ml4833 Microsemi Corporation, ml4833 Datasheet - Page 8

• Manufacturer Part Number: ml4833
• Description: Electronic Dimming Ballast Controller
• Manufacturer: Microsemi Corporation

ML4833
Figure 4 shows the output configuration for the
operational transconductance amplifiers.
A DC path to ground or V
transconductance amplifiers will introduce an offset error.
The magnitude of the offset voltage that will appear at the
input is given by V
of 0.05 µ
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.
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
8
Figure 5. Transconductance Amplifier Characteristics
IQ –
gmV
2
IN
the input referred offset will be 20mV.
Figure 4. Output Configuration
IQ +
DIS
gmV
OS
2
i
IN
of the oscillator capacitor C
= io/gm. For an io of 1µA and a gm
O
0
IN
IN
Linear Slope Region
CC
CURRENT
CURRENT
MIRROR
MIRROR
at the output of the
OUT
OUT
V
IN
Differential
io = gmV
T
.
IN
OSCILLATOR
The VCO frequency ranges are controlled by the output
of the LFB amplifier (R
LFB OUT falls in voltage, causing the C
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.
The oscillator frequency is determined by the following
equations:
and
The oscillator’s minimum frequency is set when I
where:
V
V
TH
TL
= 3.75V
= 1.25V
Figure 6. Oscillator Block Diagram and Timing
C
CLOCK
T
C
R
T
T
t
CHG
17
7
V
R
REF
T
R C In
/C
5.5mA
T T
F
F
t
T
DIS
OSC
OSC
SET
). As lamp current increases,
I
CHG
0 51
t
V
1.25/3.75
V
CHG
.
REF
REF
1
1
R C
I
I
CH
t
CH
T T
DIS
R
R
+
–
T
T
t
T
CHG
CONTROL
charging current
V
V
V
TH
REF
TL
CH
= 0
(3)
(4)
(5)