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ML4830 Ver la hoja de datos (PDF) - Micro Linear Corporation

Número de pieza
componentes Descripción
Fabricante
ML4830
Micro-Linear
Micro Linear Corporation 
ML4830 Datasheet PDF : 15 Pages
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ML4830
IC BIAS, UNDER-VOLTAGE LOCKOUT AND THERMAL
SHUTDOWN
The IC includes a shunt regulator which will limit the
voltage at VCC to 13.5 (VCCZ). The IC should be fed with
a current limited source, typically derived from the ballast
transformer auxiliary winding. When VCC is below VCCZ
– 0.7V, the IC draws less than 1.7mA of quiescent current
and the outputs are off. This allows the IC to start using a
“bleed resistor” from the rectified AC line.
VCCZ
VCC V(ON)
V(OFF)
t
15mA
ICC
1.3mA
t
Figure 3. Typical VCC and ICC waveforms when ML4830
is started with a bleed resistor from the rectified AC line
and bootstrapped from the ballast transformer.
To help reduce ballast cost, the ML4830 includes a
temperature sensor which will inhibit ballast operation if
the IC’s junction temperature exceeds 120°C. In order to
use this sensor in lieu of an external sensor, care should be
taken when placing the IC to ensure that it is sensing
temperature at the physically appropriate point in the
ballast. The ML4830’s die temperature can be estimated
with the following equation:
TJ TA × PD × 65°C / W
(7)
STARTING, RE-START, PREHEAT AND INTERRUPT
The lamp starting scenario implemented in the ML4830 is
designed to maximize lamp life and minimize ballast
heating during lamp out conditions.
The circuit in Figure 4 controls the lamp starting scenarios:
Filament preheat and Lamp Out interrupt. C(X) is charged
with a current of IR(SET) or 0.625 and discharged
4 R(SET)
through R(X). The voltage at C(X) is initialized to 0.7V
(VBE) at power up. The time for C(X) to rise to 3.4V is the
filament preheat time. During that time, the oscillator
charging
current
(ICHG)
is
2.5
R(SET)
in
both
VCO
modes.
This will produce a high frequency (or low duty cycle) for
filament preheat, but will not produce sufficient voltage to
ignite the lamp.
After cathode heating, the inverter frequency drops to
FMIN causing a high voltage to appear to ignite the lamp.
If the voltage does not drop when the lamp is supposed to
have ignited, the lamp voltage feedback coming into Pin
10 rises to above VREF, the C(X) charging current is shut off
and the inverter is inhibited until C(X) is discharged by
R(X) to the 1.2V threshold. Shutting off the inverter in this
manner prevents the inverter from generating excessive
heat when the lamp fails to strike or is out of socket.
Typically this time is set to be fairly long by choosing a
large value of R(X).
LFB OUT is ignored until C(X) reaches 6.8V threshold.
The lamps are therefore driven to full power and then
dimmed. The C(X) pin is clamped to about 7.5V.
A timing diagram of lamp ignition and restart sequences
provided by the circuit of Figure 4 is given in Figure 7.
R(X)/C(X)
12
C(X)
R(X)
6.8
.625
R(SET)
+
1.2/3.4 –
+
1.2/6.8 –
INT
10
VREF
+
HEAT
DIMMING
LOCKOUT
R INHIBIT
Q
S
Figure 4. Lamp Preheat and Interrupt Timers
Mode
Preheat
Dimming
Lock-out
Dimming
Control
PWM
50%
FM
[F(MAX) to F(MIN)]
2
D(MAX)%
F(MIN)
0 to D(MAX)%
F(MIN) to F(MAX)
Figure 5. Lamp Starting Summary
A summary of the lamp starting scenarios are given in
figure 5 for both PWM and Frequency Modulation modes.
The PWM duty cycle is defined as:
Duty Cycle = tON
tCLK
8

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