7

UCC2305

UCC3305

APPLICATIONS INFORMATION (cont.)

can be as simple as a high voltage transistor and a resis-

tor pull-up, combined with the correct choice of phases.

Regulated Lamp Input Power Gives

Constant Intensity

The LPOWER output of the UCC3305 is a voltage

roughly proportional to lamp input power. The UCC3305

regulates constant lamp power over a wide range of lamp

voltages. The range of lamp voltages which produce con-

stant lamp power is set by the limiting amplifier on

VOUTSENSE.

For inputs to VOUTSENSE below 0.5V, such as would

occur with a shorted lamp, the loop regulates constant

load current. For inputs to VOUTSENSE greater than

0.82V, as might occur with a lamp that is open or not yet

lit, the loop also regulates constant load current, but at a

lower current than for a shorted lamp. In between those

two voltages, the amplifier driving the LPOWER pin will

sum the load current and load voltage and produce a sig-

nal roughly proportional to load power. The summing am-

plifier approximates power well enough to hold power

within

±10% over a factor of two in lamp voltage.

The UCC3305 HID Controller contains a current mode

PWM similar to the industry standard UC3842 and

UCC3802 circuits. This controller uses a high gain op

amp to regulate the output of the LPOWER circuit. This

op amp drives a high speed PWM comparator, which

compares converter input current to the output of the op

amp and uses this signal to set duty cycle.

Slope Compensation

In addition to a complete current mode PWM, the

UCC3305 HID Controller contains internal slope com-

pensation, a valuable function which improves current

loop stability for high duty cycles. Slope compensation is

accomplished with an on-chip current ramp and an

off-chip resistor RSL. Larger values of RSL give more

slope compensation and a more stable feedback loop.

Powering The UCC3305

Conventional power MOSFETs require at least 8V of

gate drive to ensure high efficiency and low on resis-

tance. Despite this requirement, the UCC3305 HID Con-

troller can be used to build a ballast that will drive power

MOSFETs well with input supplies as low as 5V! The

UCC3305 does this using a charge pump.

In this typical application, power for the UCC3305 HID

Controller IC is derived from a 6.8V zener supply. This

zener regulated supply gives the application overvoltage

protection, reverse battery protection, low parts count,

and low cost. The output of the 6.8V zener supply drives

the VCC pin of the UCC3305. VCC is the input to the

UCC3305 charge pump. The charge pump generates a

regulated 10V supply on the BOOST output. This 10V

supply drives all other functions on the UCC3305.

Protection From Over Voltage

The most significant stresses in an automotive environ-

ment are the overvoltage conditions which can occur dur-

ing load dump and double-battery jump start. At these

times, the voltage into the ballast can go so high that

even the most overdesigned power stage will be dam-

aged. The UCC3305 is inherently immune to damage

from this when operated with a zener regulated supply. In

addition, the UCC3305 will protect the ballast compo-

nents by shutting down the PWM in the presence of ex-

cessive voltage on the BAT input.

This typical application shows a voltage divider consist-

ing of a 270k resistor and a 100k resistor driving the BAT

input. The threshold of the BAT input is approximately 5V,

so this divider sets the shutdown voltage at approxi-

mately 18.5V.

Programming the UCC3305

All circuitry on the UCC3305 HID Lamp Controller is op-

erated from a bias current set by the resistor from ISET

to ground. For best operation, this resistor (RSET) should

be between 75k and 150k.

Oscillator Frequency

The UCC3305 HID Lamp Controller PWM oscillator is set

by the resistor from ISET to ground and by the capacitor

from OSC to ground. Oscillator frequency can be esti-

mated by the equation:

FOSC =

2

RSET COSC

•

For operation at 100kHz, RSET should be 100k and

The PWM oscillator also determines the low frequency

lamp switching rate for AC lamps. The exact lamp switch-

ing rate is the PWM frequency divided by 512.

Lamp Temperature Compensation

Automobile headlights must come up to full intensity very

quickly, but HID lamps require many minutes to stabilize.

The UCC3305 HID Controller contains sophisticated in-

ternal circuitry to anticipate lamp temperature and also to

compensate for lamp temperature.

The circuits anticipate lamp temperature by monitoring

charge on capacitors which charge when the lamp is on

and discharge when the lamp is off. The UCC3305 HID

Controller compensates for lamp temperature by driving

the lamp with a higher lamp power when the lamp is cold

and reducing the power to a normal operating level when

the lamp is warmed up. The capacitors which set these