Reference Design Data Sheet intended for design information only.
Subjected to changes without prior notice.
5
IRPLLNR1
Shutdown
The shutdown circuitry consists of a quad comparator IC (IC2), a lamp resonance current
detection circuit* (R21, R22, R38, C16, D12) and a pull-up lamp removal circuit (R23, R24,
R25, R26, D16, C22). The current detection rectifies and integrates a measurement of the
lamp resonant current and is compared against a fixed threshold voltage. Should the current
exceed the threshold in the event of over-current due to a non-strike condition of the lamp or
non-zero voltage switching of the half-bridge due to one or more broken lamp cathodes, the
CT pin of the IR2153 is latched below the internal shutdown threshold (1/6 Vcc) and the
ballast turns off. The negative temperature coefficient of the rectifying diode of the current
measuring circuit (D12) also causes the current measurement to increase with increasing
ambient temperature, therefore latching the ballast off in the event of excessive ambient
temperatures. In the event of a lamp exchange, the latch is reset with the pull-up network at
the lamp, and the CT pin of the IR2153 is held below the internal shutdown threshold in an
unlatched
state. When a new lamp is reinserted, the ballast performs an auto restart without a
recycling of the input line voltage. For a dual lamp ballast, a second pull-up network is added
to the second lamp (R27, R28, R29, R30) and is ‘OR-ed’ together with the first lamp. If either
lamp is removed during running, the ballast turns off. Should the
upper
cathode of either lamp
break during normal operation, then the good lamp will continue running as long as the zero-
voltage switching condition on the half-bridge is fulfilled. If the total lamp resonant circuit
current for the one-lamp case is not high enough to commutate the snubber capacitor (C18)
during the deadtime of the half-bridge, then the resulting current will exceed the threshold
voltage of the current limit circuit and the half-bridge will latch off. This is a function of the DC
bus voltage, the lamp type, the lamp resonant circuit (L4, L5, C21, C23), the running
frequency (R20, C13), the snubber capacitor (C18), the current sensing resistor (R22) and the
current limit threshold (R14, R15). All of these parameters must be correctly chosen for each
new lamp type such that the lamp is driven to the manufacturers recommended lamp power
while achieving complete lamp fault protection