NCP5080

http://onsemi.com

8

Although it is possible to increase the photo feedback

sensitivity by increasing the value of the pulldown resistor,

care must be observed since such a resistor is in parallel with

the internal network as depicted in Figure 4 and the input

node might be too sensitive to the ambient noise. It is

recommended to avoid sense resistor value above 100 k

W,

although that 1 M

W is possible, the operation being rapidly

downgraded when the resistance increases beyond this

value.

The PHREF voltage is setup by the external controller, in

the 0.5 V to 1.5 V range, depending upon the need of the

application. The internal structure includes a 500 k

W

(typical) resistor network, connected between PHREF pin

and GND : the external reference source must support such

a load and a 10 k

W output impedance, or lower, is

recommended to avoid uncontrolled operation. Finally, the

IGBT signal will be switched OFF when the PHSEN signal

reaches the PHREF reference.

The function is deactivated by forcing a voltage in the

0.5 V to 1.5 V range at the PHREF pin, associated with a

GND connection to the PHSEN pin.

EN

TRGFL

Photo transistor current

Illumination = Vreference

PHSEN

IGBT

--> IGBT switched OFF

Figure 5. Typical Photo Sense Timings

READY

When the photo sense is active and the photo sense

threshold has been crossed, the photo sense feedback is

internally latched and recycling the TRGFL signal (H to L)

is necessary to reset the latch and start a new sequence .

Simplified Flash

The circuitry can be simplified when the application does

not need the multiple flashes during the same photo

sequence. In this case, the IGBT can be removed as there is

no more need to dynamically switch off the xenon tube .

Similarly, the photo sense becomes useless since there will

be no way to control the illumination once the xenon flash

has been triggered. Such a feature must be properly

deactivated to avoid uncontrolled operation during a photo

sequence: a simple resistor network fulfill such a

requirement as depicted in Figure 6.