Lighting circuit and vehicle lamp employing same
A buck converter supplies a drive current to a light source and is feedback-controlled so that the drive current comes close to a target current. An open-circuit detection circuit compares a potential difference between and input voltage and an output voltage of the buck converter with a threshold voltage.
1. A lighting circuit comprising:
a converter which has an output inductor, supplies a drive current to a light source via the output inductor, and is feedback-controlled so that the drive current comes close to a target current; and
a protection circuit which suspends a switching operation of the converter for a suspension time upon detecting a recovery from an open-circuit state to a normal state at an output terminal of the converter,
wherein the protection circuit judges that the recovery from the open-circuit state to the normal state has occurred when an output voltage of the converter has dropped rapidly.
2. The lighting circuit according to claim 1 , wherein the protection circuit suspends the switching operation of the converter for the suspension time upon detecting a recovery from a short-circuit state to the normal state at the output terminal of the converter.
3. The lighting circuit according to claim 1 , wherein the protection circuit increases or decreases a switching duty ratio of the converter slowly after a lapse of the suspension time.
4. The lighting circuit according to claim 1 , wherein the protection circuit comprises a differentiation circuit or a highpass filter which receives an output voltage of the converter, and judges that the recovery to the normal state has occurred as soon as an output signal of the differentiation circuit or the highpass filter has exceeded a prescribed value.
5. The lighting circuit according to claim 1 , wherein the protection circuit comprises:
a capacitor one end of which is grounded;
a charging resistor which is connected to the other end of the capacitor and applies, to the capacitor, a target voltage that determines the target current in the normal state; and
a discharge switch which is provided parallel with the capacitor and turned on upon the detection of the recovery to the normal state.
6. A vehicle lamp comprising:
a light source; and
the lighting circuit according to claim 1 which drives the light source.
7. A lighting circuit comprising:
a converter which has an output inductor, supplies a drive current to a light source via the output inductor, and is feedback-controlled so that the drive current comes close to a target current; and
a protection circuit which suspends a switching operation of the converter for a suspension time upon detecting a recovery from a short-circuit state to a normal state at an output terminal of the converter,
wherein the protection circuit judges that the recovery from the open-circuit state to the normal state has occurred when an output voltage of the converter has dropped rapidly.
8. A lighting circuit comprising:
a converter which has an output inductor, supplies a drive current to a light source via the output inductor, and is feedback-controlled so that the drive current comes close to a target current; and
a protection circuit which suspends a switching operation of the converter for a suspension time upon detecting a recovery from an open-circuit state to a normal state at an output terminal of the converter,
wherein the protection circuit suspends the switching operation of the converter for the suspension time upon detecting a recovery from a short-circuit state to the normal state at the output terminal of the converter, and
wherein the protection circuit judges that the recovery from the short-circuit state to the normal state has occurred when an output voltage of the converter has risen rapidly.
9. A lighting circuit comprising:
a converter which has an output inductor, supplies a drive current to a light source via the output inductor, and is feedback-controlled so that the drive current comes close to a target current; and
a protection circuit which suspends a switching operation of the converter for a suspension time when an output voltage of the converter has changed rapidly,
wherein the protection circuit judges that the recovery from the open-circuit state to the normal state has occurred when an output voltage of the converter has dropped rapidly.
10. A lighting circuit comprising:
a converter which has an output inductor, supplies a drive current to a light source via the output inductor, and is feedback-controlled so that the drive current comes close to a target current; and
a protection circuit which suspends a switching operation of the converter for a suspension time upon detecting a recovery from an open-circuit state to a normal state at an output terminal of the converter,
wherein the converter is a buck converter; and
wherein the lighting circuit further comprises an open-circuit detection circuit which compares a potential difference between an input voltage and an output voltage of the converter with a prescribed threshold voltage.
11. The lighting circuit according to claim 10 , wherein the open-circuit detection circuit comprises a pnp bipolar transistor whose emitter and base are connected to an input terminal and the output terminal of the converter, respectively.