Hi TI Support Team,
I am using MC33063 to step down ~13.5V battery voltage (Power Source/Vboard is 14.4V 2.2Ah Lead Acid Battery) to 5V. I am using 5V output to power 5 relay driver circuit. The MCU I am using to trigger transistors is 3.3V. I have attached my schematic to this thread. In idle state my circuit draws around 17-20mA (MCU + 16x2 LCD + 5mm LED).
My problem is as follows. The relay I am using is 5V relay with coil power of 0.36W. Every relay draw about 40mA in ON state when 5V is supplied from MC33063 (MC33063 supplies around 200mA for 5 of them). This is not normal operation as relay coil is expected to draw around 75mA in ON state (0.36W/5V). I have done following to debug the issue.
1. I disconnected the relays from the circuit and leave only one of them, it again drew 40mA.
2. Wthout mosfet driver I connected relay coils between MC33063 5V out and GND, again the relay coil current was 40mA.
3. I tested same relays with different power supplies. I connected them to my benchtop power supply 5V output, the relay coil draw around 75mA as expected. I connected relay coils between 7805 out and GND (in my another circuit), the relay coil current was again around 75mA as expected.
MC33063 does not supply requied current to relay coils. How should I modify my circuit that relay operate as expected? Could you please help me to solve the problem I mentioned? I cannot understand why the relay coil draw around half current when it is powered from MC33063?
I know that MC33063 is older IC, but hope you will help me with this issue.
Regards,
Ahmet