ML4831
The oscillator’s minimum frequency is set when I = 0
To help reduce ballast cost, the ML4831 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 ML4831’s die temperature can be estimated
with the following equation:
CH
where:
1
FOSC
(5)
0.51×RTCT
This assumes that t
>> t .
DIS
CHG
When LFB OUT is high, I = 0 and the minimum
CH
frequency occurs. The charging current varies according
to two control inputs to the oscillator:
TJ TA ×PD × 65°C / W
(9)
1. The output of the preheat timer
V
VCCZ
CC
2. The voltage at Pin 6 (lamp feedback amplifier
output)
V(ON)
V(OFF)
In preheat condition, charging current is fixed at
2.5
R(SET)
ICHG(PREHEAT)
=
(6)
t
t
I
CC
In running mode, charging current decreases as the V
PIN6
15mA
rises from 0V to V
of the LAMP FB amplifier. The
OH
highest frequency will be attained when I
which is attained when V
is highest,
CHG
is at 0V:
PIN6
1.3mA
5
I
=
(7)
CHG(0)
R(SET)
Highest lamp power, and lowest output frequency are
attained when V is at its maximum output voltage
OH
Figure 6. Typical V and I Waveforms when
CC
CC
PIN6
the ML4831 is Started with a Bleed Resistor from
the Rectified AC Line and Bootstrapped from an
Auxiliary Winding.
(V ).
In this condition, the minimum operating frequency of the
ballast is set per (5) above.
For the IC to be used effectively in dimming ballasts with
higher Q output networks a larger C value and lower R
T
T
STARTING, RE-START, PREHEAT AND INTERRUPT
value can be used, to yield a smaller frequency excursion
over the control range (V ). The discharge current is set
The lamp starting scenario implemented in the ML4831
is designed to maximize lamp life and minimize ballast
heating during lamp out conditions.
PIN6
to 5mA. Assuming that I
>> I :
DIS
RT
tDIS(VCO) 490 × CT
(8)
The circuit in Figure 7 controls the lamp starting scenarios:
Filament preheat and Lamp Out interrupt. C(X) is charged
IC BIAS, UNDER-VOLTAGE LOCKOUT AND THERMAL
SHUTDOWN
with a current of I
/4 and discharged through R(X).
R(SET)
The voltage at C(X) is initialized to 0.7V (V ) at power
BE
up. The time for C(X) to rise to 3.4V is the filament preheat
time. During that time, the oscillator charging current
The IC includes a shunt regulator which will limit the
voltage at V to 13.5 (V
). The IC should be fed with
CC
CCZ
(I
) is 2.5/R(SET). This will produce a high frequency
a current limited source, typically derived from the ballast
transformer auxiliary winding. When V is below
CHG
for filament preheat, but will not produce sufficient
voltage to ignite the lamp.
CC
V
– 0.7V, the IC draws less than 1.7mA of quiescent
CCZ
current and the outputs are off. This allows the IC to start
using a “bleed resistor” from the rectified AC line.
After cathode heating, the inverter frequency drops to F
MIN
causing a high voltage to appear to ignite the lamp. If the
voltage does not drop when the lamp is supposed to have
ignited, the lamp voltage feedback coming into Pin 9 rises
to above V , the C(X) charging current is shut off and the
REF
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
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
