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IRS2573D

HID BALLAST CONTROL IC

International Rectifier 

IRS2573D

Constant Power Control

During the general mode of operation and after the lamp has ignited, the IC regulates the lamp output power to a

constant level. To achieve this, the IC measures the lamp voltage and lamp current at the VSENSE and

ISENSE pins, multiplies the voltage and current together using an internal multiplier circuit to calculate power, and

regulates the output of the multiplier circuit to a constant reference voltage by increasing or decreasing the buck

on-time. If the lamp power is too low then the output of the multiplier will be below the internal reference voltage.

The operational trans-conductance amplifier (OTA) will output a sourcing current to the PCOMP pin that will

charge up the external capacitor to a higher voltage. This will increase the on-time of buck and increase the

output current to the lamp for increasing the output power. If the lamp power is too high, then the opposite will

occur. The OTA will output a sinking current to the PCOMP pin that will discharge the external capacitor to a

lower voltage. This will decrease the buck on-time and decrease the output current to the lamp for decreasing

the output power. The speed of the constant power control loop is set by the value of the external capacitor at

the PCOMP pin that determines how fast the loop will react and adjust the buck on-time over the changing load

conditions.

Current Limitation Control

The constant power control loop will increase or decrease the buck current for maintaining constant power in the

lamp load. During lamp warm-up, the lamp voltage can be very low (20V typical) and the constant power loop

will attempt to increase the buck current to several amps of current to maintain constant power. This high

current can exceed the manufacturer’s maximum current rating for the HID lamp. To prevent this condition, an

additional current limitation control loop has been included in the IC. Should the voltage at the ISENSE pin

exceed the voltage level at the OC pin, another OTA will sink current from the ICOMP pin. When the ICOMP pin

voltage decreases below the PCOMP pin voltage, then the current limitation loop will override the constant power

loop and the ICOMP pin will decrease the buck on-time. The lower of the PCOMP or ICOMP pins will override

the other and control the buck on-time. When the lamp eventually warms up and the lamp voltage increases to a

level where the lamp current is below the maximum allowable limit (Figure 8), then the ICOMP pin voltage will

increase above the PCOMP pin voltage, and the PCOMP pin will control the buck on-time again for maintaining

constant power.

V, I

Lamp Warm-up

Running

Ignition

Current

Limitation

Constant Power

VSENSE

POWER

ISENSE

t

Figure 8, VSENSE and ISENSE pins during ignition, warm-up and running modes.

Over-Voltage Fault Counter

The IC includes an over-voltage fault counter at the VSENSE pin. The over-voltage fault counter will count the

time during which an over-voltage condition at the output of the buck exists due to an open-circuit condition,

lamp extinguishes, lamp removal or end-of-life. If the voltage at the VSENSE pin remains above VOV(2/5) and

the over-voltage fault counter times out (787sec typical), then the IC will enter Fault Mode and shutdown. If the

voltage at the VSENSE pin decreases below VOV(2/5) before the over-voltage fault counter times out, then the

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