Hello, I am going to use LTC3805-5 as the controlling circuit of a flyback converter.
Firstly, I built a small circuit to get some knowledge of the chip. The schematic is shown below:
The components in the shadow are not installed on the pcb. It is really a simple circuit with only three components. The circuit has power supply and a bypass capacitor C108, a resistor R107 used to set the operating frequency, and the RUN pin of the chip is connected to the power supply directly. All the other pins are floating.
You can see the pcb in the picture below.
Then I ran a simulation of this circuit in LTspice.
Here is the circuit in LTspice.
I observed the voltage waveforms on pin FS、SSFLT and Gate. The simulation indicates the tiny circuits works and outputs the pwm with nearly 80% duty. (The blue waveform represents Gate, the green represents SSFLT, the red represents FS.)
But the practical waveforms from the oscilloscope shows the chip outputs no pwm waveform.
The voltage of FS is about 0.8V after power on which agrees with the simulation result. But SSFLT and GATE fails.
Would you please tell me how to solve the problem and make the circuit work?
The restart may happen due to noise on OC pin or due to under voltage lockout on Vcc pin.
Increasing the 5V input to 5.5V-6V level can rule out the UVLO on Vcc pin.
Adding a decoupling cap close to OC and GND pins will indicate if the problem is due to noise on OC pin.