PM6675AS datasheet(30/48 Pages) STMICROELECTRONICS

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Device description

PM6675AS

30/48

7.1.9

Gate drivers

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

side driver employes a bootstrap circuit which is supplied by the +5 V 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 feed through VCC and PGND pins.

An important feature of the PM6675AS 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 (2.5 V typ.). Similarly, when the low-side MOSFET is turned off, the

high-side one remains off until the LGATE pin voltage is above 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 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 ignition of the low-side MOSFET due to the Miller

effect.

7.1.10

Reference voltage and bandgap

The 1.237 V internal bandgap reference has a granted accuracy of ±1 % over the -25 °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 100 nF (min.) bypass capacitor toward SGND is required to enhance noise rejection.

If VREF falls below 0.8 V (typ.), the system detects a fault condition and all the circuitry is

turned off.

An internal divider derives a 0.6 V±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.

7.1.11

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

400 mV.

DRV

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driver

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