LYT4212E Power Integrations, LYT4212E Datasheet - Page 7

• Manufacturer Part Number: LYT4212E
• Description: LED Lighting Drivers HighPower LED Driver 15 W (85-132 VAC)
• Manufacturer: Power Integrations

Specifications of LYT4212E

• Rohs: yes
• Input Voltage: 85 V to 132 V
• Operating Frequency: 124 kHz to 140 kHz
• Maximum Supply Current: 1.2 mA
• Output Current: 1.17 A
• Maximum Operating Temperature: + 150 C
• Mounting Style: Through Hole
• Package / Case: eSIP-7C
• Minimum Operating Temperature: - 40 C

Diode D6, C5, C9, R19 and R20 create the primary bias supply
from an auxiliary winding on the transformer. Capacitor C8
provides local decoupling for the BYPASS pin of U1 which is the
supply pin for the internal controller. During start-up C8 is
charged to ~6 V from an internal high-voltage current source
tied to the device DRAIN pin. This allows the part to start
switching at which point the operating supply current is provided
from the bias supply via R17. Capacitor C8 also selects the
output power mode (47 µF for reduced power was selected to
reduce dissipation in U1 and increase efficiency for this design).
Feedback
The bias winding voltage is proportional to the output voltage
(set by the turns ratio between the bias and secondary
windings). This allows the output voltage to be monitored
without secondary-side feedback components. Resistor R18
converts the bias voltage into a current which is fed into the
FEEDBACK pin of U1. The internal engine within U1 combines
the FEEDBACK pin current, the VOLTAGE MONITOR pin current
and drain current information to provide a constant output
current over a 1.5:1 output voltage variation (LED string voltage
variation of ±25%) at a fixed line input voltage.
To limit the output voltage at no-load an output overvoltage
protection circuit is set by D8, C15, R22, VR4, R27, C14 and Q2.
Should the output load be disconnected then the bias voltage
will increase until VR4 conducts, turning on Q2 and reducing
the current into the FEEDBACK pin. When this current drops
below 10 µA the part enters auto-restart and switching is
disabled for 300 ms allowing time for the output and bias
voltages to fall.
Output Rectification
The transformer secondary winding is rectified by D7 and
filtered by C11 and C12. An ultrafast TO-220 diode was
selected for efficiency and the combined value of C11 and C12
were selected to give peak-to-peak LED ripple current equal to
30% of the mean value. For designs where lower ripple is
desirable the output capacitance value can be increased.
A small pre-load is provided by R23 which discharges residual
charge in output capacitors when turned off.
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TRIAC Phase Dimming Control Compatibility
The requirement to provide output dimming with low-cost,
TRIAC-based, leading edge phase dimmers introduces a
number of trade-offs in the design.
Due to the much lower power consumed by LED based lighting
the current drawn by the overall lamp is below the holding
current of the TRIAC within the dimmer. This can cause
undesirable behaviors such as limited dimming range and/or
flickering as the TRIAC fires inconsistently. The relatively large
impedance the LED lamp presents to the line allows significant
ringing to occur due to the inrush current charging the input
capacitance when the TRIAC turns on. This too can cause
similar undesirable behavior as the ringing may cause the
TRIAC current to fall to zero and turn off.
To overcome these issues simple two circuits, the SCR active
damper and R-C passive bleeder, are incorporated. The
drawback of these circuits is increased dissipation and
therefore reduced efficiency of the supply. For non-dimming
applications these components can simply be omitted.
The SCR active damper consists of components R6, C3, and
Q1 in conjunction with R8. This circuit limits the inrush current
that flows to charge C4 when the TRIAC turns on by placing R8
in series for the first ~1 ms of the TRIAC conduction. After
approximately 1 ms, Q1 turns on and bypasses R8. This keeps
the power dissipation on R8 low and allows a larger value
during current limiting. Resistor R6 and C3 provide the delay
on Q1 turn on after the TRIAC conducts. Diode D9 blocks the
charge in capacitor C4 from flowing back after the TRIAC turns
on which helps in dimming compatibility especially with high
power dimmers.
The passive bleeder circuit is comprised of R1 and C1. This
helps keep the input current above the TRIAC holding current
while the input current corresponding to the effective driver
resistance increases during each AC half-cycle.
LYT4211-4218/4311-4318
Rev. B 02/13
7