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IRPLDIM1E

IR21592 Dimming Ballast Control IC Design Kit

International Rectifier 

Ballast Design

Lamp Requirements

Before selecting component values for the bal-

last output stage and the programmable inputs

of the IR21592, the following lamp requirements

must first be defined:

Variable

I ph

t ph

V phmax

Vign

P100%

V100%

P1%

V1%

I Cathmin

Description

Filament pre-heat current

Filament pre-heat time

Maximum lamp pre-heat voltage

Lamp ignition voltage

Lamp power at 100% brightness

Lamp voltage at 100% brightness

Lamp power at 1% brightness

Lamp voltage at 1% brightness

Minimum cathode heating current

Units

Arms

s

Vpp

Vpp

W

Vpp

W

Vpp

Arms

Table I, Typical lamp requirements

Ballast Output Stage

The components comprising the output stage

are selected using a set of equations. Different

ballast operating frequencies and their respec-

tive voltages and currents are calculated. The

inductor and capacitor values are obtained us-

ing equations (2) through (7). The results of

these equations reveal the location of each

operating frequency and the corresponding volt-

ages and currents. For a given L, C, DC bus

voltage, and pre-heat current, the resulting volt-

age over the lamp during pre-heat is given as:

Vph =

VDC

π

2



+

8L

C

I

2

ph

− VDC

π

[Vpp]

(2)

www.irf.com

The resulting operating frequency during pre-

heat is given as:

f ph

=

2 I ph

πCV ph

[Hz] (3)

The resulting operating frequency during igni-

tion is given as:

f ign

=

1

2π

1+

V 4

π DC

Vign

LC

[Hz] (4)

The total load current during ignition is given

as:

Iign = fignCVign 2π

[App] (5)

The operating frequency [Hz] at maximum lamp

power is given as:

f100%

=

1

2π

1

LC

−

32P1200%

C 2V1400%

+

1





LC

−

32P1200%

C 2V1400%

2





−

1−

4



4VDC

V100%π

L2C 2

2

(6)

The cathode heating current at minimum lamp

power is given as:

I Cath1%

= V1% f1%πC

2

(7)

Design Constraints

The inductor and capacitor values should be it-

erated until the following design constraints have

been fulfilled (Table II).

Design Constraint

V ph < V phmax

f ph − fign > 5kHz

Iign < Iignmax

I ≥ I Cath1%

Cathmin

Reason

Ignition during pre-heat

Production tolerances

Inductor saturation

Lamp extinguishing during

dimming

Table II, Ballast design constraints

7

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