L6986H datasheet(29/68 Pages) STMICROELECTRONICS

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4.9

Switchover feature

The switchover maximizes the efficiency at the light-load that is crucial for LCM applications.

The switchover operation features to derive the main contribution of the supply current for the internal circuitry

from an external voltage (3 V < VBIAS < 5.5 V is typically connected to the regulated output voltage). This helps to

decrease the equivalent quiescent current seen at VIN.

In case the regulator output voltage is not compatible with the VBIAS input voltage range, it is possible to use an

auxiliary voltage source for the switchover operation. The external auxiliary voltage source must always respect

the condition 3 V < VAUX < 5.5 V, and must be derived from the L6986H power supply (VIN - pin 15) for proper

power sequencing of the internal circuits.

4.9.1

LCM

The LCM operation satisfies the high efficiency requirements of the battery powered applications.

In case the VBIAS pin is connected to the regulated output voltage (VOUT), the total current drawn from the input

voltage can be calculated as: IQVIN=IQOPVIN+ 1ηL6986H⋅VBIASVIN⋅IQOPVBIAS

(14)

where IQ OP VIN, IQ OP VBIAS are defined in Table 5. Electrical characteristics and ηL6986H is the efficiency of the

conversion in the working point.

4.9.2

LNM

is also valid when the device works in LNM and it can increase the efficiency at the medium load since the

regulator always operates in the continuous conduction mode.

4.10

Overcurrent protection

The current protection circuitry features a constant current protection, so the device limits the maximum peak

current (see Table 5. Electrical characteristics) in overcurrent condition.

The L6986H device implements a pulse by pulse current sensing on both power elements (high-side and low-side

switches) for effective current protection over the duty cycle range. The high-side current sensing is called “peak”

the low-side sensing “valley”.

The internal noise generated during the switching activity makes the current sensing circuitry ineffective for a

minimum conduction time of the power element. This time is called “masking time” because the information from

the analog circuitry is masked by the logic to prevent an erroneous detection of the overcurrent event. As a

consequence, the peak current protection is disabled for a masking time after the high-side switch is turned on,

the valley for a masking time after the low-side switch is turned on. In other words, the peak current protection can

be ineffective at extremely low duty cycles, the valley current protection at extremely high duty cycles.

The L6986H device assures an effective overcurrent protection sensing the current flowing in both power

elements. In case one of the two current sensing circuitry is ineffective because of the masking time, the device is

protected sensing the current on the opposite switch. Thus, the combination of the “peak” and “valley” current

limits assures the effectiveness of the overcurrent protection even in extreme duty cycle conditions.

The valley current threshold is designed higher than the peak to guarantee a proper operation. In case the current

diverges because of the high-side masking time, the low-side power element is turned on until the switch current

level drops below the valley current sense threshold. The low-side operation is able to prevent the high-side turn

on, so the device can skip pulses decreasing the switching frequency.

L6986H

Switchover feature

DS12905 - Rev 2

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