ml4835 Microsemi Corporation, ml4835 Datasheet - Page 10

• Manufacturer Part Number: ml4835
• Description: Compact Fluorescent Electronic Dimming Ballast Controller
• Manufacturer: Microsemi Corporation

ML4835
FUNCTIONAL DESCRIPTION
(C1) in the frequency compensation network. The
compensation network shown in Figure 5 will introduce a
zero and a pole at:
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 µW the input referred offset will be 20mV.
Capacitor C1 as shown in Figure 5 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 ML4835.
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, as illustrated in Figure 7.
END OF LAMP LIFE
At the end of a lamp’s life when the emissive material is
depleted, the arc current is rectified and high voltage
occurs across the lamp near the depleted cathode. The
ballast acts as a constant current source so power is
dissipated near the depleted cathode which can lead to
arcing and bulb cracking. Compact fluorescent lamps are
more prone to cracking or shattering because their small
diameter can’t dissipate as much heat as the larger linear
lamps. Compact fluorescents also present more of a
safety hazard since they are usually used in downlighting
systems without reflector covers.
EOL and the ML4835
The ML4835 uses a circuit that creates a DC voltage
representative of the power supplied to the lamps through
the inverter. This voltage is used by the ML4835 to latch
off the ballast when it exceeds an internal threshold. An
external resistor can be used as the “EOL latch resistor” to
set the power level trip point, as shown in by R9 in Figure
12. See Micro Linear ML4835 User Guide and
applications notes for more details. Figure 4 illustrates a
simplified model of ML4835 EOL functionality.
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
10
f
Z
=
2
p
R C
1
1 1
DIS
OS
of the oscillator capacitor C
= io/gm. For an io of 1µA and a gm
f
P
=
CC
2
p
at the output of the
R C
1
1 2
(Continued)
T
.
(2)
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:
The oscillator's start frequency can be expressed by:
Both equations assume 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
In running mode, charging current decreases as the
voltage rises from 0V to V
The charging current behavior can be expressed as:
The highest frequency is attained when I
which is attained when voltage at LFB OUT is at 0V:
1. The output of the preheat timer
2. The voltage at LFB OUT (lamp feedback amplifier
output)
F
F
F
I
I
I
t
CHG PREHEAT
CHG
CHG
OSC
CHG
MIN
START
( ) 0
(
@
=
=
=
=
=
0 51
R
t
R C In
5
CHG
.
0 51
SET
T
R
V
.
SET
5
´
)
T
1
-
1
+
=
R C
´
8
t
T
2
R
DIS

 
R R
k
25
LEAO
SET
V
T
.
±
V
T
REF
1
SET
REF
25%
T
CHG
). As lamp current decreases,
2
+
OH
+
7
II
I
´
CHG
CHG
CHG
at the LAMP FB amplifier.
C
= 0 and the minimum
T
´
´
>> t
R
R
T
T
-
-
DIS
T
V
V
CHG
charging current
.
TL
TH
is highest,
CHG
(5a)
= 0
(3)
(4)
(5)
(6)
(7)
(8)