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TPS61169: Not responding to higher freq PWM

Part Number: TPS61169

 There are another 11 LEDs in series (12 toral - set to 50mA max wire 3R9)

The LIGHTS net is direct from a 1.8V CPU, and voltage goes rail to rail (checked on scope).  When we run the PWM at around 500Hz (way below the spec) , the LEDs work and it appears to function ok.

HOwever winding up PWM  to 1000Hz its starts only working at the lower PWMs, going to max output at around 50% 

Winding up PWM to 5KHz and abive as recommended in datasheet and  the device doesn't appear to respond at all.

This trace shows the PWM when runnig at around 800Hz

  • Hi David,

    Your schematic looks OK. The cathode of LED string is connected to FB pin right?

    How much is the VBAT? What the forward voltage for your LEDs with 50mA?

    Could you help to capture the waveform combining with CTRL and FB to see if the feedback response normally?

  • Yes , connected to FB pin - works well at the lower PWM.  It takes them to max brightness (around 35V) 

    One thing (although I think my inductor hasn't got enough Isat - although speced at 1.2A - NRS4018T100MDGJ 10uH ) -  at the higher levels inductor gets hot. I don't think this is related , just more that I need a higher current inductor.

    I will tie the scope trace with FB pin tomorrow - waiting on replacement chip (decided to replace just in case it was a rogue)

  • Sorry forgot to say, voltage of VBAT is a Li-ion so typically 3.6V-4.2V

  • I have spotted the fault - a fault on the pcb, that affectively GNDed the last LED, so current wasn't going the the feedback resistor

    Thankyou for your prompt replies