Rev. A 06/10

5

LNK454/456-458/460

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This document contains information on a new product. Specifications and information herein

are subject to change without notice.

PRELIMINARY

IC Supply and BYPASS Pin

The internal 5.85 V regulator charges the bypass capacitor

connected to the BYPASS pin to 5.85 V by drawing current

from the voltage on the DRAIN pin whenever the power MOSFET

is off. The BYPASS pin is the internal supply voltage node.

When the power MOSFET is on, the device operates from the

energy stored in the bypass capacitor. Extremely low power

consumption of the internal circuitry allows LinkSwitch-PL to

operate continuously from current it takes from the DRAIN pin. A

bypass capacitor value of 1

µF is sufficient for both high

frequency decoupling and energy storage.

During phase angle dimming when the conduction angle is

small the AC input voltage is present for only short periods of

time. In that case the IC should not rely on the integrated high

voltage current source, but instead external bias circuitry should

be used to supply the IC from the output (D

4). If the output voltage is less than 7 V, external bias circuitry

should be implemented, by using a bias winding on the primary

of the transformer with a small signal rectifier and an electrolytic

capacitor with a value based on maximum IC consumption and

maximum phase dimming conduction angles.

Start-up, Switching Frequency, On-time Range

At start-up the controller uses an initial switching frequency f

MIN

and minimum on-time t

capacitor together with the energy delivery to the output LEDs

as soon as their anode-cathode threshold is reached

determines a step-by-step increase of the operating power

MOSFET switching frequency and on-time every half-cycle of

the main input voltage.

When the operating conditions (start-up or large transients)

allow for only low energy processing (low frequency and on-time),

the voltage across the input bulk capacitance will not reach zero

even if the main voltage crosses zero. During these conditions the

IC sets the reference voltage on the FEEDBACK pin to one half

of its preset level (145 mV), to avoid overshoot of the output LED

current. Once the FEEDBACK pin voltage exceeds this reduced

threshold (with the zero crossing on the bulk capacitor being

achieved), then the FEEDBACK pin voltage is restored to the

normal 290 mV level.

The steady state switching frequency and on-time is determined

by the line voltage, voltage drop across the LEDs and system

overall power transfer efficiency.

At light load when the device reaches the minimum frequency

f

cycles. In this mode of operation the input current is not power

factor corrected and the average output current is not

guaranteed to fall within the normal range. The FEEDBACK pin

cycle skipping threshold is reduced from approximately twice

the normal regulation level down to just above the level required

to limit output power delivery under these conditions. A

properly designed supply will not operate in this mode under

normal load conditions. A power supply designed correctly will

operate within the switching frequency range [f

an on-time falling between t

to a normal load.

Overload Protection

In case of overload, the system will increase the operating

frequency and on-time each AC half-cycle until the maximum

frequency and maximum on-time are reached. When this state

is reached, by the next half-cycle, the controller will enter

auto-restart protection, thus inhibiting the gate of the power

MOSFET for approximately 1.28 s if the main line frequency is

50 Hz, 1.02 s if it is 60 Hz. After this auto-restart off-time

expires, the circuit will start again, exactly as at start-up, i.e. at

f

In case of a persistent overload condition, the auto-restart duty

cycle DC

Auto-restart is inhibited during phase dimming when the TRIAC

conduction duty cycle is less than 60%.

Output Overvoltage Protection

If a no-load condition is present on the output of the supply, the

output overvoltage Zener (DZ

threshold is reached. A voltage V

appear across the FEEDBACK pin and the IC will latch off. Normal

operation will be restored once the BYPASS pin voltage drops

below 4.9 V and the IC goes through a new start-up phase.

Output Short-Circuit

If the output of the supply (i.e. the LED load) is short-circuited,

then a large amount of energy will be delivered to the sense

resistor, generating a high voltage at the FEEDBACK pin. If this

condition develops more than 2 V on the FEEDBACK pin, then

the IC will interpret this event as an output short-circuit and will

trigger latching shutdown. Normal operation will resume after

cycling the AC input such that the BYPASS pin voltage drops

below 4.9 V and the IC goes through a new start-up phase.

Safe Operating Area (SOA) Protection

If 3 consecutive cycles of the power MOSFET are prematurely

terminated due to the power MOSFET current exceeding the

current limit after the leading edge blanking time, SOA protection

mode is triggered and the IC will trigger latching shutdown.

Normal operation will resume after cycling the AC input such that

the BYPASS pin voltage drops below 4.9 V and the IC goes

through a new start-up phase.

Hysteretic Thermal Shutdown

The thermal shutdown circuitry senses the die junction

temperature. The thermal shutdown threshold is set to 142 °C

typical with a 75 °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 75 °C, at

which point the power MOSFET is re-enabled.