ML4832
FUNCTIONAL DESCRIPTION
(Continued)
The magnitude of the offset voltage that will appear at
the input is given by V
OS
= io/gm. For an io of 1mA and a
gm of 0.05 µmhos the input referred offset will be 20mV.
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 ML4832.
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.
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. As lamp current decreases, LFB
OUT 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:
F
OSC
=
and
1
t
CHG
+
t
DIS
(3)
t
CHG
=
R
T
C
T
In
V
V
REF
REF
+
I
CH
R
T
−
V
TL
+
I
CH
R
T
−
V
TH
(4)
The oscillator’s minimum frequency is set when I
CH
= 0
where:
F
OSC
≅
1
0.51
×
R
T
C
T
(5)
This assumes that t
CHG
>> t
DIS
.
When LFB OUT is high, I
CH
= 0 and the minimum
frequency occurs. The charging current varies according to
two control inputs to the oscillator:
1. The output of the preheat timer
2. The voltage at LFB OUT
In preheat condition, charging current is fixed at
I
CHG (PREHEAT)
=
25
.
R
SET
(6)
17
V
REF
V
REF
R
T
I
CHG
CONTROL
CLOCK
R
T
/C
T
8
1.25/3.75
C
T
5.5mA
+
–
t
DIS
V
TH
= 3.75V
t
CHG
C
T
V
TL
= 1.25V
Figure 5. Oscillator Block Diagram and Timing
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
