ML4835
FUNCTIONAL DESCRIPTION (Continued)
(C1) in the frequency compensation network. The
compensation network shown in Figure 5 will introduce a
zero and a pole at:
OSCILLATOR
The VCO frequency ranges are controlled by the output
of the LFB amplifier (R ). As lamp current decreases,
SET
1
1
LFB OUT falls in voltage, causing the C charging current
T
fZ
fP
(2)
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:
2 R1C1
2 R1C2
Figure 4 shows the output configuration for the
operational transconductance amplifiers.
A DC path to ground or V at the output of the
CC
1
transconductance amplifiers will introduce an offset error.
The magnitude of the offset voltage that will appear at the
FOSC
(3)
tCHG tDIS
input is given by V = io/gm. For an io of 1µA and a gm
OS
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.
and
ꢀ V
ꢃ
ICHG RT VTL
IICHG RT VTH
REF
tCHG RT CT In
(4)
ꢂV
ꢅ
ꢁ
ꢄ
REF
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.
The oscillator’s minimum frequency is set when I
where:
= 0
CHG
1
FMIN
(5)
0.51 RTCT
The oscillator's start frequency can be expressed by:
END OF LAMP LIFE
1
FSTART
(5a)
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.
0.51
R
R
CT
2 7
T
T2
Both equations assume that t
>> t
.
DIS
CHG
When LFB OUT is high, I
frequency occurs. The charging current varies according
to two control inputs to the oscillator:
= 0 and the minimum
CHG
1. The output of the preheat timer
2. The voltage at LFB OUT (lamp feedback amplifier
output)
EOL and the ML4835
In preheat condition, charging current is fixed at
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
2.5
RSET
ICHG(PREHEAT)
(6)
In running mode, charging current decreases as the
voltage rises from 0V to V
at the LAMP FB amplifier.
OH
applications notes for more details. Figure 4 illustrates a
simplified model of ML4835 EOL functionality.
The charging current behavior can be expressed as:
5V LEAO
RSET 8k 25%
ICHG
(7)
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
The highest frequency is attained when I
is highest,
CHG
which is attained when voltage at LFB OUT is at 0V:
5
discharging time t
of the oscillator capacitor C .
DIS
T
ICHG(0)
(8)
RSET
10
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
