ML4831 Ver la hoja de datos (PDF) - Micro Linear Corporation
Número de pieza
componentes Descripción
Fabricante
ML4831 Datasheet PDF : 14 Pages
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Figure 3 shows the output configuration for the
operational transconductance amplifiers.
CURRENT
MIRROR
IN
OUT
IQ + gmVIN
2
IQ – gmVIN
2
io = gmVIN
ML4831
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.
OSCILLATOR
The VCO frequency ranges are controlled by the output of
the LFB amplifier (Pin 6). As lamp current decreases, Pin 6
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.
IN
OUT
CURRENT
MIRROR
Figure 3. 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 a io of 1uA and a gm
of 0.08 µmhos the input referred offset will be 12.5mV.
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 ML4831.
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.
iO
0
Linear Slope Region
VIN Differential
Figure 4. Transconductance Amplifier Characteristics
VREF
17
R(T)
ICHG
VREF
CONTROL
R(T)/C(T)
8
+
1.25/3.75 –
C(T)
5 mA
CLOCK
tDIS
VTH = 3.75V
tCHG
C(T)
VTL = 1.25V
Figure 5. Oscillator Block Diagram and Timing
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)
7
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