ML4830
Also, in both VCO modes, the when LFB OUT is high,
= 0 and the minimum frequency occurs. The
Mode
Pin 8
Definition
I
CHG
charging current varies according to two control inputs to
the oscillator:
VCO
GND
OPEN
VREF
Frequency Modulation
Overlapping VCO F.M.
Pulse Width Modulation
VCO-O
PWM
1. The output of the preheat timer
2. The voltage at Pin 6 (lamp current output)
In preheat condition, charging current is fixed at
Table 1. ML4830 Operating Modes
2.5
R(SET)
ICHG(PREHEAT)
=
OSCILLATOR
(1)
In Table 1 above, the two VCO frequency ranges are
controlled by the output of the LFB amplifier (Pin 6). As
lamp current decreases, Pin 6 rises in voltage, causing the
C(T) charging current to decrease, thereby causing the
oscillator frequency to decrease. Since the ballast output
network attenuates high frequencies, the power to the
lamp will be increased.
In running mode, charging current decreases as the V
PIN7
rises from 0V to V
of trhe LAMP FB amplifier. The
OH
highest frequency will be attained when I
is highest,
CHG
which is attained when V
is at 0V:
PIN6
5
ICHG(0)
=
(2)
R(SET)
In PWM Mode, I
is 0 so the oscillator’s frequency is
CHG
set per (1) below.
The oscillator frequency is determined by the following
equations:
1
+ t
F
=
V
OSC
(3)
REF
t
V
18
REF
CHG
DIS
and
CONTROL
I
CHG
R(T)
C(T)
6.25 +I
3.75 +I
R
R
CHG
CHG
T
T
t
= R C In
T T
CHG
(4)
R(T)/C(T)
+
–
9
The oscillator’s minimum frequency is set when I
where:
= 0
CHG
1.25/3.75
5 mA
1
FOSC
(5)
0.51×RTCT
This assumes that t
>> t
.
DIS
CHG
Highest lamp power, and lowest output frequency are
attained when V is at its maximum output voltage
PIN6
(V ).
OH
In this condition, the minimum operating frequency of the
ballast is set per (5) above.
CLOCK
For the IC to be used effectively in dimming ballasts with
t
t
CHG
DIS
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 (V ). The discharge current is set
V
TH
PIN6
to 5mA. Assuming that I >> I :
DIS
RT
C(T)
t
490 × C
(6)
DIS(VCO)
T
V
TL
Figure 2. Oscillator Block Diagram and Timing
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
