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2) Iterate L and C to Fulfill Constraints

To select the ballast output stage inductor and

capacitor, a range of values were input into

equations (2) through (7), which have been sum-

marized in the following table:

Table IV, Ballast parameters for different C values

When compared against the lamp requirements,

a capacitor value of 6.8nF gives a lamp voltage

during pre-heat that exceeds the maximum al-

lowable specified for this lamp type. This can

ignite the lamp before the cathodes have

reached their emission temperature, drastically

reducing lamp life. The pre-heat current can

be reduced to give a lower pre-heat voltage, but

the pre-heat time must then be increased for

proper heating. Also,

is too low, which

will cause the lamp to extinguish at low light lev-

els where the arc current alone is too low to

heat the cathodes. Increasing the capacitor

value to 10nF fulfills the lamp requirements quite

well, even allowing some room in the pre-heat

voltage for the pre-heat current to be increased

and the pre-heat time shortened. During dim-

ming, however, the lamp voltage increases with

decreasing lamp power due to lamp negative

incremental impedance effects. A maximum is

reached around 10% brightness, after which the

lamp voltage decreases as the lamp is further

dimmed. The maximum filament current occurs

at the maximum lamp voltage, which for a ca-

pacitor value of 10nF, is too high and will over-

min

Cath

heat the filaments. A capacitor value of 8.2nF

was chosen which fulfills the lamp requirements

without over-heating the cathodes.

3) IR21592 Programmable Inputs

With all of the lamp requirements fulfilled, the

component values for setting the programmable

inputs of the IR21592 are calculated as:

Table V, IR21592 Programmable Inputs for T8/32W lamp.

[mH]

2.0

[nF]

6.8

8.2

[Vpp]

748

668

592

[kHz]

ign

[kHz]

ign

[App]

1.4

1.5

1.7

max

[kHz]

min

Cath

[Arms]

0.32

0.35

0.38

Equation No.

Variable

Value

(8)

100

46kHz

(8)

58kHz

(9)

100

-56.12deg

(9)

-89.27deg

(10)

FMIN

36kOhm

(11)

1.0 Ohm

(12)

IPH

22kOhm

(13)

TPH

330nF

(14)

MIN

27kOhm

(15)

MAX

24kOhm

Important Note: These design kits are intended

as a demonstration of the functionality and per-

formance of the IR21592 Dimming Ballast Con-

trol IC only. Adequate EMI filtering, line tran-

sient protection, galvanic dim control input iso-

lation, and ballast and lamp life testing are not

considered in this design.

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