L9758

Functional description

Doc ID 14273 Rev 3

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5

Functional description

5.1

General function

The L9758 is equipped with 9 linear voltage regulator. A buck boost switch mode power

supply as pre regulator for the 7 main regulators is used to reduce the power consumption in

the system.

Two standby regulators can be used to bias the system on off-mode. This to regulator’s are

equipped with a independent bandgap voltage reference. The current consumption of these

two linear regulators is specified with less than 120 µA in OFF state. I these standby

function is not used the current consumption on the battery can be reduced by not

connecting the VBAT. Under this condition the device enters immediately in the run mode,

the pin PSU_REN will lost his function. The quiescent current on the VBAT_SW can be

reduced to maximum 10 µA with 12 V battery voltage in off mode. The main regulators can

be activated with the IGN input. With a external resistor higher than 10 kohm in series to the

IGN pin a battery compliant signal can be used. In the function block diagram a resistor

value of 51 kohm is mention and a 100 nF capacitor for noise robustness on IGN.

5.2

Switching pre-regulator

The switching pre-regulator is a buck or a buck-boost current control mode regulator. The

optional boost operation for low battery conditions can be selected connecting external logic

level low side NCH FET and an external diode in series to the inductor.

The external parts required to complete the switching regulator are an inductor, recirculation

diode and input and output filtering capacitor. The compensation network is inside the

device.

With a constant switching frequency of 350 kHz, the pre-regulator controls the output

voltage (the voltage at the VB and FDBK pins) to the limits stated in the electrical

Section 5.8).

At low battery voltages, in buck configuration, the pre-regulator runs with the duty cycle up

to 100%. In buck-boost configuration normally it runs at 350 kHz but for a limited range of

input voltage it could enter in pulse skipping mode to control the output voltage.

A soft start function is implemented reducing the current limitation during the power-up

phase.

5.3

VDD5, VDDL and VCORE linear regulators

will be implemented via an external N-channel pass MOS, with the control being internal to

the IC. If the pass MOS is not used, two low current (max 30mA) regulator are available

connecting directly VDDL_FDBK to VDDL_DRV and VCORE_FDBK to VCORE_DRV with a

resistor divider.The output of the pre-regulator is used as the source of these supplies.