Canon SPEEDLITE 540EZ Service Manual page 36

Part 2: Technical Information
4. DESCRIPTION OF CIRCUITS
4.1 Starting up the Power Supply
(1) When the power is turned ON, BATT] is input to IC] pin No.4.
(2) IC1 is a step-up regulator IC that outputs a fixed 5V from pin No.5 when there is
input to pin No.4. This voltage is input to pin No.2 of IC3 and pin No.81 of IC8
(VDD).
(3) IC3 outputs the same voltage as input if the input to pin No.2 is 3V or more.
(Output is OV if input is less than 3V.)
This voltage is input to the emitter of RQ2.
(4) VDD is supplied to pin No.81 of IC8. When the main switch is turned ON, pin
No.13 becomes Low, and this signal is input to RQ2 and RQ14.
(5) By (3) and (4) above, RQ2 turns ON, turning Q5 and Q4 ON, and charging C8.
(6) This top of C8 is a line called Vcc, and is supplied to pin No.60 of IC7, pin No.1 of
IC6 and to each of the pins of LED4 as power supply.
(7) When Vcc rises, High is input to the base of Q18 via Rl, and Q18 goes ON. When
Q18 goes ON, Q2 and Q3 turn ON, and oscillation is started by T1.
* When an external power source is attached the following operations are carried out in
addition to the above operations.
(8) RQ9Q is simultaneously turned ON during operations in step (4) above. VDD (5V) is
then supplied to SEH.
(9) When SEH becomes High, the high voltage circuit in the external power supply
operates, and high-voltage is input to HV to charge C15 and C19.
¢ Flow of Operations: (1) > (2) > (3) > (4) -p (5) > (6) > (7)
(8) > (9)
Figure 2-6 Circuit 1
2-14