN
D V LYTSwitch-4
CONTROL
BP
S R FB
Figure 3a. Typical Isolated Flyback Schematic.
PI-6800-050913
AC
IN
D V LYTSwitch-4
CONTROL
BP
S R FB
Figure 3b. Typical Buck Schematic.
PI-6841-111813
LYTSwitch-4
AC
IN D V
CONTROL
BP
S R FB
Key Features
Flyback
Benefits
Provides isolated output
Supports widest range of output voltages
Very good THD performance
Limitations
Flyback transformer
Overall efficiency reduced by parasitic capacitance
and inductance in the transformer
Larger PCB area to meet isolation requirements
Requires additional components (primary clamp and bias)
Higher RMS switch and winding currents increases losses
and lowers efficiency
Buck
Benefits
Highest efficiency
Lowest component count – small size
Simple low-cost power inductor
Low drain source voltage stress
Best EMI/lowest component count for filter
Limitations
Single input line voltage range
Output voltage <0.6 × VIN(AC) × 1.41
Output voltage for low THD designs
Non-isolated
Tapped Buck
Benefits
Ideal for low output voltage designs (<20 V)
High efficiency
Low component count
Simple low-cost tapped inductor
Limitations
Designs best suited for single input line voltage
Requires additional components (primary clamp)
Non-isolated
Figure 3c. Typical Tapped Buck Schematic.
PI-6842-111813
AC
IN
D V LYTSwitch-4
CONTROL
BP
S R FB
Figure 3d. Typical Buck-Boost Schematic.
PI-6859-111813
Buck-Boost
Benefits
Ideal for non-isolated high output voltage designs
High efficiency
Low component count
Simple common low-cost power inductor can be used
Lowest THD
Limitations
Maximum VOUT is limited by MOSFET breakdown voltage
Single input line voltage range
Non-isolated
2
Rev. C 11/14
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LYT4221-4228/4321-4328
BYPASS (BP)
VOLTAGE
MONITOR (V)
FEEDBACK (FB)
REFERENCE (R)
BYPASS
CAPACITOR
SELECT
ILIM MI
1V
STOP
LOGIC
3-VT
LINE OV
SENSE
VBG
5.9 V
REGULATOR
SOFT-START
TIMER
AUTO-RESTART
COUNTER
FAULT
PRESENT
HYSTERETIC
THERMAL
SHUTDOWN
5.9 V
5.0 V
BYPASS PIN
UNDERVOLTAGE
JITTER
CLOCK
OSCILLATOR
Comparator
-
+ DFCBMOAFXF
OCP
Gate
Driver
SenseFet
LEB
+
IV PFC/CC
CONTROL
IFB
MI
CURRENT LIMIT
COMPARATOR
-
ILIM
VSENSE
FEEDBACK
SENSE
6.4 V
FBOFF
DCMAX
REFERENCE
BLOCK
VBG
IS
DRAIN (D)
PI-6843-071112
SOURCE (S)
Figure 4. Functional Block Diagram.
Pin Functional Description
DRAIN (D) Pin:
This pin is the power FET drain connection. It also provides
internal operating current for both start-up and steady-state
operation.
SOURCE (S) Pin:
This pin is the power FET source connection. It is also the
ground reference for the BYPASS, FEEDBACK, REFERENCE
and VOLTAGE MONITOR pins.
BYPASS (BP) Pin:
This is the connection point for an external bypass capacitor for
the internally generated 5.9 V supply. This pin also provides
output power selection through choice of the BYPASS pin
capacitor value.
FEEDBACK (FB) Pin:
The FEEDBACK pin is used for output voltage feedback. The
current into the FEEDBACK pin is directly proportional to the
output voltage. The FEEDBACK pin also includes circuitry to
protect against open load and overload output conditions.
REFERENCE (R) Pin:
This pin is connected to an external precision resistor and is
configured to use only 24.9 kW for non-dimming and dimming.
VOLTAGE MONITOR (V) Pin:
This pin interfaces with an external input line peak detector,
consisting of a rectifier, filter capacitor and resistors. The
applied current is used to control stop logic for overvoltage (OV),
provide feed-forward to control the output current and the
remote ON/OFF function.
Exposed Pad
(Backside) Internally
Connected to
SOURCE Pin (see
eSIP-7C Package
Drawing)
E Package (eSIP-7C)
(Top View)
Figure 5. Pin Configuration.
PI-7076-062513
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3
Rev. C 11/14

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LYT4221-4228/4321-4328
Functional Description
A LYTSwitch-4 device monolithically combines a controller and
high-voltage power FET into one package. The controller
provides both high power factor and constant current output in
a single-stage. The LYTSwitch-4 controller consists of an
oscillator, feedback (sense and logic) circuit, 5.9 V regulator,
hysteretic over-temperature protection, frequency jittering,
cycle-by-cycle current limit, auto-restart, inductance correction,
power factor and constant current control.
FEEDBACK Pin Current Control Characteristics
The figure shown below illustrates the operating boundaries of
the FEEDBACK
and below IFB(AR)
pin
the
cduervreicnet.enAtbeorsveinItFoB(SaKuIPt)os-wreitscthairnt.g
is
disabled
IFB(SKIP)
Skip-Cycle
BYPASS Pin Capacitor Power Gain Selection
LYTSwitch-4 devices have the capability to tailor the internal
gain to either full or a reduced output power setting. This allows
selection of a larger device to minimize dissipation for both
thermal and efficiency reasons. The power gain is selected with
the value of the BYPASS pin capacitor. The full power setting is
selected with a 4.7 mF capacitor and the reduced power setting
(for higher efficiency) is selected with a 47 mF capacitor. The
BYPASS pin capacitor sets both the internal power gain as well
as the over-current protection (OCP) threshold. Unlike the
larger devices, the LYT4x21 power gain is not programmable.
Use a 47 mF capacitor for the LYT4x21.
Switching Frequency
The switching frequency is 132 kHz during normal operation.
To further reduce the EMI level, the switching frequency is
jittered (frequency modulated) by approximately 5.4 kHz.
During start-up the frequency is 66 kHz to reduce start-up time
when the AC input is phase angle dimmed. Jitter is disabled in
deep dimming.
IFB
CC Control
Region
IFB(DCMAXR)
Soft-Start
Region
Soft-Start
The controller includes a soft-start timing feature which inhibits
the auto-restart protection
to distinguish start-up into
afefaatuulrte(sfhoor rtth-ecisrcouftit-)sftraormt paerlaiordge(tSOFT)
output capacitor. At start-up the LYTSwitch-4 clamps the
maximum duty cycle to reduce the output power. The total
soft-start period is tSOFT.
Remote ON/OFF and EcoSmart
The VOLTAGE MONITOR pin has a 1 V threshold comparator
connected at its input. This voltage threshold is used for
remote ON/OFF control. When a signal is received at the
VOLTAGE MONITOR pin to disable the output (VOLTAGE
MONITOR pin tied to ground through an optocoupler photo-
transistor) the LYTSwitch-4 will complete its current switching
cycle before the internal power FET is forced off.
IFB(AR)
Auto-Restart
DC10
Maximum Duty Cycle
Figure 6. FEEDBACK Pin Current Characteristic.
DCMAX
PI-6978-040213
The FEEDBACK pin current is also used to clamp the maximum
duty cycle to limit the available output power for overload and
open-loop conditions. This duty cycle reduction characteristic
also promotes a monotonic output current start-up characteristic
and helps preventing over-shoot.
The remote ON/OFF feature can also be used as an eco-mode
or power switch to turn off the LYTSwitch-4 and keep it in a
very low power consumption state for indefinite long periods.
When the LYTSwitch-4 is remotely turned on after entering this
mode, it will initiate a normal start-up sequence with soft-start
the next time the BYPASS pin reaches 5.9 V. In the worst case,
the delay from remote on to start-up can be equal to the full
discharge/charge cycle time of the BYPASS pin. This reduced
consumption remote off mode can eliminate expensive and
unreliable in-line mechanical switches.
REFERENCE Pin
The REFERENCE pin is tied to ground (SOURCE) via an external
resistor. The value selected sets the internal references and it
should be 24.9 k±1%. One percent resistors are recommended
as the resistor tolerance directly affects the output tolerance.
Other resistor values should not be used.
4
Rev. C 11/14
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LYT4221-4228/4321-4328
DV
CONTROL
BP
S R FB
PI-5435-052510
Figure 7. Remote ON/OFF VOLTAGE MONITOR Pin Control.
5.9 V Regulator/Shunt Voltage Clamp
The internal 5.9 V regulator charges the bypass capacitor
connected to the BYPASS pin to 5.9 V by drawing a current
from the voltage on the DRAIN pin whenever the power FET is
off. The BYPASS pin is the internal supply voltage node. When
the power FET is on, the device operates from the energy stored
in the bypass capacitor. Extremely low power consumption of the
internal circuitry allows LYTSwitch-4 to operate continuously from
current it takes from the DRAIN pin. A bypass capacitor value
of 47 or 4.7 mF is sufficient for both high frequency decoupling
and energy storage. In addition, there is a 6.4 V shunt regulator
clamping the BYPASS pin at 6.4 V when current is provided to
the BYPASS pin through an external resistor. This facilitates
powering of LYTSwitch-4 externally through a bias winding to
increase operating efficiency. It is recommended that the
BYPASS pin is supplied current from the bias winding for
normal operation.
Auto-Restart
In the event of an open-loop fault (open FEEDBACK pin resistor
or broken path to feedback winding), output short-circuits or an
overload condition the controller enters into the auto-restart
mode. The controller annunciates both short-circuit and
open-loop conditions once the FEEDBACK pin current falls
below the
the power
IdFBi(sARs)iptharteiosnhouldndaeftretrhtihsefasuoltft-csotnardtitpioenriothde.
To minimize
shutdown/
auto-restart circuit turns the power supply on (same as the
soft-start period) and off at an auto-restart duty cycle of
tfayuplitciasllyreDmCoAvRefdordausrinlogntgheasauthtoe-fraeuslttacrot nodff-ittiiomnep, ethrseisptos.weIfrthe
supply will remain in auto-restart until the full off-time count is
completed. Special consideration must be made to appropriately
size the output capacitor to ensure that after the soft-start
period (tSOFT) the FEEDBACK pin current is above the IFB(AR)
threshold to ensure successful power-supply start-up. After the
soft-start time period, auto-restart is activated only when the
FEEDBACK pin current falls below IFB(AR).
Over-Current Protection
The current limit circuit senses the current in the power FET.
When this current exceeds the internal threshold (ILIMIT), the power
FET is turned off for the remainder of that cycle. A leading edge
blanking circuit inhibits the current limit comparator for a short
tbimlanek(tinLEgB)taimfteerhtahse
power FET
been set so
is turned on.
that current
This leading edge
spikes caused by
capacitance and rectifier reverse recovery will not cause
premature termination of the power FET conduction.
Line Overvoltage Protection
This device includes overvoltage detection to limit the maximum
operating voltage detected through the VOLTAGE MONITOR pin.
An external peak detector consisting of a diode and capacitor is
required to provide input line peak voltage to the VOLTAGE
MONITOR pin through a resistor.
The resistor sets line overvoltage (OV) shutdown threshold which,
once exceeded, forces the LYTSwitch-4 to stop switching. Once
the line voltage returns to normal, the device resumes normal
operation. A small amount of hysteresis is provided on the OV
threshold to prevent noise-generated toggling. When the power
FET is off, the rectified DC high voltage surge capability is
increased to the voltage rating of the power FET (725 V), due to the
absence of the reflected voltage and leakage spikes on the drain.
Hysteretic Thermal Shutdown
The thermal shutdown circuitry senses the controller 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 FET is disabled and remains disabled until
the die temperature falls by 75 °C, at which point the power FET
is re-enabled.
Safe Operating Area (SOA) Protection
The device also features a safe operating area (SOA) protection
mode which disables FET switching for 40 cycles in the event
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the soft-start period when auto-restart protection is inhibited.
The SOA protection mode remains active in normal operation.
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5
Rev. C 11/14