ML4833 데이터 시트보기 (PDF) - Micro Linear Corporation
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ML4833 Datasheet PDF : 13 Pages
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ML4833
Figure 4 shows the output configuration for the
operational transconductance amplifiers.
CURRENT
MIRROR
IN
OUT
IQ
+
gmVIN
2
IQ –
gmVIN
2
io = gmVIN
IN
OUT
CURRENT
MIRROR
Figure 4. 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 VOS = io/gm. For an io of 1µA and a gm
of 0.05 µW the input referred offset will be 20mV.
Capacitor C1 as shown in Figure 3 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 ML4833.
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 5.
iO
OSCILLATOR
The VCO frequency ranges are controlled by the output
of the LFB amplifier (RSET). As lamp current increases,
LFB OUT falls in voltage, causing the CT charging current
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.
VREF
17
RT
ICHG
VREF
CONTROL
RT/CT
7
CT
5.5mA
+
1.25/3.75 –
CLOCK
tDIS
VTH = 3.75V
CT
VTL = 1.25V
tCHG
Figure 6. Oscillator Block Diagram and Timing
0
Linear Slope Region
VIN Differential
Figure 5. Transconductance Amplifier Characteristics
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 tDIS of the oscillator capacitor CT.
The oscillator frequency is determined by the following
equations:
FOSC
=
1
tCHG +
tDIS
(3)
and
tCHG
=
RTCT
In
VREF
VREF
+ ICH RT
+ ICH RT
−
−
VTL
VTH
(4)
The oscillator’s minimum frequency is set when ICH = 0
where:
FOSC
≅
1
0.51× RTCT
(5)
8
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