Datasheet

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
C
OSC should be 200pF.
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