PM6680 datasheet(29/49 Pages) STMICROELECTRONICS

29 / 49 page

PM6680

Device description

29/49

When a switching section is turned off (EN1/EN2 pins low), the controller enters in soft end

mode.The output capacitor is discharged through an internal 18

Ω P-MOSFET switch; when

the output voltage reaches 0.3V, the low-side MOSFET turns on, keeping the output to

ground. The soft end time also depends on load condition.

7.8

Gate drivers

The integrated high-current drivers allow to use different power MOSFETs. The high side

driver MOSFET uses a bootstrap circuit which is indirectly supplied by LDO5 output. The

BOOT and PHASE pins work respectively as supply and return rails for the HS driver.

The low side driver uses the internal LDO5 output for the supply rail and PGND pin as return

rail.

An important feature of the gate drivers is the adaptive anti-cross conduction protection,

which prevents high side and low side MOSFETs from being on at the same time. When the

high side MOSFET is turned off the voltage at the phase node begins to fall. The low side

MOSFET is turned on when the voltage at the phase node reaches an internal threshold.

When the low side MOSFET is turned off, the high side remains off until the LGATE pin

voltage goes approximately under 1V.

The power dissipation of the drivers is a function of the total gate charge of the external

power MOSFETs and the switching frequency, as shown in the following equation:

Equation 11

Where Vdriver is the 5V driver supply.

7.9

Reference voltage and bandgap

The 1.237V (typ.) internal bandgap voltage is accurate to ±1% over the temperature range.

It is externally available (VREF pin) and can supply up to 100

µA and can be used as a

voltage threshold for the multifunction pins FSEL and SKIP to select the appropriate working

mode. Bypass VREF to ground with a 100nF minimum capacitor.

If VREF goes below 0.87V (typ.) , the system detects a fault condition and all the circuitry is

turned off. A toggle on the input voltage (power on reset) or a toggle on SHDN pin is

necessary to restart the device.

An internal divider of the the bandgap provides a voltage reference Vr of 0.9V. This voltage

is used as reference for the linear and the switching regulators outputs. The overvoltage

protection, the undervoltage protection and the power good signals are referred to Vr.

driver

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