L6364 datasheet(37/60 Pages) STMICROELECTRONICS

37 / 60 page

Figure 14. Block diagram of DC-DC converter

VDCDC

LOUT

V5V

V3V3

VPLUS

PGND

GND

DCDC

PID

controller

DOUT

CDOUT

LDCDC

ISTARTUP

RSHUNT

5.0V lin.reg.

3.3V lin.reg.

CDCDC

This block also includes a sense resistor (RSHUNT) to measure the inductor current, a series diode to prevent

conduction during reverse polarization, and protection diodes to conduct residual inductor currents whenever

switching ceases.

The high-side switch is activated at the start of each cycle, and remains ON for a length of time determined by the

PID controller. This block monitors the voltages at VDCDC and VPLUS pins.

17.3

Converter operation

17.3.1

Startup

The converter operates only when the following operating conditions are met:

•

VV3V3 >VPOR (by means, bandgap is stable)

•

VVPLUS >VDCMIN (by means, oscillator toggling)

•

VVPLUS>VPLUSDCMIN

Operation outside these conditions is prevented to protect the IC and the surrounding circuit.

Initially the converter is in a STARTUP state. In this state an internal current source attempts to source a current

ISTARTUP from pin DOUT to pin LOUT.

Where LOUT is connected to VDCDC by an inductor, voltage at VDCDC pin is pulled up by this current: VDCDC

voltage rises until it reaches voltage VSTARTUP, where the source current is then regulated to maintain VDCDC

voltage at VSTARTUP. The converter remains in the STARTUP state until the operating conditions are met.

Bias is provided either by a HV bias circuit where the IC is in reset VV3V3 < VPOR, or by an LV bias circuit once

V3V3 is available. During the overlap period, the bias currents are doubled.

17.3.2

Entry to operation

Once the operating conditions are met, or when the DCDC:DIS bit is reset, then the converter passes through an

ARM state for 1 ms before starting operation. The switch controls are powered in this state, but set to OFF.

17.3.3

Leaving operation

If the operating conditions are no longer met, then the converter passes back through the ARM state to the

STARTUP state. If, however, the DCDC:DIS bit is set, then the converter passes back through the ARM state to

the SLEEP state. On leaving operation, any residual energy in the coil is conducted to VDCDC via the protection

diode between LOUT and PGND.

L6364

Converter operation

DS13363 - Rev 4

page 37/60

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