L6986I_V01 datasheet(9/60 Pages) STMICROELECTRONICS

9 / 60 page

Figure 5. Internal block diagram

RST

DRIVER

GND

PEAK

VALLEY

ZERO

CROSSING

LOOP

CONTROL

OSCILLATOR

RST TH.

E/A

POWER

PMOS

POWER

NMOS

SENSE

PMOS

SENSE

NMOS

Vcc

SLOPE

VIN

SYNC

COMP

GND

DRIVER

GND

VALLEY

LOOP

CONTROL

E/A

POWER

PMOS

MOS

OVP

MLF

DELAY

SS/INH

VREF

TSS

FSW

•

Embedded power elements. Thanks to the P-channel MOSFET as high-side switch the device features low-

dropout operation

•

A fully integrated sawtooth oscillator with adjustable frequency

•

A transconductance error amplifier

•

An internal feedback divider GDIV INT

•

The high-side current sense amplifier to sense the inductor current

•

A “Pulse Width Modulator” (PWM) comparator and the driving circuitry of the embedded power elements

•

The soft-start blocks to ramp the error amplifier reference voltage and so decreases the inrush current at

power-up. The SS/INH pin inhibits the device when driven low

•

The switchover capability of the internal regulator to supply a portion of the quiescent current when the VBIAS

pin is connected to an external output voltage

•

The synchronization circuitry to manage master / slave operation and the synchronization to an external

clock

•

The current limitation circuit to implement the constant current protection, sensing pulse by pulse high-side /

low-side switch current. In case of heavy short-circuit the current protection is fold back to decrease the

stress of the external components

•

A circuit to implement the thermal protection function

•

The OVP circuitry to discharge the output capacitor in case of overvoltage event

•

MLF normally connected to GND through a resistor to set the threshold of the RST comparator

•

FSW pin strapping sets the switching frequency

•

The RST open collector output

4.1.1

Power supply and voltage reference

The internal regulator block consists of a start-up circuit, the voltage pre-regulator that provides current to all the

blocks and the bandgap voltage reference. The starter supplies the start-up current when the input voltage goes

high and the device is enabled (SS/INH pin over the inhibits threshold).

The pre-regulator block supplies the bandgap cell and the rest of the circuitry with a regulated voltage that has a

very low supply voltage noise sensitivity.

L6986I

Primary side

DS13647 - Rev 2

page 9/60

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