UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
ond blank time if a true open lamp existed. If the open
lamp voltage is increased, the peak clamp circuit voltage
(equation 16) would need to be increased accordingly. A
peak VBAT-Vbuck voltage of 10.5V has been set for
open lamp detection in this example. (R10 = 2k,
R11 = 1k).
Voltage Regulator
The UCC3972/3 controller contains an internal 18V shunt
regulator that provides a 5% accurate voltage clamp for
the MOSFET gate drive while allowing the controller to
operate in applications with input voltages up to 25V.
Since only the VBAT and BUCK pins are rated for 25V,
the shunt regulator limits the voltage on the VDD and
OUT pins to 18V. The MODE, CS, and COMP pin volt-
ages are typically less than 5V. If the UCC3972/3 is to be
used in an application with input voltages greater than
18V, a resistor from VBAT to VDD is required to limit the
current into the VDD pin. The resistor should be sized to
allow sufficient current to operate the controller and drive
the external MOSFET gate, while minimizing the voltage
drop across the resistor. A bypass capacitor should be
connected at the VDD pin to provide a constant operat-
ing voltage.
Figure 9. VBUCK and MODE pin voltages during an open
lamp fault start-up.
Normal Startup
In practice, the lamp will typically strike in much less than
1 second (usually within the first few cycles) and the volt-
age at the transformer voltage will collapse to below the
open lamp trip level. Difficulty in striking the lamp usually Selecting the Shunt Resistor:
results from one or a combination of the following:
The first step in selecting the shunt resistor is to deter-
mine the current requirements for the application. With a
100kHz switching frequency and a maximum gate
charge of 11nC for the IRFL014 , the gate drive circuit re-
quires 1.1mA of average current. The UCC3972/3 re-
quires an additional maximum quiescent current of
1.5mA. The shunt resistor must therefore supply 2.6mA
of current over the operating voltage of the part.
Insufficient transformer turns ratio or input voltage.
Increase in required striking voltage at cold
temperature.
The lamp has set for a long period of time.
Transformer secondary voltage is reduced due to
voltage division between parasitic secondary
capacitance and the ballast capacitor.
The application’s maximum input voltage is 22V. With a
regulator clamp voltage of 18V, the maximum value for
the shunt resistor becomes 1.5kW [(22-18)V/2.6mA]. This
resistor will minimize losses at maximum input voltage,
but could produce a 4V drop (from VBAT to VDD) even
when the regulator is not clamped. This drop reduces the
available gate drive voltage, leaving only 6V with the
minimum input voltage of 10V. Since the efficiency of the
shunt regulator is not of primary importance when the
charger is running, a smaller value of shunt resistor is se-
lected to improve the available gate drive voltage. A
470W shunt resistor will produce a maximum 1.2V drop
from VBAT to VDD when the shunt regulator is not
clamped. When the regulator is clamped at 18V and the
charger voltage is at its maximum of 22V, the power
across the shunt resistor will be 35mW [(4V x 4V)/470].
Setting the Open Lamp Trip Level
The buck voltage is monitored by an internal 7V com-
parator to detect an open lamp. The actual trip voltage
across the resonant tank is set with an external resistor
divider R10 and R11.
VOPENLAMP =VIN –VBUCK
(18)
æ
ç
è
ö
R10+ R11
÷
=
·7V PEAK
ø
R10
R10 and R11 should be in the 1kW-5kW range, to guar-
antee sharp zero crossing edges at the buck pin of the
IC. In most applications the peak clamp voltage would be
set to a higher level than the open lamp trip voltage, en-
suring the converter would shut down after the one sec-
14
TI [ TEXAS INSTRUMENTS ]
