ML4830
IC BIAS, UNDER-VOLTAGE LOCKOUT AND THERMAL
SHUTDOWN
This will produce a high frequency (or low duty cycle) for
filament preheat, but will not produce sufficient voltage to
ignite the lamp.
The IC includes a shunt regulator which will limit the
voltage at VCC to 13.5 (V ). The IC should be fed with
CCZ
After cathode heating, the inverter frequency drops to
a current limited source, typically derived from the ballast
transformer auxiliary winding. When VCC is below V
F
causing a high voltage to appear to ignite the lamp.
MIN
If the voltage does not drop when the lamp is supposed to
have ignited, the lamp voltage feedback coming into Pin
CCZ
– 0.7V, the IC draws less than 1.7mA of quiescent current
and the outputs are off. This allows the IC to start using a
“bleed resistor” from the rectified AC line.
10 rises to above V , the C(X) charging current is shut off
REF
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).
VCCZ
V(ON)
V
CC
V(OFF)
LFB OUT is ignored until C(X) reaches 6.8V threshold.
The lamps are therefore driven to full power and then
dimmed. The C(X) pin is clamped to about 7.5V.
t
t
A timing diagram of lamp ignition and restart sequences
provided by the circuit of Figure 4 is given in Figure 7.
15mA
I
CC
1.3mA
.625
R(SET)
R(X)/C(X)
Figure 3. Typical V and I waveforms when ML4830
is started with a bleed resistor from the rectified AC line
and bootstrapped from the ballast transformer.
+
CC
CC
12
HEAT
C(X)
–
1.2/3.4
1.2/6.8
R(X)
6.8
To help reduce ballast cost, the ML4830 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 ML4830’s die temperature can be estimated
with the following equation:
+
–
DIMMING
LOCKOUT
R
S
INHIBIT
INT
Q
–
10
+
V
REF
TJ TA ×PD × 65°C / W
(7)
Figure 4. Lamp Preheat and Interrupt Timers
STARTING, RE-START, PREHEAT AND INTERRUPT
Mode
PWM
50%
FM
The lamp starting scenario implemented in the ML4830 is
designed to maximize lamp life and minimize ballast
heating during lamp out conditions.
[F(MAX) to F(MIN)]
2
Preheat
Dimming
Lock-out
The circuit in Figure 4 controls the lamp starting scenarios:
Filament preheat and Lamp Out interrupt. C(X) is charged
D(MAX)%
0 to D(MAX)%
F(MIN)
Dimming
Control
IR(SET)
0.625
R(SET)
F(MIN) to F(MAX)
with a current of
or
and discharged
4
Figure 5. Lamp Starting Summary
through R(X). The voltage at C(X) is initialized to 0.7V
BE
filament preheat time. During that time, the oscillator
(V ) at power up. The time for C(X) to rise to 3.4V is the
A summary of the lamp starting scenarios are given in
figure 5 for both PWM and Frequency Modulation modes.
The PWM duty cycle is defined as:
2.5
R(SET)
charging current (I
)is
inboth VCO modes.
CHG
tON
Duty Cycle =
tCLK
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
