ml4832 Microsemi Corporation, ml4832 Datasheet - Page 8

• Manufacturer Part Number: ml4832
• Description: Electronic Dimming Ballast Controller
• Manufacturer: Microsemi Corporation

ML4832
FUNCTIONAL DESCRIPTION
The magnitude of the offset voltage that will appear at
the input is given by V
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
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
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:
8
C
R
T
T
17
8
V
R
REF
T
/C
5.5mA
T
DIS
OS
of the oscillator capacitor C
= io/gm. For an io of 1mA and a
I
CHG
1.25/3.75
T
charging current to
+
–
Figure 5. Oscillator Block Diagram and Timing
CONTROL
V
REF
(Continued)
T
.
and
The oscillator’s minimum frequency is set when I
where:
This assumes that t
When LFB OUT is high, I
frequency occurs. The charging current varies according to
two control inputs to the oscillator:
In preheat condition, charging current is fixed at
V
V
1. The output of the preheat timer
2. The voltage at LFB OUT
TH
TL
= 3.75V
= 1.25V
C
CLOCK
T
t
CHG
R C In
I
T
F
CHG
CHG PREHEAT
F
OSC
t
DIS
OSC
T
(
>> t
CH

 
0 51
t
V
V
DIS
.
CHG
= 0 and the minimum
REF
REF
.
)
1
1
R C
I
I
t
CH
R
T
CH
DIS
. 25
SET
T
R
R
t
CHG
T
T
V
V
TH
TL
CH
= 0
(3)
(4)
(5)
(6)