Hi,
I have an application where I need to control voltage across a load with constant resistance (about 14 Ohm) from about 2 to almost 24V with constant supply voltage 24V. We are trying TPS54335 runing at 100kHz. I have a trouble to achieve stable output voltage below about 3V - the PWM becomes to flicker. The load is actually a magnetic lens requiring high current stability therefore any mode with random skipping of pulses would be detrimental for current stability in order of ppm.
I have a trouble to understand the datasheet and the prototype behavior. The datasheet says that regulation down to 0.8V shall be possible in the text, minimum on time about 100ns (Figure 11). On the other hand Figure 12 indicates minimum controllable duty cycle should be below about 5% at temperatures <50C that presents on time about 500ns at working frequency. It would fit our application giving output voltage about 1.5V.
The PWM in the prototype becomes to be unstable below duty cycle about 7% with 14 Ohm load (works in DCM) but decreasing load resistance to about 6 Ohm the mode changes to CCM and 5% duty cycle can be achieved.
Does anyone have an explanation and/or some advice how to achieve stable PWM at low duty cycles please?
Thank you,
Bohus, FEI company