ml4831 Microsemi Corporation, ml4831 Datasheet - Page 8

• Manufacturer Part Number: ml4831
• Description: Electronic Ballast Controller
• Manufacturer: Microsemi Corporation

ML4831
The oscillator’s minimum frequency is set when I
where:
This assumes that t
When LFB OUT is high, I
frequency occurs. The charging current varies according
to two control inputs to the oscillator:
In preheat condition, charging current is fixed at
In running mode, charging current decreases as the V
rises from 0V to V
highest frequency will be attained when I
which is attained when V
Highest lamp power, and lowest output frequency are
attained when V
(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 can be used, to yield a smaller frequency excursion
over the control range (V
to 5mA. Assuming that I
IC BIAS, UNDER-VOLTAGE LOCKOUT AND THERMAL
SHUTDOWN
The IC includes a shunt regulator which will limit the
voltage at V
a current limited source, typically derived from the ballast
transformer auxiliary winding. When V
V
current and the outputs are off. This allows the IC to start
using a “bleed resistor” from the rectified AC line.
8
CCZ
OH
1. The output of the preheat timer
2. The voltage at Pin 6 (lamp feedback amplifier
).
– 0.7V, the IC draws less than 1.7mA of quiescent
output)
CC
to 13.5 (V
PIN6
I
OH
CHG PREHEAT
CHG
t
F
DIS VCO
OSC
is at its maximum output voltage
I
of the LAMP FB amplifier. The
CHG( )
(
(
>> t
DIS
PIN6
CCZ
CH
PIN6
0
0 51
>> I
)
DIS
.
= 0 and the minimum
). The IC should be fed with
). The discharge current is set
is at 0V:
.
490
R SET
)
RT
(
1
R C
5
:
R SET
T T
(
2 5
T
)
C
.
value and lower R
T
CC
)
CHG
is below
is highest,
CH
= 0
PIN6
(5)
(6)
(7)
(8)
T
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:
STARTING, RE-START, PREHEAT AND INTERRUPT
The lamp starting scenario implemented in the ML4831
is designed to maximize lamp life and minimize ballast
heating during lamp out conditions.
The circuit in Figure 7 controls the lamp starting scenarios:
Filament preheat and Lamp Out interrupt. C(X) is charged
with a current of I
The voltage at C(X) is initialized to 0.7V (V
up. The time for C(X) to rise to 3.4V is the filament preheat
time. During that time, the oscillator charging current
(I
for filament preheat, but will not produce sufficient
voltage to ignite the lamp.
After cathode heating, the inverter frequency drops to F
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
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).
CHG
V
I
the ML4831 is Started with a Bleed Resistor from
CC
CC
the Rectified AC Line and Bootstrapped from an
Figure 6. Typical V
) is 2.5/R(SET). This will produce a high frequency
V(OFF)
V(ON)
1.3mA
VCCZ
15mA
REF
, the C(X) charging current is shut off and the
R(SET)
T
J
Auxiliary Winding.
T
/4 and discharged through R(X).
A
CC
P
and I
D
65
CC
C W
Waveforms when
/
BE
) at power
t
t
MIN
(9)