PM6685TR View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
PM6685TR
STMicroelectronics
'PM6685TR' PDF : 52 Pages View PDF
PM6685
Device description
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 Qg of the external
power MOSFETs and of the switching frequency, as shown in the following equation:
Pdriver = Vdriver × Qg × fsw
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 ±100µA and can be used as a
voltage threshold for the multifunction pins FSEL, SKIP and LDO3_SEL 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 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.
7.10
Internal linear regulators
The PM6685 has two linear regulators providing respectively 5V(LDO5) and 3.3V(LDO3) at
±2% accuracy. High side drivers, low side drivers and most of internal circuitry are supplied
by LDO5 output through VCC pin (an external RC filter may be applied between LDO5 and
VCC). Both linear regulators can provide an average output current of 50mA and a peak
output current of 100mA. Bypass both LDO5 and LDO3 outputs with a minimum 1µF
ceramic capacitor and a 4,7µF tantalum capacitor (ESR < 2Ω). If the 5V output goes below
4V, the system detects a fault condition and all the circuitry is turned off. A power on reset or
a toggle on SHDN pin is necessary to restart the device.
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