here is the output voltage ripple test result with very light load(the output current maybe lower than 1mA),is it normal?
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The output voltage waveform at 1mA load current should be the normal behavior of this device.
TPS61040-Q1 operates in a pulse frequency modulation (PFM) scheme with constant peak current control. The converter monitors the output voltage, and as soon as the feedback voltage fails below the reference voltage of typically 1.233V, the internal switch turns on and the current ramps up. The switch turns off as soon as the inductor current reaches the internally set peak current. When the load current is very low, the device will turn on until the current ramps up to peak current limit. Then the lower side switch turns off. Inductor current will charge output capacitor through rectifier diode. The output voltage ramps up. Then the device stops switching and output caps discharge until FB fails below 1.233V.
So if the output caps value is higher, the output voltage ripple will be reduced. The Cff cap won't help decrease output voltage ripple in low load current condition.