LYT2002-2005
LYTSwitch-2
Family
Energy-Efficient, Accurate Primary-Side Regulation
CC/CV Switcher for LED Lighting Applications
Product Highlights
Accurate CC Regulation, Meets ±3% in a Typical
Design
1
Controller Automatically Compensates For:
•
Transformer inductance variation
•
External component changes with temperature
•
Input line voltage variations
AC
IN
LYTSwitch-2
S
This enhances production yield
•
•
•
•
•
D
FB
BP
Cost-Effective, Small Size Designs
Eliminates the optocoupler and secondary CC control circuitry
Eliminates control loop compensation circuitry
Frequency-jitter greatly reduces EMI filter cost
Programmable switching frequency reduces transformer size
725 V switching power MOSFET enables clampless flyback designs
PI-7037a-051914
Figure 1.
Typical Flyback Implementation – Not a Simplified Circuit.
Advanced Protection/Safety Features
•
Auto-restart protection reduces power delivered by >90% for output
short-circuit and control loop faults (open and short-circuit components)
•
Hysteretic thermal shutdown – with automatic recovery
•
Meets high-voltage creepage requirements between DRAIN and all
other pins both on the PCB and at the package
Output Power Table
2
Product
5
LYT2002D
LYT2003D
LYT2004D
LYT2004E/K
LYT2005E/K
90-308 VAC
Enclosed Bulb
3
5W
6W
7W
9W
10 W
Ballast Driver
4
6W
7W
8W
10 W
12 W
EcoSmart™–
Energy Efficient
•
No-load consumption <30 mW
1
•
No current sense resistors – maximizes efficiency
Green Package
Applications
•
All parts are halogen free and RoHS compliant
•
LED bulbs, down lights, luminaires, ballasts and T8 tubes
Description
The LYTSwitch™-2 family of ICs dramatically simplifies low power CC
LED drivers by eliminating the optocoupler and secondary control
circuitry. The family introduces a revolutionary control technique which
provides accurate output current regulation, compensating for
transformer and external component variations, and device parameter
tolerances as well as input voltage variations.
The device incorporates a high-voltage switching MOSFET, an ON/OFF
control state-machine, a high-voltage switched current source for
self-biasing, frequency jitter to reduce EMI, cycle-by-cycle current limit
and hysteretic thermal shutdown circuitry into a monolithic IC. This high
level of integration enables cost-effective designs with very few
external components, reducing solution cost and driver size.
LYTSwitch-2 parts minimize energy consumption when the output is
unloaded. Practical designs can easily achieve less than 30 mW
no-load consumption.
The 725 V power MOSFET used in LYTSwitch-2 devices increases the
circuits’ ability to withstand input surge. In addition, each package is
designed to maximize the creepage distance between high-voltage pins
and logic level inputs. This increased pin spacing increases driver
lifetime and reliability in polluted environments. These in-built protec-
tion features also protect the entire circuit from excess temperature
operation, increasing lifetime in thermally challenging environments.
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Table 1. Output Power Table.
Notes:
1. Nominal input and bias supply applied to BYPASS pin.
2. Performance for typical design.
3. Maximum continuous power in a typical non-ventilated bulb measured at +50
°C ambient, device T
J
≤ 100 °C.
4. Maximum practical continuous power in an open frame design with adequate
heat sinking, measured at +50 °C.
5. Packages: D: SO-8C, E: eSIP-7C, K: eSOP-12B.
September 2015
This Product is Covered by Patents and/or Pending Patent Applications.
LYT2002-2005
BYPASS
(BP)
+
REGULATOR
6V
+
DRAIN
(D)
FEEDBACK
(FB)
V
TH
-
D
t
SAMPLE-OUT
Q
FB
OUT
STATE
MACHINE
I
LIM
DC
MAX
Reset
V
ILIMIT
Drive
6V
5V
-
6.5 V
INDUCTANCE
CORRECTION
FB
t
SAMPLE-INPUT
DC
MAX
FAULT
AUTO-RESTART
OPEN-LOOP
THERMAL
SHUTDOWN
t
SAMPLE-OUT
t
SAMPLE-INPUT
OSCILLATOR
+
SAMPLE
DELAY
SOURCE
(S)
V
ILIMIT
LEADING
EDGE
BLANKING
SOURCE
(S)
CONSTANT
CURRENT
I
LIM
-
Current Limit
Comparator
PI-7302-061214
Figure 2.
Functional Block Diagram.
Pin Functional Description
DRAIN (D) Pin:
Power MOSFET drain connection. It also provides internal
operating current during start-up and in steady-state operation.
BYPASS (BP) Pin:
Connection point for the external 1
mF
bypass capacitor connected to
the internally generated 6 V supply.
FEEDBACK (FB) Pin:
E Package
(eSIP-7C)
Exposed Pad
(On Back Side)
Internally
Connected to
SOURCE Pin
D Package (SO-8C)
8S
7S
6S
FB 1
12345
S
NC
NC
BP
FB
7
D
BP 2
Controls switching of the power MOSFET during normal opera-
tion. This pin senses the AC voltage on the bias winding.
Input is used to regulate both the output voltage in CV mode
and output current in CC mode based on voltage across the
bias winding in the flyback portion of the switching cycle. The
internal inductance correction circuit uses voltage on the bias
winding during forward part of the switching cycle to sense the
bulk capacitor voltage.
SOURCE (S) Pin:
D4
5S
Exposed Pad (On Bottom)
Internally Connected to
SOURCE Pin
K Package
(eSOP-12B)
12 S
11 S
10 S
9S
8S
7S
PI-6906-051614
FB 1
BP 2
NC 3
NC 4
D6
Connected to the MOSFET source and is used for high-voltage
power and control circuit common returns.
Figure 3.
Pin Configuration.
2
Rev. B 09/15
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LYT2002-2005
LYTSwitch-2 Functional Description
The LYTSwitch-2 IC combines a high-voltage power MOSFET switch
with a power supply controller in one device. Similar to the
LinkSwitch™-LP and TinySwitch™-III ICs it uses an ON/OFF control to
regulate the output voltage. In addition, the switching frequency is
modulated to regulate the output current to provide a constant current
characteristic. The LYTSwitch-2 controller consists of an oscillator,
feedback (sense and logic) circuit, 6 V regulator, over-temperature
protection, frequency jittering, current limit circuit, leading-edge
blanking, inductance correction circuitry, frequency control for
constant current regulation and ON/OFF state-machine for CV control.
Inductance Correction Circuitry
If the primary magnetizing inductance is either too high or low the
converter will automatically compensate for this by adjusting the
oscillator frequency. Since this controller is designed to operate in
discontinuous-conduction mode the output power is directly propor-
tional to the set primary inductance and its tolerance can be completely
compensated with adjustments to the switching frequency.
Constant Current (CC) Operation
As the output voltage and therefore the flyback voltage across the
bias winding ramps up, the FEEDBACK pin voltage increases. The
switching frequency is adjusted as the FEEDBACK pin voltage increases
to provide a constant output current regulation. The constant current
circuit and the inductance correction circuit are designed to operate
concurrently in the CC region.
Constant Voltage (CV) Operation
As the FEEDBACK pin approaches 2 V from the constant current
regulation mode, the power supply transitions into CV operation.
The switching frequency at this point is at its maximum value,
corresponding to the peak power point of the CV/CC characteristic.
The controller regulates the FEEDBACK pin voltage to remain at
FEEDBACK pin threshold (V
FBTH
) using an ON/OFF state-machine.
The FEEDBACK pin voltage is sampled 2.5
ms
after the turn-off of the
high-voltage switch. At light loads the current limit is also reduced to
decrease the transformer flux density and the FEEDBACK pin
sampling is done earlier.
Auto-Restart and Open-Loop Protection
In the event of a fault condition such as an output short or an
open-loop condition the LYTSwitch-2 IC enters into an appropriate
protection mode.
In the event the FEEDBACK pin voltage during the flyback period falls
below 0.7 V before the FEEDBACK pin sampling delay (~2.5
ms)
for a
duration in excess of ~450 ms (auto-restart on-time (t
AR-ON
) the
converter enters into auto-restart, wherein the power MOSFET is
disabled for 1.2 seconds. The auto-restart alternately enables and
disables the switching of the power MOSFET until the fault condition
is removed.
In addition to the conditions for auto-restart described above, if the
sensed FEEDBACK pin current during the forward period of the
conduction cycle (switch “on” time) falls below 120
mA,
the converter
annunciates this as an open-loop condition (top resistor in potential
divider is open or missing) and reduces the auto-restart time from
450 ms to approximately 6 clock cycles (90
ms),
whilst keeping the
disable period of 2 seconds.
Over-Temperature Protection
The thermal shutdown circuitry senses the die temperature. The
threshold is set at 142 °C typical with a 60 °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 60 °C, at which point the MOSFET 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 capaci-
tance and rectifier reverse recovery time will not cause premature
termination of the MOSFET conduction. The LYTSwitch-2 IC also
contains a “di/dt” correction feature to minimize CC variation across the
input line range.
6 V Regulator
The 6 V regulator charges the bypass capacitor connected to the
BYPASS pin to 6 V by drawing a current from the voltage on the
DRAIN pin, whenever the MOSFET is off. The BYPASS pin is the
internal supply voltage node. When the MOSFET is on, the device
runs off of the energy stored in the bypass capacitor. Extremely low
power consumption of the internal circuitry allows the LYTSwitch-2 IC
to operate continuously from the current drawn from the DRAIN pin.
A bypass capacitor value of 1
mF
is sufficient for both high frequency
decoupling and energy storage.
3
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Rev. B 09/15
LYT2002-2005
Applications Example
R1
3.9 kΩ
1/8 W
R3
220 kΩ
L1
3.9 mH
C3
470 pF
1 kV
2
9
1
T1
EE19
10
22 V - 48 V,
180 mA
D3
US1G
C6
100
µF
63 V
R10
1.2 MΩ
TP3
RTN
TP4
C7
1 nF
500 V
BR1
B10S-G
1000 V
L
TP1
90 - 265
VAC
N
TP2
F1
2A
C1
12
µF
400 V
R4
560
Ω
R5
560
Ω
D1
S1ML
5
R6
2
Ω
1/8 W
4
C2
12
µF
400 V
LYTSwitch-2
U1
LYT2004E
D
FB
BP
S
D2
BAV21W-7-F
R7
60.4 kΩ
1%
1/8 W
R2
3.9 kΩ
1/8 W
C4
1
µF
50 V
R9
12 kΩ C5
1/8 W 1
µF
50 V
R8
6.98 kΩ
1%
1/8 W
PI-7280-051414
L2
3.9 mH
Figure 4.
Energy Efficient 8.6 W LED Power Supply (>86 % Average Efficiency, <30 mW No-load Input Power).
Circuit Description
This circuit shown in Figure 4 is configured as a primary-side
regulated flyback power supply utilizing the LYT2004E from the
LYTSwitch-2 family of ICs. This type of LED driver design is typical
for an external ballast application where safety isolation is required
while power factor correction is not. The output can drive an LED
load from 48 V to 22 V with a constant output current of 180 mA ±5%
across input range of 90 VAC to 265 VAC and ambient temperature
range of 0 ºC to 60 ºC. It has an average efficiency of >86% and
<30 mW no-load input power measured at nominal input voltages
(i.e. 115 VAC and 230 VAC). This design easily meets the most
stringent current energy efficiency requirements.
Input Filter
AC input power is rectified by bridge diode BR1. The rectified DC is
filtered by the bulk storage capacitors C1 and C2. Inductor L1, L2,
C1 and C2 form a pi (π)
filter, which attenuates conducted differen-
tial-mode EMI. Resistors R1 and R2 placed across the inductors
damp the Q to improve frequency noise filtering without reducing low
frequency noise attenuation. A small value Y capacitor (C7) across
the transformer was used to reducer common-mode noise currents.
The fuse F1 provides protection against catastrophic failure. This
can be replaced by a fusible resistor for cost reduction but should be
suitably rated (and typically a wire wound type) to withstand the
instantaneous dissipation experienced during input capacitor charging
when first connected to the AC line.
LYT2004 Primary
The LYTSwitch-2 family (U1) incorporates the power switching device,
oscillator, CC/CV control engine, start-up, and protection functions.
The integrated 725 V power MOSFET provides a large drain voltage
margin in universal input AC applications, increasing reliability and
also reducing the output diode voltage stress by permitting the use of
higher transformer turns ratios. The device is completely self-
powered from the BYPASS pin and decoupling capacitor C4.
The optional bias supply formed by D2 and C5 and R6 provides
operating current to U1 via resistor R9. This reduces the no-load
consumption from 200 mW to less than 30 mW. The bias supply also
increases light load efficiency.
The rectified and filtered input voltage is applied to one side of the
primary winding of T1. The other side of the transformer’s primary
winding is driven by the integrated power MOSFET in U1. The
leakage inductance drain voltage spike is limited by an RCD-R clamp
consisting of D1, R3, R4, R5, and C3.
Output Rectification
The output from the transformer is rectified by D3, a 1 A, 400 V
ultrafast recovery type diode (for higher efficiency), and filtered by
C6. In this application C6 was sized to meet a (typical) ripple
requirement of less than 10% without the need for an additional LC
post filter.
4
Rev. B 09/15
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LYT2002-2005
A pre-load resistor R10 was employed to discharge the output
capacitor and extinguish the LED light immediately after turn-off. The
resistor will also keep the output from rising higher than the permitted
maximum output voltage (usually determined by the output capacitor
voltage rating) when the load is disconnected.
Output Regulation
The LYTSwitch-2 family regulates the output using ON/OFF control in
the constant voltage (CV) regulation region of the output characteris-
tic and frequency control for the constant current (CC) region. The
feedback resistors (R7 and R8) were selected using standard 1%
resistors to center both the nominal output voltage and constant
current regulation thresholds. Resistor R6 acts as filter to limit the
voltage spike (caused by the coupling of the bias winding to the
primary winding), improving regulation.
LYTSwitch-2 Layout Considerations
Circuit Board Layout
The LYTSwitch-2 family of ICs present a highly integrated power
supply solution that integrates, both, the controller and the high-
voltage power MOSFET onto a single die. The presence of high
switching currents and voltages together with analog signals makes
it especially important to follow good PCB design practice to ensure
stable and trouble free operation of the power supply. See Figures 5
and 6 for a recommended circuit board layout for LYTSwitch-2.
When designing a printed circuit board layout for the LYTSwitch-2 based
power supply, it is important to follow these guidelines:
Single Point Grounding
Use a single point (Kelvin) connection at the negative terminal of the
input filter capacitor for the LYTSwitch-2 SOURCE pin and bias
winding return. This improves surge capabilities by returning surge
currents from the bias winding directly to the input filter capacitor.
Bypass Capacitor
The BYPASS pin capacitor should be located as close as possible to
the SOURCE and BYPASS pins for effective noise decoupling.
Feedback Resistors
Place the feedback resistors (R7 and R8) very close to the FEEDBACK
pin of the LYTSwitch-2 device. This minimizes noise coupling.
Thermal Considerations (D and K Package)
The copper area connected to the SOURCE pins provides heat sinking.
A good estimate of expected power dissipation is to assume is that
the LYTSwitch-2 will dissipate 5% of the output power. Provide
enough copper area to keep the SOURCE pin temperature below
100 °C. Higher temperatures are allowable but output current (CC)
tolerance will increase. In this case a maximum SOURCE pin tempera-
ture below 100 °C is recommended to provide margin for part-to-part
R
DS(ON)
variation.
Secondary Loop Area
To minimize leakage inductance and EMI the area of the loop
contained within the connections between the secondary winding
(T1), the output diode (D3) and the output filter capacitor (C6) should
be minimized. In addition, sufficient copper area should be to the
rectifier diode for heat sinking preferably connected to the quiet
cathode terminal. A large anode area can increase high frequency
radiated EMI.
Electrostatic Discharge Spark Gap
A trace is placed at one of the AC line inputs to form one electrode of
a spark gap. The other electrode on the secondary is formed by the
output return node. The spark gap directs most ESD energy from the
secondary back to the AC input during a surge event. The trace from
the AC input to the spark gap electrode should be spaced away from
other traces to prevent unwanted arcing occurring and possible circuit
damage. If R1 and R2 are removed additional spark gaps across the
EMI filter inductors (L1 and L2) to prevent excessive build-up of
voltage across them during surge.
Key Application Considerations
Output Power Table
The data sheet maximum output power table (Table 1) represents the
maximum practical continuous output power that can be obtained
under the following assumed conditions:
1.
The minimum DC bus voltage is 100 V at 90 VAC input. The value
of the input capacitance should be made large enough to meet
this requirement for AC input designs – typically 2-3
mF/W
for
low-line or universal input designs and 1-2
mF/W
for high-line input
designs.
The secondary output rectifier diode should withstand peak
inverse voltage (PIV) for 55 V output voltage for open load
condition.
Assume efficiency of >80%.
Discontinuous mode operation (K
P
>1.3).
The LYTSwitch-2 part is either board mounted with SOURCE pins
soldered to a sufficient area of copper to keep the SOURCE pin
temperature at or below 100 °C, or (in the case of the E package)
attached to a sufficiently sized heat sink to limit device tempera-
ture to below 110 °C.
Ambient temperature of less than 50 °C for open frame designs
and an internal enclosure temperature of 60 °C for enclosed
ballast-type designs.
2.
3.
4.
5.
6.
Note: Higher output powers are achievable if an output CC tolerance
> ±10% is acceptable, and allowing the device to be operated at a
higher SOURCE pin temperature.
Output Tolerance
LYTSwitch-2 K and E package parts provides an overall CC mode
output current tolerance of ±5% including line voltage, normal
board-to-board component variation and across a temperature range
of 0 °C to 110 °C. For the D package (SO-8) additional CC variance
may occur due to stress caused by manufacturing (i.e. solder-wave
immersion or I
R
reflow).
A sample power supply build is recommended to verify production
tolerances for each design.
BYPASS Pin Capacitor Selection
A 1
mF
BYPASS pin capacitor is recommended. The capacitor voltage
rating should be greater than 7 V. The capacitor can be ceramic or
electrolytic but tolerance of capacitor should be ≤ ±50%. The
capacitor must be physically located close to the LYTSwitch-2 BYPASS
pin for effective noise decoupling.
5
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Rev. B 09/15
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