PM6680ATR datasheet(26/48 Pages) STMICROELECTRONICS

26 / 48 page

Device description

PM6680A

26/48

Figure 35.

Soft start waveforms

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 0.3 V, 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 approximatively under 1 V.

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 5 V driver supply.

driver

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