ML4835
FUNCTIONAL DESCRIPTION (Continued)
Highest lamp power, and lowest output frequency are
attained when voltage at LFB OUT is at its maximum
The circuit in Figure 10 controls the lamp starting
scenarios: Filament preheat and lamp out interrupt. C is
X
output voltage (V ).
charged with a current of I
/4 and discharged through
OH
R(SET)
R . The voltage at C is initialized to 0.7V (V ) at power
X
X
BE
In this condition, the minimum operating frequency of the
ballast is set per equation 5 above.
up. The time for C to rise to 4.75V is the filament preheat
X
time. During that time, the oscillator charging current
(I
) is 2.5/R . This will produce a high frequency for
CHG
SET
For the IC to be used effectively in dimming ballasts with
filament preheat, but will not produce sufficient voltage
to ignite the lamp or cause significant glow current.
higher Q output networks a larger C value and lower R
T
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 5.5mA.
After cathode heating, the inverter frequency drops to
START
F
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 C charging current is shut
Assuming that I
>>I :
RT
DIS
X
off and the inverter is inhibited until C is discharged by
X
tDIS(VCO) 600 CT
(9)
R to the 1.25V threshold. Shutting off the inverter in this
X
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
IC BIAS, UNDER-VOLTAGE LOCKOUT AND
THERMALSHUTDOWN
large value of R .
X
The IC includes a shunt clamp which will limit the
voltage at V to 15V (V
). The IC should be fed with
LFB OUT is ignored by the oscillator until INTERRUPT is
CC
CCZ
a current limited source, typically derived from the
ballast transformer auxiliary winding. When V is below
above 1.25V The C pin is clamped to about 7.5V.
X
CC
V
CCZ
– 1.1V, the IC draws less than 0.55mA of quiescent
Care should also be taken not to turn on the VCCZ clamp
so as not to dissipate excessive power in the IC. This will
cause the temp sensor to become active at a lower
ambient temperature.
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 ML4835 includes a
temperature sensor which will inhibit ballast operation if
the IC’s junction temperature exceeds 130°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 ML4835’s die temperature can be estimated
with the following equation:
A summary of the operating frequencies in the various
operating modes is shown below.
OPERATING MODE
OPERATING FREQUENCY
[F(MAX) to F(MIN)]
2
Preheat
After
Preheat
F(START)
TJ TA (PD 65 C / W)
(10)
Dimming
Control
F(MIN) to F(MAX)
STARTING, RE-START, PREHEATAND INTERRUPT
The lamp starting scenario implemented in the ML4835
is designed to maximize lamp life and minimize ballast
heating during lamp out conditions.
0.625
R
SET
R /C
X
X
+
–
10
9
HEAT
R
X
C
X
1.25/4.75
1.0/1.25
LEA_ENB OR
DIMMING LOCKOUT
+
–
INTERRUPT
S
Q
INHIBIT
+
–
R
1.25/6.75
Figure 10. Lamp Preheat and Interrupt Timers
11
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