EFP01 datasheet(18/158 Pages) SILABS

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3.2.2.2 Peak Current Configuration

DCDC B has configurable peak current adjustment, set by the BK_IPK and BK_IPK_EM2 fields in the BK_IPK register.

In EM0, IPK_BASE is determined by: IPK _ BASE = 0.090A + (0.009A × BK _ IPK )

In EM2, IPK_BASE is determined by: IPK _ BASE = 0.090A + 0.009A × (4 × BK _ IPK _ EM 2 + 2)

The DCDC B buck converter peak current is determined by: Peak Current

(A)= IPK_BASE + 25ns ×(VDDB-VOB)

L B

Typically, the peak current can be lowered in EM2 for improved efficiency.

Note: The Maximum Output Current formula below provides useful guidance but is not a guarantee of performance. The final authority

for the Maximum Output Current will be the Output Load Current specifications in the Electrical Characteristics tables.

The DCDC B buck converter maximum output current is determined by: Maximum Output Current (A) = 0.5 × IPK _ BASE

3.2.2.3 Current Limiting

When powering EFP01 from weak power sources or sources with a high internal impedance, peak load currents at the converter output

can cause a large supply voltage droop at the input, increasing the risk of an unintended power-on reset. To alleviate these issues, it

may be necessary to limit the input current.

Note: Because enabling the current limit will essentially current-starve the output, the system designer should validate the application

under the worst-case output load current.

On EFP01, the current limit is controlled by forcing a minimum time between the start of pulse events and is configured by the

BK_IRI_CON field in BK_CTRL2. BK_IRI_CON can be calculated based on the mode, the input voltage (VDDB), output voltage (VOB),

DCDC B inductor value (LB), peak current (IPK, set by BK_IPK), and the desired limited battery current (IBATT_LIMIT) as follows:

IBATT_LIMIT =

L × IPK 2

2 × TSW × (VDDB - VOB)

where TSW is programmed by BK_IRI_CON as follows:

TSW = 300ns × (2 × BK_IRI_CON + 1)

The

maximum

output

load

current

going

limited

accordingly,

and

can

determined

by:

ILOAD_MAX =

VDDB

VOB

× efficiency × IBATT_LIMIT

Note that the required BK_IRI_CON setting for a given current limit varies with the VDDB voltage and the peak current, both variables

that can change over battery voltage. An application requiring a fixed current limit over the entire range of the battery may need to

occassionally measure the VDDB voltage and recalculate / reprogram the BK_IRI_CON setting.

3.2.2.4 TON Maximum Limiting

In Buck mode, the pulse frequency modulation (PFM) pulse on time (TON) required to charge the inductor is roughly (L × IPK)/(VDDB-

VOB ). As VDDB approaches VOB, the resulting TON can become quite large, resulting in large output ripple voltage or, in the worst

case, the DCDC stalling altogether.

When the BK_TON_MAX field in the BK_CTRL1 register is set to a non-zero value, the PFM pulse TON is limited to no more than 70 ns

× (1 + 4 × BK_TON_MAX), which terminates the inductor charging before the programmed peak current value is reached. When oper-

ating in this TON limiting mode, the maximum available load current and the actual peak current values are reduced from the otherwise

expected value.

Because the time measuring mechanism costs some power, BK_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 - VOB is very small) and there is no LDO enabled in parallel with the

DCDC output.

Comparison plots of TON limiting can be found in Figure 3.3 DCDC A TON Max Limiting Example on page 14.

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