I am struggling to get a 24V battery to 11.5V step-down to have a stable switching waveform, although the output 11.5V is relatively stable. The design needs to work from 7V to 58V, 600mA maximum current, where at low battery voltages the output voltage just tracks the battery voltage as it cannot achieve 11.5V (no boost).
A slightly simplified circuit is shown below, based upon the TI WebBench design:
What I am finding is that the PH output pin beats, actually turning off the output briefly: yellow trace is the unstable PH voltage, orange trace is the switcher inductor L11 current, nominally 250mA rms with dips to zero 0mA and up to 500mA (still not saturating). The switching frequency varies and is never the predicted 250kHz, always lower (120kHz-ish to 190kHz-ish)
I have tried various compensation circuits, switcher diode, inductor, output capacitor combinations, adding close high frequency decoupling and bulk decoupling to the VIN pin. The enable, boot and soft start pins seem at the expected voltages.
The layout is poor, as we are very space constrained, but I have tried putting the compensation directly between pin 8 (COMP) and 9 (GND), the RTCLK resistor directly between pin 5 (RTCLK) and pin 9 (GND), and adding input decoupling directly between pins 2 (VIN) and pin 9(GND).
I will try a different RTCLK value (not a preferred solution) and moving the SENSE resistors closer to the IC...
Any other ideas what could be causing the instability?
