Rev. B 09/15

3

LYT2002-2005

www.power.com

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.