ML4833
Figure 4 shows the output configuration for the
operational transconductance amplifiers.
OSCILLATOR
The VCO frequency ranges are controlled by the output
of the LFB amplifier (R
SET
). As lamp current increases,
LFB OUT falls in voltage, causing the C
T
charging current
to increase, thereby causing the oscillator frequency to
increase. Since the ballast output network attenuates high
frequencies, the power to the lamp will be decreased.
CURRENT
MIRROR
IN
OUT
gmV
IN
2
io = gmV
IN
IQ +
IQ –
gmV
IN
2
17
V
REF
I
CHG
V
REF
CONTROL
R
T
IN
OUT
CURRENT
MIRROR
C
T
7
R
T
/C
T
1.25/3.75
+
–
Figure 4. Output Configuration
A DC path to ground or V
CC
at the output of the
transconductance amplifiers will introduce an offset error.
The magnitude of the offset voltage that will appear at the
input is given by V
OS
= io/gm. For an io of 1µA and a gm
of 0.05 µW the input referred offset will be 20mV.
Capacitor C1 as shown in Figure 3 is used to block the
DC current to minimize the adverse effect of offsets.
Slew rate enhancement is incorporated into all of the
operational transconductance amplifiers in the ML4833.
This improves the recovery of the circuit in response to
power up and transient conditions. The response to large
signals will be somewhat non-linear as the transconductance
amplifiers change from their low to high transconductance
mode. This is illustrated in Figure 5.
i
O
5.5mA
Figure 5. Transconductance Amplifier Characteristics
BALLAST OUTPUT SECTION
The IC controls output power to the lamps via frequency
modulation with non-overlapping conduction. This means
that both ballast output drivers will be low during the
discharging time t
DIS
of the oscillator capacitor C
T
.
8
W
CLOCK
t
DIS
V
TH
= 3.75V
t
CHG
C
T
V
TL
= 1.25V
Figure 6. Oscillator Block Diagram and Timing
V
IN
Differential
Linear Slope Region
0
The oscillator frequency is determined by the following
equations:
F
OSC
=
and
V
+
I R
−
V
TL
t
CHG
=
R
T
C
T
In
REF CH T
V
REF
+
I
CH
R
T
−
V
TH
1
t
CHG
+
t
DIS
(3)
(4)
The oscillator’s minimum frequency is set when I
CH
= 0
where:
F
OSC
≅
1
0.51
×
R
T
C
T
(5)
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
