UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
The current sense resistor R
S
provides direct control of
lamp current. Since the current sense resistor voltage is
controlled to a 1.5V reference, its power loss is inversely
proportional to its value at a given lamp current.
Synchronizing the Stages
An internal comparator at the BUCK node is used to syn-
chronize the PWM buck frequency to twice the resonant
tank frequency. Synchronization is accomplished with
sync pulse that is generated each time the BUCK node
voltage is within 1.0V of VBAT; the UCC3972/3 uses this
sync pulse to reset the PWM oscillator’s saw-tooth ramp.
The syn circuit will operate at 2 X the transform switching
frequency.
Buck Stage Design
The PWM output controls current in the buck inductor.
The UCC3972/3’s buck power stage differs from a tradi-
tional buck topology in a few respects:
•
The topology is inverted using a ground referenced
N-Channel MOSFET rather than a VDD referenced
P-Channel.
•
The output voltage is a full wave rectified sinewave at
the switching frequency, rather than DC.
Referring back to Fig. 1, when OUT turns S
BUCK
on, the
BUCK node voltage V
BUCK
is placed across the inductor.
This voltage is typically positive and current ramps up in
the inductor (it is possible for the BUCK node voltage to
go negative if VBAT is low and the lamp current is near
is
turned
off,
maximum).
When
S
BUCK
VBAT-V
BUCK
+V
DBUCK
is placed across the inductor with
opposite polarity. As with any buck converter, the
volt-seconds across the inductor must be reversed on
each switching cycle to maintain constant current. The
duty cycle (D) relationship is complicated somewhat by
the fact the output voltage is changing within a switching
cycle. The equations below determine the relationship
between on and off times in continuous conduction mode
where T is the switching period, D = t
ON
/T, and t
OFF
= T-
t
ON
.
t
ON
0
V
BUCK
_
AVE
=
V
BAT
-
=
V
BAT
-
V
SEC
·
2
N
·p
(9)
820
·
2
=
V
BAT
-
5. 5
·
Volts
67
· p
The approximate on time using the maximum 22V input
voltage (V
BUCK_AVE
= 16.4), a 100kHz switching fre-
quency (two times the resonant frequency), and ignoring
the diode drop can be calculated from the following:
VBAT
-
V
BUCK
_
AVE
t
ON
=
T
-
t
ON
V
BUCK
_
AVE
(10)
The resulting on time is 2.5ms. A 150mH inductor will re-
sult in a peak to peak ripple current of 280mA. Average
inductor current (with maximum lamp current) can be cal-
culated by taking the lamp power divided by the tank effi-
ciency and the RMS buck voltage.
I
BUCK
=
æ
V
LAMP
·
I
LAMP
ö æ
2
·
N
ö
375
·
0. 005
·
2
·
67
÷=
ç
÷·ç
ç
÷ ç
÷
0. 8
·
820
è
Efficiency
ø è
V
SEC
ø
=
380
mA
The resulting inductor ripple is less than 50%. A list of
possible inductors are given below along with ESR and
current rating (losses in the inductor are calculated with
RMS current).
The choice of a MOSFET for the buck switch should take
into consideration conduction and switching losses. The
R
DS(on)
and gate charge are typically at odds, however,
where minimizing one will typically result in the other in-
creasing. An International Rectifier IRFL014 was se-
lected (SOT-223 package) in this application with a gate
charge of 11nC and R
DS(on)
of 0.2W. A Schottky diode
should be used for the buck diode in order to minimize
forward drop.
(11)
ò
V
BUCK
·
dt
=
ò
(
VBAT
-
V
BUCK
t
ON
T
+
V
D
)
·
dt
(8)
Vendor
Table 3. Inductor Suppliers
L
Part Number
ESR
0.38
0.175
0.4
0.73
Current
Rating
1A
0.72A
0.85A
0.4A
Selecting the buck inductor:
Maximum ripple current in the inductor occurs when fre-
quency and duty cycle are at a minimum, which corre-
sponds to VBAT and lamp current being a maximum.
The average value of V
BUCK
at rated lamp current is
equal to:
Coilcraft
(847) 639-6400
Coiltronics (407)
241-7876
Sumida
(847) 956-0666
Toko (847) 297-0070
150mH DO3316-154
150mH
CTX150-4
150mH CDR125-151
150mH
646CY-151
9
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