I want to use FM31256-G with external generator.
I have built a test schematic (please look at the attachment).
So when i'm disabling 5V source i observe too high current flow through the R1 resistor and Supercap C2 is discharging very quickly. I've measured the current through R1 and it was about 700uA when C2 was fully charged.
I've made an experiment and disconnected X1 and X2 from D3 and D4. After that current trough R1 became about 3 uA.
So i observe high current consumption only when external generator is connected to FM31256-G.
Do you have any ideas how to solve that?
It is not recommended to power the external oscillator and other external circuitry through a supercapacitor. Please refer to our AN401. So, please remove D3 and D4 from the Vbak line and provide supply to them separately. You can use a simple 32.768 kHz crystal instead of an external oscillator.
Also, according to the schematic given in the App note, you have to connect a current limiting resistor before the Schottky diode, and there is no resistor like 1K in series to the supercapacitor. There is no 0.1 uF capacitor in parallel to the supercapacitor. So, please correct your schematic as per the App note.
thank you for your reply.
According to the appnote AN402 i used an external oscillator with invertor to provide clock to X1 and X2 (i didn't want to use crystal since they have a big dependency on temperature).
Since i'm using an external oscillator i can't disconnect it from Vbak because i want my clock to continue counting even when there is not main 5V supply.
I've placed a current limiting resistor before the shottky diode and removed 1K resistor which was in series with supercap and also removed a 0.1uF capacitor. But situation is still the same - when 5V supply is disabled i observe a high current through the supercap and very quick discharging.
Please look at the attached schematic.
I have already tested that.
When i disconnect D3 and D4 i have about 1.5 uA consumption trough Vbak.
But also if I connect D3 and D4 to Vbak but disconnect them from FM31256 X1 and X2 I also have about quite small 4uA consumption.
But when i connect D3 and D4 to FM31256 X1 and X2 i'm getting quite high current about 700uA.
i've made another measurement.
Please take a look at my testing schematic.
I've added 100 Ohm resistors in series with X1 and X2.
I've measured the AC voltage drop on the R2 and R3 by using oscilloscope when 5V power is applied. It was about 30mV (if you need i can provide waveforms). It means that current consumption through X1 and X2 pins is about 300uA which in sum makes about 600 uA for both.
That's why my generator (D3) and invertor (D4) start consuming a lot when i'm connecting them to FM31256.
Is that correct behavior of this device with external generator connected? Datasheet doesn't specify leakage current through this pins.
Does it mean that this current can be so big?
Could you please apply the input Voltage to D1 (Currently 5V is applied) within RTC backup voltage range, VBAK ( 1.55V – 3.75V@+25 C to +85 C and 1.90V – 3.75V @–40 C to +25 C. Looks like input Voltage to VBAK pin is out of datasheet specified range which might be triggering some unknown behavior on the chip.
You can try with typical 3V or best option available on your board within VBAK max limit (3.75V).