ml4831 Microsemi Corporation, ml4831 Datasheet - Page 7

• Manufacturer Part Number: ml4831
• Description: Electronic Ballast Controller
• Manufacturer: Microsemi Corporation

Figure 3 shows the output configuration for the
operational transconductance amplifiers.
A DC path to ground or VCC 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
of 0.08 mhos the input referred offset will be 12.5mV.
Capacitor C1 as shown in Figure 2 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 ML4831.
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 4.
Figure 4. Transconductance Amplifier Characteristics
IQ –
gmV
2
IN
Figure 3. Output Configuration
IQ +
gmV
OS
2
i
O
IN
= io/gm. For a io of 1uA and a gm
0
Linear Slope Region
IN
IN
CURRENT
CURRENT
MIRROR
MIRROR
OUT
OUT
V
IN
Differential
io = gmV
IN
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
OSCILLATOR
The 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.
The oscillator frequency is determined by the following
equations:
and
V
V
TH
TL
= 3.75V
= 1.25V
Figure 5. Oscillator Block Diagram and Timing
C(T)
CLOCK
C(T)
R(T)
t
CHG
17
8
V
R(T)/C(T)
REF
DIS
R C In
T T
F
5 mA
t
DIS
OSC
of the oscillator capacitor C
I
CHG
t
V
1.25/3.75
V
CHG
REF
REF
1
I
I
CH
t
CH
DIS
R
R
+
–
T
T
t
CHG
CONTROL
V
V
V
ML4831
REF
TH
TL
T
.
(3)
(4)
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