ML4833
This assumes that t
CHG
>> t
DIS
.
When LFB OUT is high, I
CH
= 0 and the minimum
frequency occurs. The charging current varies according
to two control inputs to the oscillator:
1. The output of the preheat timer
2. The voltage at LFB OUT (lamp feedback amplifier
output)
In preheat condition, charging current is fixed at
T
J
≅
T
A
+
(P
D
+
65
°
C / W)
(9)
V
CC
VCCZ
V(ON)
V(OFF)
I
CHG (PREHEAT)
=
2.5
R
SET
(6)
I
CC
5.5mA
t
In running mode, charging current decreases as the
voltage rises from 0V to V
OH
at the LAMP FB amplifier.
The highest frequency will be attained when I
CHG
is
highest, which is attained when voltage at LFB OUT
is at 0V:
0.34mA
t
I
CHG(0)
=
5
R
SET
(7)
Highest lamp power, and lowest output frequency are
attained when voltage at LFB OUT is at its maximum
output voltage (V
OH
).
In this condition, the minimum operating frequency of the
ballast is set per equation 5 above.
For the IC to be used effectively in dimming ballasts with
higher Q output networks a larger C
T
value and lower R
T
value can be used, to yield a smaller frequency excursion
over the control range (voltage at LFB OUT). The
discharge current is set to 5mA. Assuming that I
DIS
>>
I
RT
:
t
DIS(VCO)
≅
600
×
C
T
Figure 7. Typical V
CC
and I
CC
Waveforms when
the ML4833 is Started with a Bleed Resistor from
the Rectified AC Line and Bootstrapped from an
Auxiliary Winding.
STARTING, RE-START, PREHEAT AND INTERRUPT
The lamp starting scenario implemented in the ML4833
is designed to maximize lamp life and minimize ballast
heating during lamp out conditions.
The circuit in Figure 8 controls the lamp starting
scenarios: Filament preheat and lamp out interrupt. C
X
is
charged with a current of I
R(SET)
/4 and discharged through
R
X
. The voltage at C
X
is initialized to 0.7V (V
BE
) at power
up. The time for C
X
to rise to 4.8V is the filament preheat
time. During that time, the oscillator charging current
(I
CHG
) is 2.5/R
SET
. This will produce a high frequency for
filament preheat, but will not produce sufficient voltage
to ignite the lamp or cause significant glow current.
After cathode heating, the inverter frequency drops to F
MIN
causing a high voltage to appear to ignite the lamp. If
lamp current is not detected when the lamp is supposed
to have ignited, the lamp voltage feedback coming into
pin 8 remains below 1.25V, the C
X
charging current is
shut off and the inverter is inhibited until C
X
is discharged
by R
X
to the 1.2V threshold. Shutting off the inverter in
this manner prevents the inverter from generating
excessive heat when the lamp fails to strike or is out of
socket. Typically this time is set to be fairly long by
choosing a large value of R
X
.
(8)
IC BIAS, UNDER-VOLTAGE LOCKOUT AND
THERMAL SHUTDOWN
The IC includes a shunt clamp which will limit the
voltage at V
CC
to 15V (V
CCZ
). The IC should be fed with
a current limited source, typically derived from the
ballast transformer auxiliary winding. When V
CC
is below
V
CCZ
– 1.1V, the IC draws less than 0.48mA of quiescent
current and the outputs are off. This allows the IC to start
using a “bleed resistor” from the rectified AC line.
To help reduce ballast cost, the ML4833 includes a
temperature sensor which will inhibit ballast operation if
the IC’s junction temperature exceeds 120°C. In order to
use this sensor in lieu of an external sensor, care should
be taken when placing the IC to ensure that it is sensing
temperature at the physically appropriate point in the
ballast. The ML4833’s die temperature can be estimated
with the following equation:
9
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
