IRS2158D(S)

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© 2009 International Rectifier

21

FMIN

VCO

CPH

CO

M

LO

MLS

OSC.

HO

MHS

VS

Half-

Bridge

Output

D

V

V

Load

Return

Half-

Bridge

Driver

IRS2158D

M1

R

PH

R

FMIN

C

VCO

C

PH

VCC

R

CPH

4

3

5

MODE

16

15

11

10

CS

12

R

CS

C

CS

R

3

+

-

1.2V

M2

1.2K

Figure 5: Ignition circuitry.

The over-current threshold on pin CS (VCSTH+) will protect the ballast against a non-strike or open-filament

lamp fault condition. The voltage on pin CS is defined by the lower half-bridge MOSFET current flowing

through the external current sensing resistor RCS. This resistor programs the maximum peak ignition current

(and therefore peak ignition voltage) of the ballast output stage. Should this voltage exceed the internal

VCSTH+ threshold of 1.2 V, the ignition regulation circuit discharges the VCO voltage slightly to increase the

frequency slightly (see Figure 6). This cycle-by-cycle feedback from the CS pin to the VCO pin will adjust

the frequency each cycle to limit the amplitude of the current for the entire duration of ignition mode.

t

t

t

VS

LO

HO

1.2V

5V

Figure 6: Ignition regulation timing diagram.

The IRS2158D remains in IGNITION mode until the voltage at CPH reaches 1/2*VCC at which point it will

switch into PRE-RUN mode.

Pre-Run Mode

In PRE-RUN mode the fault counter is disables so that the IRS2158D will enter FAULT mode if a single

event occurs where VCS>1.2 V (VCSTH+). This allows the ballast to shut down if the resonant inductor

saturates during ignition.