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TPS62125: TPS62125: stable using 56uH inductor?

Part Number: TPS62125
Other Parts Discussed in Thread: TPS629210-Q1, TPS629210, TPS629206, TPS629203

Hi

I am designing a power supply for Xbee3 module.  Digi's documents recommend to keep switching above 500kHz if using a switcher. Xbee3 module draws 18mA in Rx and 130mA in Tx at 3.3V.

My power source is 12V battery (10V min - 14.8V max).

Best strategy is to drop 12V to 4V with buck and then filter out the ripple with an LDO to 3.3V.  But it would be great if I could avoid the extra expense of the LDO and just drop from 12V to 3.3V with the switcher.

RE: TPS62125

Webench shows that if I use an L of 22uH (max recommended L in data sheet), at 18mA Fo = 262kHz, Vout p-p 3.12mV. Efficiency 86.9%

If I custom the webench design with an L of 56uH, at 18mA , Fo = 485kHz, Vout p-p 1.38mV. Efficiency 88.7%.  This gets me close to 500kHz with good efficiency and low ripple.

No mention of phase margin in the webench report. How would 56uH effect the stability?

I see that the load transient suffers with 56uH compared to 22uH, but wonder whether the load transient of an LDO would be much better; considering it would also be impacted by the line transient (voltage drop from load transient of the upstream switching supply)

Looking at the efficiency graph, you can see there is a trough at around 28mA (77%) where the switch mode changes from PWM to PFM. Given that the tolerance of the inductor is +/- 20%, in some circumstances it may be possible that a 20% higher value of 68uH may cause the switcher to remain in PWM at 18mA in which case the efficiency would only be around 70%.  I wounder how the switcher would behave if operated continuously in that transition region between PWM and PFM modes?

Do you think I should I just use an LDO downstream of the switcher?

Or perhaps I should be looking for something that operates in FPWM, slightly above 500kHz, that would still give me good efficiency at 18mA? Do you know of any?

Cheers, regards Matt.

WBDesign246_Load Transient-3.pdf