PM6675 datasheet(28/47 Pages) STMICROELECTRONICS

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Device description

PM6675

28/47

6.1.9

Gate drivers

The integrated high-current gate drivers allow using different power MOSFETs. The high-

side driver uses a bootstrap circuit which is supplied by the +5V rail. The BOOT and PHASE

pins work respectively as supply and return path for the high-side driver, while the low-side

driver is directly fed through VCC and PGND pins.

An important feature of the PM6675 gate drivers is the Adaptive Anti-Cross-Conduction

circuitry, which prevents high-side and low-side MOSFETs from being turned 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 only when the voltage at the PHASE node reaches

an internal threshold (2.5V typ.). Similarly, when the low-side MOSFET is turned off, the

high-side one remains off until the LGATE pin voltage is above 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 22

The low-side driver has been designed to have a low-resistance pull-down transistor

(0.6

Ω typ.) in order to prevent undesired start-up of the low-side MOSFET due to the Miller

effect.

6.1.10

Reference voltage and bandgap

The 1.237V internal bandgap reference has a granted accuracy of ±1% over the 0°C to

85°C temperature range. The VREF pin is a buffered replica of the bandgap voltage. It can

supply up to ±100

µA and is suitable to set the intermediate level of NOSKIP multifunction

pin. A 100nF (min.) bypass capacitor toward SGND is required to enhance noise rejection. If

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

turned off.

An internal divider derives a 0.6V±1% voltage (Vr) from the bandgap. This voltage is used

as reference for both the switching and the linear sections. The Over-Voltage Protection, the

Under-Voltage Protection and the Power-Good signals are also referred to Vr.

6.1.11

Switching section OV and UV protections

When the switching output voltage is about 115% of its nominal value, a latched Over-

Voltage Protection (OVP) occurs. In this case the synchronous rectifier immediately turns on

while the high-side MOSFET turns off. The output capacitor is rapidly discharged and the

load is preserved from being damaged. The OVP is also active during the soft start. Once

an OVP has taken part, a toggle on SWEN pin or a Power-On-Reset is necessary to exit

from the latched state.

When the switching output voltage is below 70% of its nominal value, a latched Under-

Voltage Protection occurs. This event causes the switching section to be immediately

disabled and both switches to be opened. The controller performs a Soft-End and the output

is eventually kept to ground, turning the low side MOSFET on when the voltage is lower than

400mV.

The Under-Voltage Protection circuit is enabled only at the end of the soft-start. Once an

UVP has taken part, a toggle on SWEN pin or a Power-On-Reset is necessary to clear the

fault state and restart the section.

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