Rev. B 09/15

5

LYT2002-2005

www.power.com

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.

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.

2. The secondary output rectifier diode should withstand peak

inverse voltage (PIV) for 55 V output voltage for open load

condition.

3. Assume efficiency of >80%.

4. Discontinuous mode operation (K

P >1.3).

5. 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.

6. Ambient temperature of less than 50 °C for open frame designs

and an internal enclosure temperature of 60 °C for enclosed

ballast-type designs.

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.

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.