A6984 datasheet(31/47 Pages) STMICROELECTRONICS

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A6984

Device description

4.6

Switchover feature

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

The main switching controller is supplied by the VCC pin regulator

An integrated LDO regulates VCC = 3.3 V if VBIAS voltage is < 2.4 V.

VCC is connected to VBIAS through a MOSFET switch if VBIAS > 3.2 V and the embedded

LDO is disabled to increase the light load efficiency.

4.6.1

LCM

LCM operation satisfies the requirements of battery-powered applications where it is crucial

to increase efficiency at the light load.

In order to minimize the regulator quiescent current request from the input voltage, the

VBIAS pin can be connected to an external voltage source in the range 3 V < VBIAS < 5.5 V.

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:

Equation 23

where IQ OP VIN, IQ OP VBIAS are defined in Table 5: Electrical characteristics on page 7 and



A6984 is the efficiency of the conversion in the working point.

4.6.2

LNM

Equation 23 is also valid when the device works in LNM and it can boost the efficiency at

medium load since the regulator always operates in continuous conduction mode.

4.7

Overcurrent protection

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

the maximum current (see Table 5: Electrical characteristics on page 7) in overcurrent

condition.

The low-side switch pulse-by-pulse current sensing, called “valley”, implements the constant

current protection. In overcurrent condition the internal logic keeps the low-side switch

conducting as long as the switch current is higher than the valley current threshold.

As a consequence, the maximum DC output current is:

Equation 24

Q VIN

Q OP VIN



A6984

-----------------

BIAS

-------------- IQ OP VBIAS



MAX

VALLEY_TH

RIPPLE

--------------------

VALLEY_TH

OUT

–

------------------------------ TON



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