UCC1973 datasheet(9/26 Pages) TI2

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UCC1972/3

UCC2972/3

UCC3972/3

The current sense resistor RS 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 SBUCK on, the

BUCK node voltage VBUCK 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

maximum).

When

SBUCK

turned

off,

VBAT-VBUCK+VDBUCK 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 = tON/T, and tOFF =T-

tON.

()

VBAT V

BUCK

·=

òò

(8)

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 VBUCK at rated lamp current is

equal to:

BUCK AVE

BAT

SEC

BAT

820

olts

(9)

The approximate on time using the maximum 22V input

voltage (VBUCK_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

(10)

The resulting on time is 2.5

ms. 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.

Efficiency

BUCK

LAMP

SEC

çç

÷÷· ·

çç

÷ =

·· ·

375 0 005 2 67

0 8 820

380

(11)

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

RDS(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 RDS(on) of 0.2W. A Schottky diode

should be used for the buck diode in order to minimize

forward drop.

APPLICATION INFORMATION (cont.)

Vendor

Part Number

ESR

Current

Rating

Coilcraft

(847) 639-6400

150

mH DO3316-154 0.38

Coiltronics (407)

241-7876

150

CTX150-4

0.175

0.72A

Sumida

(847) 956-0666

150

mH CDR125-151

0.4

0.85A

Toko (847) 297-0070 150

mH 646CY-151

0.73

0.4A

Table 3. Inductor Suppliers

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