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LTC3805-5 seems working incorrectly

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 OC, ISENSE and FB pins should be connected as in the application circuit. The MOSFET can be left out in order to see the gate pulses if debugging is required.



  • After a few error-and-try, here is the conclusion: in order to make the chip run with minimum external circuit, the following conditions should be met.

    • a bypass capacitor should be connected to the power pin of the chip
    • a capacitor should be connected to SSFLT pin to set the soft start time
    • a resistor should be connected to FS pin to set the operating frequency
    • the ITH, OC, ISENSE and FB pins should be grounded
    • finally and most importantly, the GND pin of the chip must be well connected to the ground of the power source, the pad of the GND pin should be through-hole if the chip is soldered by hand

    If the chip does not output something at the GATE pin, the connection betwent the GND pin and ground should be checked.