ML4831
Figure 3 shows the output configuration for the
operational transconductance amplifiers.
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
.
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.
V
REF
CURRENT
MIRROR
IN
OUT
IQ +
gmV
IN
IQ –
2
gmV
IN
2
io = gmV
IN
17
IN
OUT
CURRENT
MIRROR
V
REF
R(T)
I
CHG
CONTROL
R(T)/C(T)
Figure 3. Output Configuration
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
OS
= io/gm. For a io of 1uA and a gm
of 0.08
µmhos
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.
i
O
C(T)
8
1.25/3.75
+
–
5 mA
CLOCK
t
DIS
V
TH
= 3.75V
t
CHG
C(T)
V
TL
= 1.25V
0
Linear Slope Region
V
IN
Differential
Figure 5. Oscillator Block Diagram and Timing
The oscillator frequency is determined by the following
equations:
F
OSC
=
and
Figure 4. Transconductance Amplifier Characteristics
1
t
CHG
+
t
DIS
(3)
V
+
I R
−
V
TL
t
CHG
=
R
T
C
T
In
REF CH T
V
REF
+
I
CH
R
T
−
V
TH
(4)
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
