AN2982

Ballast stage design

Doc ID 15599 Rev 1

21/31

2.5

IC power supply design

The IC current consumption depends on both the PFC frequency and half-bridge frequency.

In the worst case, the mean value of the PFC frequency is equal to 162 kHz and the

maximum half-bridge frequency is equal to 90 kHz.

With these values the maximum supply current is approximately equal to:

Equation 53

This current is delivered to the IC by means of a charge pump connected to the middle point

of the half bridge.

Figure 14.

Charge pump network and startup

The current delivered by the charge pump capacitor is approximately:

Equation 54

where:

Equation 55

and rearranging the terms:

Equation 56

The startup network works also as a detector of lamp presence. This feature can be

implemented by placing one or both of the lamp cathodes along the startup network path. If

the lamp is absent, the IC cannot start.

mA

11

56

.

4

f

0373

.

0

f

0192

.

0

I

kHz

mA

pre

kHz

PFC

cc

≈

+

+

⋅

=

cc

run

rise

pk

,

cp

cp

I

f

T

I

I

≥

⋅

⋅

=

rise

out

cp

pk

,

cp

T

V

C

I

=

→

=

⋅

≥

pF

558

f

V

I

C

run

OUT

cp

cp

C

cp

1nF

=