ML4832 Datasheet, PDF (9/15 Pages) Micro Linear Corporation – Electronic Dimming Ballast Controller

FUNCTIONAL DESCRIPTION (Continued)
In running mode, charging current decreases as the V_PIN6
rises from 0V to V_OH of the LAMP FB amplifier. The
highest frequency will be attained when I_CHG is highest,
which is attained when LFB OUT is at 0V:
I_CHG(0)
=
5
R_SET
(7)
Highest lamp power, and lowest output frequency are
attained when LFB OUT is at its maximum output voltage
(V_OH).
In this condition, the minimum operating frequency of the
ballast is set per (5) above.
For the IC to be used effectively in dimming ballasts with
higher Q output networks a larger C_T value and lower R_T
value can be used, to yield a smaller frequency excursion
over the control range (V_{LFB OUT}). The discharge current is
set to 5.5mA. Assuming that I_DIS >> I_RT:
t_DIS(VCO) ≅ 600 × C_T
(8)
IC BIAS, UNDER-VOLTAGE LOCKOUT AND THERMAL
SHUTDOWN
The IC includes a shunt regulator which will limit the
voltage at V_CC to 15V (V_CCZ). The IC should be fed with
a current limited source, typically derived from the ballast
transformer auxiliary winding. When V_CC is below
V_CCZ – 1.1V, the IC draws less than 0.48mA of quiescent
current and the outputs are off. This allows the IC to start
using a “bleed resistor” from the rectified AC line.
ML4832
To help reduce ballast cost, the ML4832 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 ML4832’s die temperature can be estimated
with the following equation:
T_J ≅ T_A × P_D × 65° C / W
(9)
STARTING, RE-START, PREHEAT AND INTERRUPT
The lamp starting scenario implemented in the ML4832
is designed to maximize lamp life and minimize ballast
heating during lamp out conditions.
The circuit in Figure 7 controls the lamp starting scenarios:
Filament preheat and lamp out interrupt. C_X is charged
with a current of I_RSET/4 and discharged through R_X. The
voltage at C_X is initialized to 0.7V (V_BE) at power up. The
time for C_X to rise to 4.8V is the filament preheat time.
During that time, the oscillator charging current (I_CHG) is
2.5/R_SET. This will produce a high frequency for filament
preheat, but will not produce sufficient voltage to ignite
the lamp.
After cathode heating, the inverter frequency drops to
F_MIN 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 9
rises to above V_REF, 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
VCC VCCZ
V_ON
VOFF
ICC
t
5.5mA
0.34mA
t
Figure 6. Typical VCC and ICC Waveforms when
the ML4832 is Started with a Bleed Resistor from
the Rectified AC Line and Bootstrapped from an
Auxiliary Winding.
CX
RX
RX/CX
10
6.8
0.625
RSET
+
1.2/4.8 –
+
1.2/6.8 –
INT
9
–
VREF
+
HEAT
DIMMING
LOCKOUT
R INHIBIT
Q
S
Figure 7. Lamp Preheat and Interrupt Timers
9