EFP01 datasheet(13/158 Pages) SILABS

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3.2.1.5 TON Maximum Limiting

Buck Mode TON_MAX

In Buck mode, the pulse frequency modulation (PFM) pulse on time (TON) required to charge the inductor is given by:

TON =

L × IPK

VDDB - VOA

As VDDB approaches VOA, the resulting TON can become quite large, resulting in sizable output ripple voltage

or, in the worst case, the DCDC stalling altogether.

When the BB_TON_MAX field in the LDOC_BB_CTRL register is set to a non-zero value, the PFM pulse TON will be limited as shown:

TON_MAX = 70 ns + (1 + (4 × BB_TON_MAX )) A limited TON will terminate the inductor charging before the programmed peak current

value is reached, and limit the maximum available load current and the actual peak current values.

Because the time-measuring circuitry required to limit TON costs some small amount of power, BB_TON_MAX should be set to set a

nonzero value only under specific conditions:

• If the subsequent reduction in max available load current can be tolerated and the reduction in ripple voltage is desired.

• When operating with low input voltage headroom (i.e., VDDB - VOA is very small) and there is no LDO enabled in parallel with the

DCDC output.

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