ML4832
FUNCTIONAL DESCRIPTION (Continued)
In running mode, charging current decreases as the V
To help reduce ballast cost, the ML4832 includes a
PIN6
rises from 0V to V
of the LAMP FB amplifier. The
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 ML4832’s die temperature can be estimated
with the following equation:
OH
highest frequency will be attained when I
is highest,
CHG
which is attained when LFB OUT is at 0V:
5
RSET
ICHG(0)
=
(7)
TJ TA × PD × 65° C / W
(9)
Highest lamp power, and lowest output frequency are
attained when LFB OUT is at its maximum output voltage
(V ).
OH
STARTING, RE-START, PREHEAT AND INTERRUPT
In this condition, the minimum operating frequency of the
ballast is set per (5) above.
The lamp starting scenario implemented in the ML4832
is designed to maximize lamp life and minimize ballast
heating during lamp out conditions.
For the IC to be used effectively in dimming ballasts with
higher Q output networks a larger C value and lower R
The circuit in Figure 7 controls the lamp starting scenarios:
T
T
value can be used, to yield a smaller frequency excursion
over the control range (V ). The discharge current is
Filament preheat and lamp out interrupt. C is charged
X
with a current of I
/4 and discharged through R . The
LFB OUT
RSET
X
set to 5.5mA. Assuming that I >> I :
voltage at C is initialized to 0.7V (V ) at power up. The
DIS
RT
X
BE
time for C to rise to 4.8V is the filament preheat time.
X
During that time, the oscillator charging current (I
) is
CHG
tDIS(VCO) 600 × CT
(8)
2.5/R . This will produce a high frequency for filament
SET
preheat, but will not produce sufficient voltage to ignite
the lamp.
IC BIAS, UNDER-VOLTAGE LOCKOUT AND THERMAL
SHUTDOWN
After cathode heating, the inverter frequency drops to
MIN
The IC includes a shunt regulator which will limit the
F
causing a high voltage to appear to ignite the lamp.
voltage at V to 15V (V
). The IC should be fed with
CC
CCZ
If the voltage does not drop when the lamp is supposed to
have ignited, the lamp voltage feedback coming into pin 9
a current limited source, typically derived from the ballast
transformer auxiliary winding. WhenV is below
CC
rises to above V , the C charging current is shut off and
REF
X
V
CCZ
– 1.1V, the IC draws less than 0.48mA of quiescent
the inverter is inhibited until C is discharged by R to the
X
X
current and the outputs are off. This allows the IC to start
using a “bleed resistor” from the rectified AC line.
1.2V threshold. Shutting off the inverter in this manner
prevents the inverter from generating excessive heat when
V
CC
VCCZ
V
ON
0.625
R
SET
V
OFF
R /C
X X
+
–
10
HEAT
C
X
1.2/4.8
1.2/6.8
R
X
t
t
6.8
I
CC
5.5mA
+
–
DIMMING
LOCKOUT
R
S
INHIBIT
0.34mA
INT
Q
–
9
+
V
REF
Figure 6. Typical VCC and ICC Waveforms when
the ML4832 is Started with a Bleed Resistor from
the Rectified AC Line and Bootstrapped from an
Auxiliary Winding.
Figure 7. Lamp Preheat and Interrupt Timers
9
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
