Canon SPEEDLITE 540EZ Service Manual page 48

Part 2: Technical Information
4.13 Oscillation Interruption
(1) The voltage level of VCC is monitored by pin No.33 of IC7.
(If this voltage falls below 3V, the following operations are carried out in order to
prevent malfunction due to the drop in VCC voltage.)
(2) When Vcc falls below 3V, pin No.43 of IC7 becomes High.
(3) This turns RQ13 and RQ1 ON.
(4) When RQ1 turns ON, Q@2 and Q3 go OFF to stop oscillation.
(5) When the voltage of Vcc returns to 3V or more, pin No.43 of IC7 goes Low again to
resume oscillation.
(6) In order to to stabilize operation (in particular, flash level control) immediately after
the main flash, pin No.60 of IC8 becomes momentarily High at the same time as
the main flash to interrupt oscillation by the same operation as above. (This is
because the fluctuations in the VCC level influence the flash control level.)
(7) When the secondary voltage of Tl becomes approx. 340V or more, High is input to
the base of Q22 via ZD8.
(8) This turns Q22 and RQ1 ON, and the same operation as in step (4) above stops
oscillation.
e Flow of Operation: 1. (1) > (2) > (3) > (4) - (5) when VCC is 3V or more, restored
— oscillation resumed
2. On the sequence (6) — (3) — (4), pin No.60 of IC8 becomes
Low, and oscillation is resumed.
3. (7) > (8) > (4) - oscillation is resumed at secondary side of
T-1 becoming approx. 340 V or more.
(6)
Figure 2-17 Circuit 13
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