18

LT1786F

Dual Transformer CCFL Power Supply

Space constraints may dictate utilization of two small

transformers instead of a single, larger unit. Although this

approach is somewhat more expensive, it can solve space

problems and offers other attractive advantages. Figure

2’s approach is essentially a “grounded lamp” LT1786F-

based circuit. The transistors drive two transformer pri-

maries in parallel. The transformer secondaries, stacked

in series, provide the output. The relatively small trans-

formers, each supplying half the load power, may be

located directly at the lamp terminals. Aside from the

obvious space advantage (particularly height), this

arrangement minimizes parasitic wiring losses by elimi-

nating high voltage lead length. Additionally, although the

lamp receives differential drive, with its attendant low

parasitic losses, the feedback signal is ground referred.

Thus, the stacked secondaries afford floating lamp oper-

ating efficiency with grounded mode current certainty and

line regulation.

L1 is directly driven, with winding 4-5 furnishing feedback

in the normal fashion. L3, “slaved” to L1’s and L3’s

interconnects must be laid out for low inductance to

maintain waveform purity. The traces should be as wide as

possible (e.g., 1/8") and overlaid to cancel inductive

effects.

16

15

14

13

12

11

10

9

WIDE TRACE

WIDE TRACE

SEE TEXT AND NOTES

1

2

3

4

5

6

7

8

DIO

AGND

SHDN

SMBSUS

BULB

BAT

ROYER

SCL

PGND

ADR

LT1786F

LAMP

10

6

10

6

L1

L3

NC

NC

750

Ω

MBRS130L

L2

BAT54

3

21

3

2

1

5

4

5

4

+

0.1

µF*

0.1

µF*

1000pF

220k

100k

2.2

µF

16V

1

µF

1786 F02

2.2

µF

27pF

+

2.2

µF

Q1-Q2

ZDT1048

SHUTDOWN

SDA

L1, L3 = COILTRONICS CTX110605

OVERLAY INDICATED TRACES BETWEEN L1 AND L3

L2 = COILTRONICS CTX100-4

* = WIMA MKI OR MKP-20

= PANASONIC ECH-U

COILTRONICS (561) 241-7876

5V

+

TO

SMBus

HOST

Figure 2. Dual Transformers Save Space and Minimize parasitic Losses While Maintaining

Current Accuracy and Line Regulation. Trade-Off Is Increased Cost