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RD-0613

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8. Functional Description

The IRPLLNR5 reference design consists of an EMI filter, an active power factor correction section, a

ballast control section, and a resonant lamp output stage. The active power factor correction section is a

boost converter operating in critical conduction, free-running frequency mode. The ballast control

section provides frequency modulation control of a traditional RCL lamp resonant output circuit and is

easily adaptable to a wide variety of lamp types. The ballast control section also provides the necessary

circuitry to perform lamp fault detection, shutdown, and auto-restart.

Reference Design Overview

This demo-board is designed for a single 54 W TL5 lamp with voltage mode heating. TL5 lamps are

popular due to their low profile and high lumen/watt output. These lamps, however, can be more

difficult to control due to their higher ignition and running voltages. A typical ballast output stage using

current-mode filament heating (filament placed inside L-C tank) will result in excessive filament current

during running. The output stage has therefore been configured for voltage-mode filament heating using

secondary windings off of the resonant inductor LRES. The lamp has been placed outside the under-

damped resonant circuit loop, which consist of LRES and CRES. The filament heating during preheat

can be adjusted with the capacitors CH1 and CH2. The result is a more flexible ballast output stage

necessary for fulfilling the lamp requirements. The DC blocking capacitor, CDC, is also placed outside

the under-damped resonant circuit loop such that it does not influence the natural resonance frequency

of LRES and CRES. The snubber capacitor, CSNUB, serves as EMI reduction and a charge pump for

supplying the IRS2168D.

The IRS2168D ballast control IC is used to program the ballast operating points and protect the ballast

against conditions such as lamp strike failures, low DC bus or lamp failure during normal operations. It

is also used to regulate the DC bus and for power factor correction to give high power factor and low

harmonic distortion of the ballast AC input current.

Power Factor Correction Section

The power factor correction section contained in the IRS2168D controls a boost topology circuit

operating in critical conduction mode. This topology is designed to step-up and regulate the output DC

bus voltage while drawing sinusoidal current from the line (low THD) which is “in phase” with the AC

input line voltage. The power factor correction section also includes over-current protection of the

boost MOSFET to prevent damage that can occur during boost inductor saturation.

Ballast Control Section

The ballast control section of the IRS2168D ballast control IC contains an oscillator, a high-voltage

half-bridge gate driver, and lamp fault protection circuitry. Please refer to the datasheet of this IC for

the block diagram and the state diagram. The following is a breakdown of the different modes of

operation for the ballast.

Startup Mode

When power is initially applied to the ballast, the voltage on the VCC pin of the IRS2168D begins to

increase. The voltage for the IRS2168D is derived from the current supplied from the rectified AC line

through the startup resistor RSUPPLY. During this initial startup when the VCC voltage of the

IRS2168D is below the rising under-voltage lock-out threshold (UVLO+), the IC is in UVLO Mode and

draws micro-power current at VCC. The micro-power current of the IRS2168D allows for the use of a