LYT1402-1604
Constant output current regulation is achieved through the FEED-
BACK (FB) pin directly sensing the drain current during the FET
on-time using external current sense resistors (RFB) R5 and R6 and
comparing the voltage drop to a fixed internal reference voltage
(VFB(REF)) of absolute value 279 mV typical. RFB can be estimated by
the given equation;
A small output pre-load resistor R8 discharges the output capacitor
when the driver is turned off, giving a relatively quick and smooth
decay of the LED light. Recommended pre-load power dissipation is
≤ 0.5% of the output power.
Key Design Considerations
Device Selection
RFB = VFB(REF) /k # IOUT
The data sheet power table (Table 2) represents the maximum
practical continuous output power that can be delivered in an open
frame design with adequate heat sinking.
Where: k is the ratio between IPK and IOUT; such that k = 3 for
LYT-14xx, and k = 3.6 for LYT-16xx)
RDK-464 is a universal input 8 W driver for bulb application, where
the operating temperature is high and a relatively low THD less than
25% is desired for universal input application. LYT1603D was chosen
based on these conditions.
Trimming RFB may be necessary to center IOUT at the nominal output
LED voltage.
The MULTIFUNCTION (M) pin monitors the line for any line overvolt-
age event. When the internal MOSFET is in on-state, the MULTI-
FUNCTION pin is shorted internally to the SOURCE (S) pin in order to
detect the rectified input line voltage derived for the voltage across
the inductor, i.e. (VIN – VOUT) and current flowing out of the MULTI-
FUNCTION pin is defined by resistor R7, thus the line over voltage
detection is calculated as follows;
Output Power Table
Optimized for Smallest Components
Product
VOUT ≤ 30 V
45 V ≤ VOUT ≤ 55 V
LYT1402D
LYT1403D
LYT1404D
4.0 W
7.5 W
11 W
8.0 W
15 W
22 W
VLINE(OVP) = IIOV # R7 + VOUT
Where: R7 is assumed to be 402 kΩ ±1%.
Once the detected current exceeds the input overvoltage threshold
(IIOV) of 1 mA typical, the IC will inhibit switching instantaneously and
initiate auto-restart to protect the internal MOSFET of the IC.
Optimized for Lowest THD
Product
VOUT ≤ 30 V
VOUT ≥ 55 V
The MULTIFUNCTION (M) pin also monitors the output for any
overvoltage and undervoltage event. When the internal MOSFET is in
off-state, the output voltage is sensed via divider resistors R4 and R7
across the inductor voltage of T3. When an output open-load condition
occurs, the voltage at the MULTIFUNCTION pin will rise abruptly and
when it exceeds the VOOV threshold of 2.4 V typical, the IC will inhibit
switching and initiate auto-restart to limit the output voltage from
further rising. The overvoltage cut-off is typically set to 120% of the
output voltage, which is equivalent to 2 V target at the MULTIFUNC-
TION pin (VOUT(OVP) = VOUT × 2.4 V / 2 V). If desired, higher overvolt-
age cut-off can be set with lower MULTIFUNCTION pin voltage target.
Resistor R7 is set to a fixed value of 402 kΩ ±1% and R4 will determine
the output overvoltage limit. Any output short-circuit at the output
will be detected once the MULTIFUNCTION pin voltage falls below the
undervoltage threshold (VOUV) of 1 V typical, then the IC will inhibit
switching and initiate auto-restart to limit the average input power of
less than 1 W, preventing any component from overheating.
LYT1602D
LYT1603D
4.0 W
7.5 W
11 W
8.0 W
15 W
22 W
LYT1604D
Table 2. Output Power Table.
Magnetic Selection
The core is a small size EE10 with ferrite core material and open
winding window that allowed better convection cooling for the
winding.
To ensure proper magnetic design and accurate output current
regulation, it is recommended that the LYTSwitch-1 PIXls spreadsheet
located at PI Expert web site (https://piexpertonline.power.com/site/
login) should be used for magnetic calculations.
EMI Considerations
Total input capacitance affects PF and THD – increasing the value will
degrade performance. LYTSwitch-1’s control engine allows operating
in critical conduction mode with variable frequency and variable
on-time provides low EMI and enables the use of small and simple pi
(π) filter. It also allows simple magnetic construction where the main
winding can be wound continuously using the automated winding
approach preferred for low-cost manufacturing. The recommended
location of the EMI filter is after the bridge rectifier. This allows the
use of regular film capacitors as opposed to more expensive safety
rated X-capacitors that would be required if the filter is placed before
the bridge.
R4 can be calculated as follows;
R4 = 2V # R7/ VOUT - 2V
This is also applicable to Low-Side Configuration Buck topology
(see application note AN-67).
Another function of the MULTIFUNCTION (M) pin is for zero current
detection (ZCD). This is to ensure operation in critical conduction
mode. The inductor demagnetization is sensed when the voltage
across the inductor begins to collapse towards zero as flywheel diode
(D1) conduction expires.
Thermal and Lifetime Considerations
Output Stage
Lighting applications present thermal challenges to the driver. In
many cases the LED load dissipation determines the working ambient
temperature experienced by the drive. Thermal evaluation should be
performed with the driver inside the final enclosure. Temperature
has a direct impact on driver and LED lifetime. For every 10 °C rise in
temperature, component life is reduced by a factor of 2. Therefore, it
is important to verify and optimize the operating temperatures of all
components.
During the switching off-state, free-wheeling diode D1 rectifies the
voltage across T3 and the output filtered by C6. An ultrafast 1 A,
600 V with 75 ns reverse recovery time (tRR) diode was selected for
efficiency and good regulation. The value of the output capacitor C8
was 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.
4
Rev. B 07/16
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